New Horizons

This article is about the space probe. For other uses, seeNew Horizons (disambiguation).

New Horizons is an interplanetary space probe that waslaunched as part of NASA's New Frontiers program.[1]Engineered by the Johns Hopkins University AppliedPhysics Laboratory (APL) and the Southwest ResearchInstitute (SwRI), with a team led by S. Alan Stern, thespacecraft was launched to study Pluto, its moons and theKuiper belt, performing ybys of the Pluto system andone or more other Kuiper belt objects (KBOs).[2][3][4][5]

New Horizons is the result of many years of work on mis-sions to send a spacecraft to Pluto, starting in 1990 withPluto 350, with Alan Stern and Fran Bagenal of the PlutoUnderground, and in 1992 with the Jet Propulsion Lab-oratory's Pluto Fast Flyby; the latter inspired by a USPSstamp that branded Pluto as Not Yet Explored. The am-bitious mission aimed to send a lightweight, cost-eectivespacecraft to Pluto, later evolving into a Kuiper belt ob-ject mission named Pluto Kuiper Express. However, be-cause of underwhelming support from NASA and grow-ing projected cost, the project was eventually cancelledaltogether in 2000.Following backlash from the cancellation, the New Fron-tiers program was established for missions that t in be-tween the big budgets of the Flagship Program and thelow budgets of the Discovery Program. The AppliedPhysics Laboratory, with a team consisting of formerPluto Kuiper Express team members, won a competitionto fund their New Horizons project, based on work left ofrom Pluto Kuiper Express, under the New Frontiers pro-gram. However, funding for the mission was not secureduntil after a nancial stando between the team and then-NASA Administrator Sean O'Keefe. After three years ofconstruction, and several delays at the launch site, NewHorizons was launched on January 19, 2006, from CapeCanaveral, directly into an Earth-and-solar-escape trajec-tory with an Earth-relative speed of about 16.26 kilome-ters per second (58,536 km/h; 36,373 mph); it set therecord for the highest launch speed of a human-made ob-ject from Earth.After a brief encounter with asteroid 132524 APL, NewHorizons proceeded to Jupiter, making its closest ap-proach on February 28, 2007, at a distance of 2.3 millionkilometers (1.4 million miles). The Jupiter yby provideda gravity assist that increased New Horizons ' speed by4 km/s (14,000 km/h; 9,000 mph). The encounter wasalso used as a general test of New Horizons ' scientic ca-

pabilities, returning data about the planets atmosphere,moons, and magnetosphere. Most of the post-Jupiter voy-age was spent in hibernation mode to preserve on-boardsystems, except for brief annual checkouts.[6] On Decem-ber 6, 2014, New Horizons was brought back online forthe Pluto encounter, and instrument check-out began.[7]On January 15, 2015, the New Horizons spacecraft be-gan its approach phase to Pluto. On July 14, 2015 11:49UTC (07:49 EDT), it ew 12,500 km (7,800 mi) abovethe surface of Pluto,[8][9] making it the rst spacecraftto explore the dwarf planet.[5][10] Thirteen hours later at00:52:37 UTC (20:52:37 EDT)[11] NASA received therst communication from the probe following a yby atthe time expected. Engineering data indicated that theyby was successful and the probe operated in all respectsas expected.[12][13][14]

1 HistoryExploring Pluto was contemplated since its discovery byClyde Tombaugh in 1930. One of many possibilities forthe Voyager 1 spacecraft after its yby of Saturn in 1980was to use Saturn as a slingshot towards Pluto for a ybyas early as March 1986.[15] However, scientists on theVoyager program decided that a yby of Titan duringthe Saturn encounter would be a more important scien-tic objective. A yby of Pluto was impossible, as notonly did the spacecraft make a close approach of Titan, itwas also on a trajectory that slingshot the spacecraft up-wards out of the ecliptic.[16] Because no mission to Plutowas planned by any space agency at the time, it would beleft unexplored by interplanetary spacecraft for years tocome. Shortly after Voyager 2 ' s yby of Neptune and itsndings at Triton in August 1989, scientists sought inter-est in a mission to Pluto and further studies for the exis-tence of a Kuiper belt and Kuiper belt objects, potentiallysimilar to Triton.[17]

1.1 The Pluto Underground, Pluto 350 andMariner Mark II

In May 1989, a group of scientists and engineers, in-cluding Alan Stern and Fran Bagenal, formed an alliancecalled the Pluto Underground. It was named in homageof the Mars Underground, another group of scientiststhat successfully lobbied for the restart of missions toMars, following the lack of such since the Viking pro-gram. The group started a letter writing campaign which

1

2 1 HISTORY

One of the many early concepts for a mission to Pluto was to senda Mariner Mark II spacecraft. The idea would later be ruled outin favour of a smaller, less expensive spacecraft similar to thePluto 350 concept.

aimed to bring to attention Pluto as a viable target forexploration.[18] In 1990, because of pressure from thescientic community, including those of the Pluto Under-ground, engineers at NASA decided to look into conceptsfor a mission to Pluto. At the time, it was thought that theatmosphere of Pluto would freeze and fall to the surfaceduring winter, and so a lightweight spacecraft was desir-able, as it would be able to reach Pluto before such anevent would occur. One of the earliest concepts was fora 40-kilogram spacecraft that would reach Pluto in 5 to 6years. The idea was shortly scrapped, however, becauseof the infeasibility of miniaturizing scientic instrumentsaboard such a spacecraft to that size.[19]

Another mission concept, known as Pluto 350, was de-veloped by Robert Farquhar of the Goddard Space FlightCenter, with Alan Stern and Fran Bagenal of the PlutoUnderground, who both served as study scientists for theproject. Pluto 350 aimed to send a spacecraft, weighing350 kilograms, to Pluto.[17] The spacecrafts minimalisticdesign was so to allow it to travel faster and be more cost-eective, in-contrast to most other big-budget projectsNASA were developing at the time, such as Galileo andCassini. Pluto 350, however, would later become contro-versial among mission planners at NASA, who consideredthe project to be too small and too high-risk. An alternateplan which was considered at one point was to send toPluto a conguration of the Mariner Mark II spacecraft,which would weigh 2,000 kilograms and cost US$3.2 bil-lion, in sharp contrast to Pluto 350 ' s $543 million cost.[19]While both projects competed for approval, Pluto 350was more favored by NASA mission planners, who werestarting to adopt smaller missions such as Mars Pathnderand NEAR Shoemaker.[17]

1.2 USPS postage stamp and Pluto FastFlyby

Main article: Pluto Fast FlybyFor many years, people had waved that stamp around as

The infamous Not Yet Explored USPS stamp that inspired sci-entists to research into and lobby for a mission to Pluto.

sort of a call to arms, as a motivating graphic: Not yetexplored. That stamp had been in so many presentationsby that point, I knew it would please people to have it goalong.S. Alan Stern on the USPS stamp[19]

In October 1991, the United States Postal Service re-leased a series of stamps commemorating NASA's explo-ration of the Solar System.[20] The series featured a stampfor all planets, displaying an image of the planet and high-lighting an associated spacecraft which was sent to it. Thestamp for Pluto, however, depicted a featureless sphere,presented with the phrase not yet explored in place ofthe name of a spacecraft. The stamps were unveiled ina ceremony at the Jet Propulsion Laboratory. Two sci-entists who attended the event, World Space Foundationpresident Robert Staehle and JPL scientist Stacy Wein-stein, were inspired by Plutos status on the stamp; suchthat they started to inquire about the feasibility of sendinga spacecraft to Pluto.[21][22] Engineers at the Jet Propul-sion Laboratory, inspired by the Not Yet Explored sta-tus of Pluto, also started to put forward ideas about a mis-sion to Pluto.[23][24]

In 1992, Staehle, with the help of JPL engineers and stu-dents from the California Institute of Technology, formedthe Pluto Fast Flyby project. The mission heralded thesame ideology as the Pluto 350 concept: small in sizeand cost-eective in scope, so that the spacecraft wouldbe able to get to Pluto faster and be aordable to de-velop and launch. Described as a radical mission con-cept, the mission would see two spacecraft being sent toPluto. Both spacecraft were to weigh only around 35-50kilograms each (including 7 kg worth of scientic instru-ments), and the project would cost less than US$500 mil-lion to develop, excluding launch costs.[21] Described byStaehle as a faster, better, [and] cheaper approach than

1.4 Alan Stern and New Horizons 3

the Pluto 350 and Mariner Mark II projects, it caught theattention of then-NASA Administrator Daniel S. Goldin,who ordered all work on both Pluto 350 and MarinerMark II to cease and shift all resources to the new PlutoFast Flyby project instead.[17]

During the development of Pluto Fast Flyby, however,there were multiple concerns from both NASA, Admin-istrator Goldin and the missions development team. Asresearch and development into the mission progressed,the projects size, scope, and budget all expanded. Ad-ditionally, morale among the team and personnel work-ing on interplanetary missions was low following the lossof the Mars Observer spacecraft during its attemptedAreocentric orbit insertion in August 1993. Alan Sternwould later cite that event as a signicant factor towardsthe low enthusiasm for the Pluto Fast Flyby project.[19]The launch of the spacecraft were intended to be per-formed using Titan IV rockets, which would have costUS$400 million each, thus raising the budget to overUS$1 billion.[17] Because of growing budget constraints,the dual-spacecraft concept was scrapped in favor ofsending a singular spacecraft to Pluto. The project wasstill too expensive, however, in the eyes of Administra-tor Goldin.[19] Alan Stern, as a compromise, reached anagreement with Russian Space Research Institute sci-entists in Moscow, in which Pluto Fast Flyby wouldbe launched atop a Proton rocket, saving NASA overUS$400 million in launch costs. The proposal was for-warded to Administrator Goldin, but he vetoed the pro-posal, recommending instead that the JPL look into thefeasibility of Pluto Fast Flyby being launched aboard asmaller rocket, such as the Delta II.[17]

1.3 Kuiper belt, Pluto Kuiper Express andcancellation

Main article: Pluto Kuiper ExpressDuring the course of the late 1990s, a number of Trans-

Concept art for the Pluto Kuiper Express mission; the last itera-tion of the original Pluto mission concepts, eventually cancelledin 2000.

Neptunian objects were discovered, conrming the exis-tence of a Kuiper belt. Interest in a mission to the Kuiperbelt arose to such that NASA instructed the JPL to re-purpose the mission as not only a Pluto yby, but also aKuiper belt object (KBO) yby. The mission was thusre-branded the Pluto Kuiper Express, after briey beingbilled as Pluto Express prior to the revision of the mission.The weight of the spacecraft was raised again, this timeto 175 kilograms, and NASA allowed further liberty withthe projects budget.[17] However, it was later decided byGoldin that Pluto Kuiper Express was of low importance,and thus cut funding to the project drastically. Eventually,despite ocial selection of scientic instruments and theappointment of several investigators, then-Science Mis-sion Directorate Edward J. Weiler ordered the cancella-tion of the entire Pluto and Kuiper belt mission in 2000,citing growing budget constraints, which had plagued theproject since its inception in 1992. At the time of cancel-lation, the projected costs surpassed $1 billion.[19][25]

1.4 Alan Stern and New Horizons

Main article: New Frontiers programThe cancellation of Pluto Kuiper Express angered most

Early concept art of the New Horizons spacecraft. The mis-sion, led by the Applied Physics Laboratory and S. Alan Stern,would become the rst mission to Pluto successfully funded andlaunched, after years of delays and cancellations.

of the scientic community, which led to most groups,such as The Planetary Society, lobbying NASA for a re-boot of Pluto Kuiper Express or, at the very least, a restartof a mission to Pluto. Internal divisions within NASA, in-cluding its Scientic Advisory Council, also voiced sup-port for a Pluto mission.[19] In response to the backlashcaused by the cancellation of Pluto Kuiper Express, Weilerdecided to inaugurate a new class of missions that wouldt between the big-budget Flagship Program and the low-budget Discovery Program, creating a compromise formissions such as the former Pluto Kuiper Express, whichproved to be too expensive for the Discovery Program.A competition was held, in which NASA would select a

4 2 MISSION PROFILE

mission concept to fund as part of the rst mission of theNew Frontiers program.[25]

Stamatios Tom Krimigis, head of the Applied PhysicsLaboratory's space division, one of many entrants in theNew Frontiers Program competition, formed the NewHorizons team with Alan Stern in December 2000. Ap-pointed as the projects principal investigator, Stern wasdescribed by Krimigis as the personication of the Plutomission.[25] New Horizons was based largely on Sternswork since Pluto 350 and involved most of the team fromPluto Kuiper Express.[17] The New Horizons proposal wasone of ve that were ocially submitted to NASA. It waslater selected as one of two nalists to be subject to athree-month concept study, in June 2001. The other -nalist, POSSE (Pluto and Outer Solar System Explorer),was a separate, but similar Pluto mission concept by theUniversity of Colorado Boulder, led by principal inves-tigator Larry W. Esposito, and supported by the JPL,Lockheed Martin and the University of California.[26]However, the APL, in addition to being supported byPluto Kuiper Express developers at the Goddard SpaceFlight Center and Stanford University,[26] were at an ad-vantage; they had recently developed NEAR Shoemakerfor NASA, which had successfully entered orbit around433 Eros earlier in the year, and would later land on theasteroid to scientic and engineering fanfare.[27]

In November 2001, New Horizons was ocially selectedfor funding as part of the New Frontiers program.[28]However, the new NASA Administrator appointed by theBush Administration, Sean O'Keefe, was not supportiveof New Horizons, and eectively cancelled it by not in-cluding it in NASAs budget for 2003. Weiler promptedStern to lobby for the funding of New Horizons in hopes ofthe mission appearing in the Planetary Science DecadalSurvey; a prioritized wish list, compiled by the UnitedStates National Research Council, that reects the opin-ions of the scientic community. After an intense cam-paign to gain support for New Horizons, the Planetary Sci-ence Decadal Survey of 2003-2013 was published in thesummer of 2002. New Horizons topped the list of projectsconsidered the highest priority among the scientic com-munity in the medium-size category; ahead of missions tothe Moon, and even Jupiter. Weiler stated that it was a re-sult that "[his] administration was not going to ght.[25]Funding for the mission was nally secured following thepublication of the report, and Sterns team were nallyable to start building the spacecraft and its instruments,with a planned launch in January 2006 and arrival at Plutoin 2015.[25]

2 Mission proleNew Horizons is the rst mission in NASAs New Fron-tiers mission category, larger and more expensive thanthe Discovery missions but smaller than the Flagship Pro-gram. The cost of the mission (including spacecraft and

Artists impression of New Horizons ' close encounter with thePlutoCharon system.

instrument development, launch vehicle, mission opera-tions, data analysis, and education/public outreach) is ap-proximately $700 million over 15 years (20012016)[29]The spacecraft was built primarily by Southwest Re-search Institute (SwRI) and the Johns Hopkins AppliedPhysics Laboratory. The missions principal investigatoris Alan Stern of the Southwest Research Institute (for-merly NASA Associate Administrator).After separation from the launch vehicle, overall con-trol was taken by Mission Operations Center (MOC)at the Applied Physics Laboratory in Howard County,Maryland. The science instruments are operated atClyde Tombaugh Science Operations Center (T-SOC) inBoulder, Colorado.[30] Navigation is performed at variouscontractor facilities, whereas the navigational positionaldata and related celestial reference frames are providedby the Naval Observatory Flagsta Station through Head-quarters NASA and JPL; KinetX is the lead on the NewHorizons navigation team and is responsible for planningtrajectory adjustments as the spacecraft speeds towardthe outer Solar System. Coincidentally the Naval Ob-servatory Flagsta Station was where the photographicplates were taken for the discovery of Plutos moonCharon; and the Naval Observatory is itself not far fromthe Lowell Observatory where Pluto was discovered.New Horizons was originally planned as a voyage to theonly unexplored planet in the Solar System. When thespacecraft was launched, Pluto was still classied as aplanet, later to be reclassied as a dwarf planet by theInternational Astronomical Union (IAU). Some membersof the New Horizons team, including Alan Stern, dis-

5agree with the IAU denition and still describe Pluto asthe ninth planet.[31] Plutos satellites Nix and Hydra alsohave a connection with the spacecraft: the rst letters oftheir names (N and H) are the initials of New Horizons.The moons discoverers chose these names for this rea-son, plus Nix and Hydras relationship to the mythologi-cal Pluto.[32]

In addition to the science equipment, there are severalcultural artifacts traveling with the spacecraft. These in-clude a collection of 434,738 names stored on a compactdisc,[33] a piece of Scaled Composites's SpaceShipOne,[34]a Not Yet Explored USPS stamp,[22][23] and a Flag ofthe United States, along with other mementos.[35]

About 1 ounce (30 g) of Clyde Tombaugh's ashes areaboard the spacecraft, to commemorate his discovery ofPluto in 1930.[36][37] A Florida-state quarter coin, whosedesign commemorates human exploration, is included,ocially as a trim weight.[38] One of the science packages(a dust counter) is named after Venetia Burney, who, asa child, suggested the name Pluto after its discovery.

3 Goal

View of Mission Operations at the Applied Physics Laboratory inLaurel, MD. (July 14, 2015).

The goal of the mission is to understand the formation ofthe Pluto system, the Kuiper belt, and the transformationof the early Solar System.[39] The spacecraft will studythe atmospheres, surfaces, interiors and environments ofPluto and its moons. It will also study other objects in theKuiper belt.[40] By way of comparison, New Horizons willgather 5,000 times as much data at Pluto as Mariner didat Mars.[41]

Some of the questions the mission will attempt to answerare: What is Plutos atmosphere made of and how doesit behave? What does its surface look like? Are therelarge geological structures? How do solar wind particlesinteract with Plutos atmosphere?[42]

Specically, the missions science objectives are to:[43]

map the surface composition of Pluto and Charon characterize the geology and morphology of Pluto

and Charon

characterize the neutral atmosphere of Pluto and itsescape rate

search for an atmosphere around Charon

map surface temperatures on Pluto and Charon

search for rings and additional satellites aroundPluto

conduct similar investigations of one or more Kuiperbelt objects

4 Design and construction

4.1 Spacecraft subsystems

New Horizons at the Kennedy Space Center in 2005

The spacecraft is comparable in size and general shape toa grand piano and has been compared to a piano gluedto a cocktail bar-sized satellite dish.[44] As a point ofdeparture, the team took inspiration from the Ulyssesspacecraft,[45] which also carried a radioisotope ther-moelectric generator (RTG) and dish on a box-in-boxstructure through the outer Solar System. Many sub-systems and components have ight heritage from APLsCONTOUR spacecraft, which in turn had heritage fromAPLs TIMED spacecraft.New Horizons body forms a triangle, almost 0.76 m(2.5 ft) thick. (The Pioneers have hexagonal bodies,whereas the Voyagers, Galileo, and CassiniHuygens havedecagonal, hollow bodies.) A 7075 aluminium alloy tubeforms the main structural column, between the launch ve-hicle adapter ring at the rear, and the 2.1 m (6 ft 11 in)radio dish antenna axed to the front at side. Thetitanium fuel tank is in this tube. The RTG attaches witha 4-sided titanium mount resembling a gray pyramid orstepstool. Titanium provides strength and thermal isola-tion. The rest of the triangle is primarily sandwich pan-els of thin aluminium facesheet (less than 164 in or 0.40

6 4 DESIGN AND CONSTRUCTION

mm) bonded to aluminium honeycomb core. The struc-ture is larger than strictly necessary, with empty space in-side. The structure is designed to act as shielding, reduc-ing electronics errors caused by radiation from the RTG.Also, the mass distribution required for a spinning space-craft demands a wider triangle.The interior structure is painted black to equalize tem-perature by radiative heat transfer. Overall, the space-craft is thoroughly blanketed to retain heat. Unlike thePioneers and Voyagers, the radio dish is also enclosed inblankets that extend to the body. The heat from the RTGadds warmth to the spacecraft while it is in the outer SolarSystem. While in the inner Solar System, the spacecraftmust prevent overheating, hence electronic activity is lim-ited, power is diverted to shunts with attached radiators,and louvers are opened to radiate excess heat. While thespacecraft is cruising inactively in the cold outer SolarSystem, the louvers are closed, and the shunt regulatorreroutes power to electric heaters.

4.1.1 Propulsion and attitude control

New Horizons has both spin-stabilized (cruise) and three-axis stabilized (science) modes controlled entirely withhydrazine monopropellant. Additional post launch delta-v of over 290 m/s (1,000 km/h; 650 mph) is providedby a 77 kg (170 lb) internal tank. Helium is used as apressurant, with an elastomeric diaphragm assisting ex-pulsion. The spacecrafts on-orbit mass including fuel isover 470 kg (1,040 lb) on the Jupiter yby trajectory, butwould have been only 445 kg (981 lb) for the backup di-rect ight option to Pluto. Signicantly, had the backupoption been taken, this would have meant less fuel forlater Kuiper belt operations.There are 16 thrusters on New Horizons: four 4.4 N (1.0lbf) and twelve 0.9 N (0.2 lbf) plumbed into redundantbranches. The larger thrusters are used primarily for tra-jectory corrections, and the small ones (previously usedon Cassini and the Voyager spacecraft) are used primar-ily for attitude control and spinup/spindown maneuvers.Two star cameras are used to measure the spacecraft at-titude. They are mounted on the face of the spacecraftand provide attitude information while in spin-stabilizedor 3-axis mode. In between the time of star camera read-ings, spacecraft orientation is provided by dual redundantminiature inertial measurement units. Each unit containsthree solid-state gyroscopes and three accelerometers.Two Adcole Sun sensors provide attitude determination.One detects the angle to the Sun, whereas the other mea-sures spin rate and clocking.

4.1.2 Power

A cylindrical radioisotope thermoelectric generator(RTG) protrudes in the plane of the triangle from onevertex of the triangle. The RTG will provide about 250

New Horizons ' RTG

W, 30 V DC at launch, and is predicted to drop approxi-mately 5% every 4 years, decaying to 200 W by the timeof its encounter with the Plutonian system in 2015. TheRTG, model "GPHS-RTG, was originally a spare fromthe Cassini mission. The RTG contains 10.9 kg (24 lb)of plutonium-238 oxide pellets. Each pellet is clad iniridium, then encased in a graphite shell. It was devel-oped by the U.S. Department of Energy at the Materialsand Fuels Complex, a part of the Idaho National Labo-ratory.[46] Less power than the original design goal wasproduced because of delays at the United States Depart-ment of Energy, including security activities, that delayedproduction. The mission parameters and observation se-quence had to be modied for the reduced wattage; still,not all instruments can operate simultaneously. The De-partment of Energy transferred the space battery programfrom Ohio to Argonne in 2002 because of security con-cerns. There are no onboard batteries. RTG output isrelatively predictable; load transients are handled by a ca-pacitor bank and fast circuit breakers.The amount of radioactive plutonium in the RTG is 10.9kg (24 lb), about one-third the amount on board theCassiniHuygens probe when it launched in 1997. Thatlaunch was protested by some. The United States Depart-ment of Energy estimated the chances of a launch acci-dent that would release radiation into the atmosphere at1 in 350, and monitored the launch[47] as it always doeswhen RTGs are involved. It was estimated that a worst-

4.1 Spacecraft subsystems 7

case scenario of total dispersal of on-board plutoniumwould spread the equivalent radiation of 80% the averageannual dosage in North America from background radi-ation over an area with a radius of 105 km (65 mi).[48]

4.1.3 Flight computer

The spacecraft carries two computer systems: the Com-mand and Data Handling system and the Guidance andControl processor. Each of the two systems is dupli-cated for redundancy, for a total of four computers. Theprocessor used for its ight computers is the Mongoose-V, a 12 MHz radiation-hardened version of the MIPSR3000 CPU. Multiple clocks and timing routines are im-plemented in hardware and software to help prevent faultsand downtime. To conserve heat and mass, spacecraftand instrument electronics are housed together in IEMs(integrated electronics modules). There are two redun-dant IEMs. Including other functions such as instrumentand radio electronics, each IEM contains 9 boards.[49]

There have been two sang events, that sent the space-craft into safe mode:

On March 19, 2007 the Command and Data Han-dling computer experienced an uncorrectable mem-ory error and rebooted itself, causing the spacecraftto go into safe mode. The craft fully recoveredwithin two days, with some data loss on Jupitersmagnetotail. No impact on the subsequent missionwas expected.[50]

On July 4, 2015 there was a CPU sang eventcaused by over assignment of commanded scienceoperations.[51][52][53]

4.1.4 Telecommunications and data handling

New Horizons ' antennas

Communication with the spacecraft is via X band. Thecraft had a communication rate of 38 kbit/s at Jupiter;at Plutos distance, a rate of approximately 1 kbit/s pertransmitter is expected. Besides the low bandwidth,Plutos distance also causes a latency of about 4.5 hours

(one-way). The 70 m (230 ft) NASA Deep Space Net-work (DSN) dishes are used to relay commands once itis beyond Jupiter. The spacecraft uses dual redundanttransmitters and receivers, and either right- or left-handcircular polarization. The downlink signal is ampliedby dual redundant 12-watt traveling-wave tube ampliers(TWTAs) mounted on the body under the dish. The re-ceivers are new, low-power designs. The system can becontrolled to power both TWTAs at the same time, andtransmit a dual-polarized downlink signal to the DSN thatnearly doubles the downlink rate. DSN tests early in themission with this dual polarization combining techniquewere successful, and the capability is now considered op-erational (when the spacecraft power budget permits bothTWTAs to be powered).In addition to the high-gain antenna, there are two backuplow-gain antennas and a medium-gain dish. The high-gain dish has a Cassegrain reector layout, compositeconstruction, and a 2.1-meter (7 ft) diameter providingover 42 dBi of gain, has a half-power beam width of abouta degree. The prime-focus, medium-gain antenna, with a0.3-meter (1 ft) aperture and 10 half-power beam width,is mounted to the back of the high-gain antennas sec-ondary reector. The forward low-gain antenna is stackedatop the feed of the medium-gain antenna. The aft low-gain antenna is mounted within the launch adapter at therear of the spacecraft. This antenna was used only forearly mission phases near Earth, just after launch and foremergencies if the spacecraft had lost attitude control.New Horizons recorded scientic instrument data to itssolid-state memory buer at each encounter, then trans-mitted the data to Earth. Data storage is done on two low-power solid-state recorders (one primary, one backup)holding up to 8 gigabytes each. Because of the extremedistance from Pluto and the Kuiper belt, only one buerload at those encounters can be saved. This is becauseNew Horizons will require approximately 16 months afterit has left the vicinity of Pluto (or future target object) totransmit the buer load back to Earth.[54] At Plutos dis-tance transmissions from the space probe back to earthtook four hours 25 minutes to traverse 4.7 billion km ofspace.[55]

Part of the reason for the delay between the gathering ofand transmission of data is because all of the New Hori-zons instrumentation is body-mounted. In order for thecameras to record data, the entire probe must turn, andthe one-degree-wide beam of the high-gain antenna willalmost certainly not be pointing toward Earth. Previousspacecraft, such as the Voyager program probes, had a ro-tatable instrumentation platform (a scan platform) thatcould take measurements from virtually any angle with-out losing radio contact with Earth. New Horizons ' elim-ination of excess mechanisms was implemented to saveweight, shorten the schedule, and improve reliability dur-ing its 15-year lifetime.The Voyager 2 spacecraft experienced platform jamming

8 4 DESIGN AND CONSTRUCTION

at Saturn; the demands of long time exposures at Uranusled to modications of the mission such that the entireprobe was rotated to make the time exposure photos atUranus and Neptune, similar to how New Horizons willrotate.

4.2 Science payload

New Horizons carries seven instruments: three optical in-struments, two plasma instruments, a dust sensor and aradio science receiver/radiometer. The instruments areto be used to investigate the global geology, surface com-position, surface temperature, atmospheric pressure, at-mospheric temperature and escape rate of Pluto and itsmoons. The rated power is 21 watts, though not all instru-ments operate simultaneously.[56] In addition, New Hori-zons has an Ultrastable Oscillator subsystem, which maybe used to study and test the Pioneer anomaly towards theend of the spacecrafts life.[57]

4.2.1 Long-Range Reconnaissance Imager(LORRI)

LORRIlong-range camera

The Long-Range Reconnaissance Imager (LORRI) is along-focal-length imager designed for high resolution andresponsivity at visible wavelengths. The instrument isequipped with a 10241024 pixel by 12-bits-per-pixelmonochromatic CCD imager with a 208.3 mm (8.20 in)aperture giving a resolution of 5 rad (~1 arcsec).[58] TheCCD is chilled far below freezing by a passive radiatoron the antisolar face of the spacecraft. This temperaturedierential requires insulation, and isolation from the restof the structure. The RitcheyChretien mirrors and me-tering structure are made of silicon carbide, to boost sti-ness, reduce weight, and prevent warping at low tempera-tures. The optical elements sit in a composite light shield,and mount with titanium and berglass for thermal isola-tion. Overall mass is 8.6 kg (19 lb), with the optical tubeassembly (OTA) weighing about 5.6 kg (12 lb),[59] for oneof the largest silicon-carbide telescopes own at the time

(now surpassed by Herschel). For viewing on public websites the 12-bit per pixel LORRI images are converted to8-bit per pixel JPEG images.[58] These public images donot contain the full dynamic range of brightness informa-tion available from the raw LORRI images les.[58]

Principal investigator: Andy Cheng, AppliedPhysics Laboratory, Data: LORRI image search atjhuapl.edu[60]

4.2.2 Solar Wind At Pluto (SWAP)

SWAPSolar Wind At Pluto

Solar Wind At Pluto (SWAP) is a toroidal electrostaticanalyzer and retarding potential analyzer (RPA), thatmakes up one of the two instruments comprising NewHorizons ' Plasma and high-energy particle spectrome-ter suite (PAM), the other being PEPSSI. SWAP mea-sures particles of up to 6.5 keV and, because of the ten-uous solar wind at Plutos distance, the instrument is de-signed with the largest aperture of any such instrumentever own.

Principal investigator: David McComas, SouthwestResearch Institute

4.2.3 Pluto Energetic Particle Spectrometer ScienceInvestigation (PEPSSI)

Pluto Energetic Particle Spectrometer Science Investiga-tion (PEPSSI) is a time of ight ion and electron sensorthat makes up one of the two instruments comprising NewHorizons ' plasma and high-energy particle spectrome-ter suite (PAM), the other being SWAP. Unlike SWAP,which measures particles of up to 6.5 keV, PEPSSI goesup to 1 MeV.

Principal investigator: Ralph McNutt Jr., AppliedPhysics Laboratory

4.2 Science payload 9

4.2.4 Alice

Alice is an ultraviolet imaging spectrometer that is oneof two photographic instruments comprising New Hori-zons ' Pluto Exploration Remote Sensing Investigation(PERSI); the other being the Ralph telescope. It resolves1,024 wavelength bands in the far and extreme ultravi-olet (from 50180 nm), over 32 view elds. Its goal isto determine the atmospheric composition of Pluto. ThisAlice instrument is derived from another Alice aboard theESA's Rosetta spacecraft.

Principal investigator: Alan Stern, Southwest Re-search Institute

4.2.5 Ralph telescope

Ralphtelescope and color camera

The Ralph telescope, 6 cm (2.4 in) in aperture, is one oftwo photographic instruments that make up New Hori-zons ' Pluto Exploration Remote Sensing Investigation(PERSI), with the other being the Alice instrument.Ralph has two separate channels: a visible-light CCDimager (MVIC- Multispectral Visible Imaging Camera)with broadband and color channels, and a near-infraredimaging spectrometer, LEISA (Linear Etalon ImagingSpectral Array). LEISA is derived from a similar instru-ment on the EO-1 mission. Ralph was named after Al-ices husband on The Honeymooners, and was designedafter Alice.[61]

Principal investigator: Alan Stern, Southwest Re-search Institute

4.2.6 Venetia Burney Student Dust Counter (VB-SDC)

The Venetia Burney Student Dust Counter (VBSDC),built by students at the University of Colorado Boulder,will operate continuously to make dust measurements.[62]It consists of a detector panel, about 460 mm 300mm (18 in 12 in), mounted on the anti-solar face ofthe spacecraft (the ram direction), and an electronics

VBSDCVenetia Burney Student Dust Counter

box within the spacecraft. The detector contains four-teen polyvinylidene diuoride (PVDF) panels, twelve sci-ence and two reference, which generate voltage whenimpacted. Eective collecting area is 0.125 m2 (1.35sq ft). No dust counter has operated past the orbit ofUranus; models of dust in the outer Solar System, espe-cially the Kuiper belt, are speculative. The VBSDC isalways turned on measuring the masses of the interplan-etary and interstellar dust particles (in the range of nano-and picograms) as they collide with the PVDF panelsmounted on the New Horizons spacecraft. The measureddata is expected to greatly contribute to the understandingof the dust spectra of the Solar System. The dust spec-tra can then be compared with those from observationsof other stars, giving new clues as to where Earth-likeplanets can be found in the universe. The dust counter isnamed for Venetia Burney, who rst suggested the namePluto at the age of 11. A thirteen-minute short lmabout the VBSDC garnered an Emmy Award for studentachievement in 2006.[63]

Principal investigator: Mihaly Horanyi, University ofColorado Boulder

4.2.7 Radio Science Experiment (REX)

The Radio Science Experiment (REX) will use an ultra-stable crystal oscillator (essentially a calibrated crystal ina miniature oven) and some additional electronics to con-duct radio science investigations using the communica-tions channels. These are small enough to t on a singlecard. Because there are two redundant communicationssubsystems, there are two, identical REX circuit boards.

Principal investigators: Len Tyler and Ivan Linscott,Stanford University

REX information, from a BOINC screensaver

BOINC screensaver, Charon Radio Science

10 5 JOURNEY TO PLUTO

REX information, link to PDF of Radio ScienceProposal

5 Journey to Pluto

5.1 Launch

Launch of New Horizons. The Atlas V rocket on thelaunchpad (left) and lift o from Cape Canaveral. NewHorizons ' launch was the fastest ever to date.

On September 24, 2005 the spacecraft arrived at theKennedy Space Center on board a C-17 GlobemasterIII for launch preparations.[64] The launch of New Hori-zons was originally scheduled for January 11, 2006, butwas initially delayed until January 17, 2006 to allow forborescope inspections of the Atlas V's kerosene tank.Further delays related to low cloud ceiling conditionsdownrange, and high winds and technical dicultiesunrelated to the rocket itselfprevented launch for a fur-ther two days.[65][66] Although there were backup launchopportunities in February 2006 and February 2007, only

the rst twenty-three days of the 2006 window permittedthe Jupiter yby. Any launch outside that period wouldhave forced the spacecraft to y a slower trajectory di-rectly to Pluto, delaying its encounter by 24 years. Theprobe nally lifted o from Pad 41 at Cape CanaveralAir Force Station, Florida, directly south of Space Shut-tle Launch Complex 39, at 14:00 EST on January 19,2006.[67][68]

The Centaur second stage reignited at 14:30 EST (19:30UTC), followed by the ATK Star 48B third stage, suc-cessfully sending the probe on a solar-escape trajectory.New Horizons took only nine hours to reach the Moonsorbit, passing lunar orbit before midnight EST that day.The probe was launched by a Lockheed Martin Atlas V551 rocket, with a third stage added to increase the he-liocentric (escape) speed. This was the rst launch of theAtlas V 551 conguration, which uses ve solid rocketboosters, and the rst Atlas V with a third stage. Pre-vious ights had used zero, two, or three solid boosters,but never ve. The vehicle, AV-010, weighed 570,000 kg(1.26 million lb) at lift-o, and had earlier been slightlydamaged when Hurricane Wilma swept across Floridaon October 24, 2005. One of the solid rocket boosterswas hit by a door. The booster was replaced with anidentical unit, rather than inspecting and requalifying theoriginal.[69]

New Horizons is often given the title of Fastest SpacecraftEver Launched, and left Earth faster than any other space-craft to date. It is also the rst spacecraft launched di-rectly into a solar escape trajectory, which requires anapproximate speed of 16.5 km/s (59,000 km/h; 37,000mph), plus aerodynamic and gravity losses, all to be pro-vided by the launcher. But it is not the fastest space-craft to leave the Solar System. This record is held byVoyager 1, currently traveling at 17.145 km/s (61,720km/h; 38,350 mph) relative to the Sun. Voyager 1 at-tained greater hyperbolic excess velocity from Jupiter andSaturn gravitational slingshots than New Horizons. Otherspacecraft, such as the Helios probes, can also be mea-sured as the fastest objects, because of their orbital speedrelative to the Sun at perihelion. Because they remain insolar orbit, their specic orbital energy relative to the Sunis lower than New Horizons and other articial objects es-caping the Solar System.The Star 48B third-stage is also on a hyperbolic SolarSystem escape trajectory, and reached Jupiter before theNew Horizons spacecraft. The Star 48B is expected tocross Plutos orbit on October 15, 2015.[70] Because it isnot in controlled ight, it did not receive the correct grav-ity assist, and will only pass within 200 million km (120million mi) of Pluto.[70] The Centaur stage did not achievesolar escape velocity, and is in heliocentric orbit.[71] Thelaunch was dedicated to the memory of launch conductorDaniel Sarokon, who was described by space program of-cials as one of the most inuential people in the historyof space travel.[72]

5.2 Inner Solar System 11

5.2 Inner Solar System

5.2.1 Trajectory corrections

On January 28 and 30, 2006, mission controllers guidedthe probe through its rst trajectory-correction maneuver(TCM), which was divided into two parts (TCM-1A andTCM-1B). The total velocity change of these two cor-rections was about 18 meters per second (65 km/h; 40mph). TCM-1 was accurate enough to permit the can-cellation of TCM-2, the second of three originally sched-uled corrections.[73] On March 9, 2006, controllers per-formed TCM-3, the last of three scheduled course cor-rections. The engines burned for 76 seconds, adjustingthe spacecrafts velocity by about 1.16 m/s (4.2 km/h; 2.6mph).[74] Further trajectory maneuvers were not neededuntil September 25, 2007 (seven months after the Jupiteryby), when the engines were red for 15 minutes and37 seconds, changing the spacecrafts velocity by 2.37m/s (8.5 km/h; 5.3 mph),[75] followed by another TCM,almost three years later on June 30, 2010, that lasted35.6 seconds, when New Horizons had already reachedthe halfway point (in time traveled) to Pluto.[76]

5.2.2 In-ight tests and Mars orbit

During the week of February 20, 2006, controllers con-ducted initial in-ight tests of three onboard science in-struments, the Alice ultraviolet imaging spectrometer,the PEPSSI plasma-sensor, and the LORRI long-rangevisible-spectrum camera. No scientic measurements orimages were taken, but instrument electronics, and inthe case of Alice, some electromechanical systems wereshown to be functioning correctly.[77] On April 7, 2006,the spacecraft passed the orbit of Mars, moving at roughly21 km/s (76,000 km/h; 47,000 mph) away from the Sunat a solar distance of 243 million kilometers.[78][79][80]

5.2.3 Asteroid 132524 APL

Asteroid 132524 APL viewed by New Horizons in June2006

First images of Pluto in September 2006

Because of the need to conserve fuel for possible en-counters with Kuiper belt objects subsequent to the Plutoyby, intentional encounters with objects in the asteroidbelt were not planned. After launch, the New Horizonsteam scanned the spacecrafts trajectory to determine ifany asteroids would, by chance, be close enough for ob-servation. In May 2006 it was discovered that New Hori-zons would pass close to the tiny asteroid 132524 APLon June 13, 2006. Closest approach occurred at 4:05UTC at a distance of 101,867 km (63,297 mi). The as-teroid was imaged by Ralph (use of LORRI was not pos-sible because of proximity to Sun), which gave the teama chance to test Ralph's capabilities, and make observa-tions of the asteroids composition as well as light andphase curves. The asteroid was estimated to be 2.5 km(1.6 mi) in diameter.[81][82][83] The spacecraft success-fully tracked the asteroid over June 1012, 2006. Thisallowed the mission team to test the spacecrafts abilityto track rapidly moving objects. Images were obtainedthrough the Ralph telescope.[83]

5.2.4 First Pluto sighting

The rst images of Pluto from New Horizons were ac-quired September 2124, 2006, during a test of theLORRI. They were released on November 28, 2006.[84]The images, taken from a distance of approximately4,200,000,000 km (2.6109 mi) or 28.07 AU, conrmedthe spacecrafts ability to track distant targets, critical formaneuvering toward Pluto and other Kuiper belt objects.

12 5 JOURNEY TO PLUTO

5.3 Jupiter encounter

See also: Exploration of Jupiter

Jupiter viewed by LORRI in January 2007

Ralph sees Jupiter in infrared in February 2007.

New Horizons used LORRI to take its rst photographsof Jupiter on September 4, 2006 from a distance of 291million kilometers (181 million miles).[85] More detailedexploration of the system began in January 2007 with aninfrared image of the moon Callisto, as well as severalblack-and-white images of Jupiter itself.[86] New Hori-zons received a gravity assist from Jupiter, with its closestapproach at 05:43:40 UTC on February 28, 2007 whenit was 2.3 million kilometers (1.4 million miles) fromJupiter. The yby increased New Horizons ' speed by 4km/s (14,000 km/h; 9,000 mph), accelerating the probeto a velocity of 23 km/s (83,000 km/h; 51,000 mph) rela-tive to the Sun and shortening its voyage to Pluto by threeyears.[87]

The yby was the center of a 4-month intensive obser-vation campaign lasting from January to June. Beingan ever-changing scientic target, Jupiter has been ob-served intermittently since the end of the Galileo mis-

sion in September 2003. Knowledge about Jupiter ben-eted from the fact that New Horizons ' instrumentswere built using the latest technology, especially in thearea of cameras, representing a signicant improvementover Galileo's cameras, which were modied versions ofVoyager cameras, which, in turn, were modied Marinercameras. The Jupiter encounter also served as a shake-down and dress rehearsal for the Pluto encounter. Be-cause Jupiter is much closer to Earth than Pluto, the com-munications link can transmit multiple loadings of thememory buer; thus the mission returned more data fromthe Jovian system than it is expected to transmit fromPluto.[88]

One of the main goals during the Jupiter encounter wasobserving its atmospheric conditions and analyzing thestructure and composition of its clouds. Heat-inducedlightning strikes in the polar regions and waves thatindicate violent storm activity were observed and mea-sured. The Little Red Spot, spanning up to 70% ofEarths diameter, was imaged from up close for the rsttime.[87]

Recording from dierent angles and illumination con-ditions, New Horizons took detailed images of Jupitersfaint ring system, discovering debris left over from re-cent collisions within the rings or from other unexplainedphenomena. The search for undiscovered moons withinthe rings showed no results. Travelling through Jupitersmagnetosphere, New Horizons collected valuable particlereadings.[87] Bubbles of plasma that are thought to beformed from material ejected by the moon Io, were no-ticed in the magnetotail.[89]

The Tvashtar volcano erupting on Io during yby

5.3.1 Jovian moons

The four largest moons of Jupiter were in poor positionsfor observation; the necessary path of the gravity-assist

5.4 Outer Solar System 13

maneuver meant that New Horizons passed millions ofkilometers from any of the Galilean moons. Still, its in-struments were intended for small, dim targets, so theywere scientically useful on large, distant moons. Em-phasis was put on Jupiters innermost Galilean moon, Io,whose active volcanoes shoot out tons of material intoJupiters magnetosphere, and further. Out of eleven ob-served eruptions, three were seen for the rst time. Thatof Tvashtar reached an altitude of up to 330 kilometers.The event gave scientists an unprecedented look into thestructure and motion of the rising plume and its subse-quent fall back to the surface. Infrared signatures of a fur-ther 36 volcanoes were noticed.[87] Callistos surface wasanalyzed with LEISA, revealing how lighting and viewingconditions aect infrared spectrum readings of its surfacewater ice.[90] Minor moons such as Amalthea had their or-bit solutions rened. The cameras determined their posi-tions, acting as reverse optical navigation.

5.4 Outer Solar System

After passing Jupiter, New Horizons spent most of itsjourney towards Pluto in hibernation mode: redundantcomponents as well as guidance and control systems wereshut down to extend their life cycle, decrease opera-tion costs and free the Deep Space Network for othermissions.[91] During hibernation mode, the onboard com-puter monitored the probes systems and transmitted asignal back to Earth: a green code if everything wasfunctioning as expected or a red code if mission con-trols assistance was needed.[91] The probe was activatedfor about two months a year so that the instruments couldbe calibrated and the systems checked. The rst hiberna-tion mode cycle started on June 28, 2007,[91] the secondcycle began on December 16, 2008,[92] the third cycle onAugust 27, 2009,[93] and the fourth cycle on August 29,2014 after a 10-week test.[94]

New Horizons crossed the orbit of Saturn on June 8,2008,[95] and Uranus on March 18, 2011.[96] After as-tronomers announced the discovery of two new moons inthe Pluto system, Kerberos and Styx, mission plannersstarted contemplating the possibility of the probe run-ning into unseen debris and dust left over from earliercollisions with the moons. A study based on 18 monthsof computer simulations, Earth-based telescope obser-vations and occultations of the Pluto system revealedthat the possibility of a catastrophic collision with de-bris or dust was less than 0.3% on the probes sched-uled course.[97][98] If the hazard increased, New Horizonscould have used one of two possible contingency plans,the so-called SHBOTs (Safe Haven by Other Trajecto-ries): the probe could have continued on its present tra-jectory with the antenna facing the incoming particles sothe more vital systems would be protected, or, it couldhave positioned its antenna to make a course correctionthat would take it just 3000 km from the surface of Plutowhere it was expected that the atmospheric drag would

clean the surrounding space of possible debris.[98]

While in hibernation mode in July 2012, New Horizonsstarted gathering scientic data with SWAP, PEPSSI andVBSDC. Although it was originally planned to activatejust the VBSDC, other instruments were powered on theinitiative of principal investigator Alan Stern who de-cided they could use the opportunity to collect valuableheliospheric data. Before activating the other two instru-ments, ground tests were conducted to make sure thatthe expanded data gathering in this phase of the mis-sion would not limit available energy, memory and fuelin the future and that all systems are functioning duringthe yby.[99] The rst set of data was transmitted in Jan-uary 2013 during a three-week activation from hiberna-tion. The command and data handling software was up-dated to address the problem of computer resets.[100]

5.4.1 Possible Neptune trojan targets

Other possible targets were Neptune trojans. Theprobes trajectory to Pluto passed near Neptunes trailingLagrange point ("L5"), which may host hundreds of bod-ies in 1:1 resonance. In late 2013, New Horizons passedwithin 1.2 AU (180,000,000 km; 110,000,000 mi) ofthe high-inclination L5 Neptune trojan 2011 HM102,[101]which was identied shortly before by the New HorizonsKBO Search Survey team while searching for more distantobjects for New Horizons to y by after its 2015 Plutoencounter. At that range, 2011 HM102 would have beenbright enough to be detectable by New Horizons ' LORRIinstrument; however, the New Horizons team eventuallydecided that they would not target 2011 HM102 for obser-vations because the preparations for the Pluto approachtook precedence.[102]

5.4.2 Observations of Pluto and Charon 201314

Images from July 1 to 3, 2013 by LORRI were the rstby the probe to resolve Pluto and Charon as separateobjects.[103] On July 14, 2014, mission controllers per-formed a sixth trajectory-correction maneuver (TCM)since its launch to enable the craft to reach Pluto.[104] Be-tween July 1924, 2014, New Horizons ' LORRI snapped12 images of Charon revolving around Pluto, cover-ing almost one full rotation at distances ranging fromabout 429,000,000 km (267,000,000 mi) to 422,000,000km (262,000,000 mi).[105] In August 2014, astronomersmade high-precision measurements of Plutos locationand orbit around the Sun using the Atacama Large Mil-limeter/submillimeter Array (ALMA) to help NASAsNew Horizons spacecraft accurately home in on Pluto.[106]On December 6, 2014, mission controllers sent a signalfor the craft to wake up from its nal Pluto-approachhibernation and begin regular operations. The crafts re-sponse that it was awake arrived to Earth at 9:30 PMEST.[107][108][109]

14 5 JOURNEY TO PLUTO

5.5 Pluto approach

Timeline: Pluto science operation

January and February 2015: New Horizons takes imagesof Pluto, Charon, Nix, and Hydra

The PlutoCharon system imaged by New Horizons onJune 29, 2015

Distant-encounter operations at Pluto began on January4, 2015.[110] At this date images of the targets with theonboard LORRI imager plus Ralph telescope would onlybe a few pixels in width. Investigators began taking Plutoand background stareld images to assist mission naviga-tors in the design of course-correcting engine maneuversthat would precisely modify the trajectory of New Hori-zons to aim the approach.On January 15, 2015, NASA gave a brief update of thetimeline of the approach and departure phases.[111]

On February 12, 2015, NASA released new images ofPluto (taken from January 25 to 31) from the approachingprobe.[112][113] New Horizons was more than 203,000,000km (126,000,000 mi) away from Pluto when it begantaking the photos, which showed Pluto and its largestmoon, Charon. The exposure time was too short to seePlutos smaller, much fainter, moons. Investigators com-piled a series of images of the moons Nix and Hydrataken from January 27 through February 8, 2015, begin-ning at a range of 201,000,000 km (125,000,000 mi).[114]Pluto and Charon appear as a single overexposed objectat the center. The right side image has been processedto remove the background stareld. The yet smaller twomoons, Kerberos and Styx were seen on photos takenon April 25.[115] Starting May 11 a hazard search was

Animation of the PlutoCharon system in January 2015

performed, by looking for until then unknown objectsthat could be a danger to the spacecraft, such as ringsor more moons, which were possible to avoid by a coursechange.[116]

Also in regards to the approach phase during January2015, on August 21, 2012, the team announced that theywould also spend mission time attempting long-range ob-servations of the Kuiper belt object temporarily desig-nated VNH0004, when the object was at a distance fromNew Horizons of 75 gigameters (0.50 AU).[117] The ob-ject would be too distant to resolve surface features ortake spectroscopy, but it would be able to make observa-tions that cannot be made from Earth, namely a phasecurve and a search for small moons. A second objectwas planned to be observed in June 2015, and a third inSeptember after the yby; the team hoped to observe adozen such objects through 2018.[117] On April 15, 2015,Pluto was imaged showing a possible polar cap.[118]

5.5.1 Software glitch

On July 4, 2015, New Horizons experienced a softwareanomaly and went into safe mode, preventing the space-craft from performing scientic observations until en-gineers could resolve the problem.[119][120] On July 5,NASA announced that the problem was determined tobe a timing aw in a command sequence used to preparethe spacecraft for its yby, and the spacecraft would re-sume scheduled science operations on July 7. The scienceobservations lost because of the anomaly were judged tohave no impact on the missions main objectives and min-imal impact on other objectives.[121]

The timing aw consisted of performing two taskssimultaneouslycompressing previously acquired datato release space for more data, and making a secondcopy of the approach command sequencethat together

6.1 Objectives 15

overloaded the spacecrafts primary computer. After theoverload was detected, the spacecraft performed as de-signed: it switched from the primary computer to thebackup computer, entered safe mode, and sent a distresscall back to Earth. The distress call was received theafternoon of July 4, which alerted engineers that theyneeded to contact the spacecraft to get more informationand resolve the issue. The resolution was that the problemhappened as part of preparations for the approach, andwas not expected to happen again because no similar taskswere planned for the remainder of the encounter.[121][122]

6 Pluto system encounter

Eyes on the Solar System simulation of New Horizons Pluto en-counter.

Pluto viewed by New Horizons on July 13, 2015

Charon viewed by New Horizons on July 13, 2015The closest approach of the New Horizons spacecraft

Alan Stern and the New Horizons team celebrate after the space-craft successfully completed its yby of Pluto.

to Pluto occurred at 11:49 UTC on July 14, 2015 at arange of 7,750 miles (12,472 km) from the surface[123]and 8,487 miles (13,658 km) from the center of Pluto.Telemetry data conrming a successful yby and ahealthy spacecraft were received on Earth from the vicin-ity of the Pluto system on July 15, 2015, 00:52:37UTC,[124] after 22 hours of planned radio silence dueto the spacecraft being pointed toward the Pluto system.Mission managers estimated a one in 10,000 chance thatdebris could have destroyed it during the yby, preventingit from sending science data to Earth.[125] The rst detailsof the encounter were received later in the day, but thedownload of the complete data will take 16 months, andanalysis of the data will take longer.

6.1 ObjectivesThe missions science objectives are grouped in three dis-tinct priorities. The primary objectives are required byNASA and a loss of any of these objectives will con-stitute a partial failure of the mission, while it is ex-pected, but not demanded, that most of the secondaryobjectives will be met. The tertiary objectives are de-sired. These objectives may be attempted, though theymay be skipped in favor of the above objectives. Anobjective to measure any magnetic eld of Pluto wasdropped. A magnetometer instrument could not be im-plemented within a reasonable mass budget and schedule,and SWAP and PEPSSI could do an indirect job detect-ing some magnetic eld around Pluto.

Primary objectives (required) Characterize the global geology and morphol-

ogy of Pluto and Charon Map chemical compositions of Pluto and

Charon surfaces Characterize the neutral (non-ionized)

atmosphere of Pluto and its escape rate Secondary objectives (expected)

16 6 PLUTO SYSTEM ENCOUNTER

Characterize the time variability of Plutossurface and atmosphere

Image select Pluto and Charon areas in stereo Map the terminators (day/night border) of

Pluto and Charon with high resolution Map the chemical compositions of select Pluto

and Charon areas with high resolution Characterize Plutos ionosphere (upper layer

of the atmosphere) and its interaction with thesolar wind

Search for neutral species such as molecularhydrogen, hydrocarbons, hydrogen cyanideand other nitriles in the atmosphere

Search for any Charon atmosphere Determine bolometric Bond albedos for Pluto

and Charon Map surface temperatures of Pluto and

Charon Map any additional surfaces of outermost

moons: Nix, Hydra, Kerberos, and Styx

Tertiary objectives (desired) Characterize the energetic particle environ-

ment at Pluto and Charon Rene bulk parameters (radii, masses) and or-

bits of Pluto and Charon Search for additional moons and any rings

6.2 Flyby details

Pluto yby trajectory on July 14, 2015

New Horizons passed within 12,500 km (7,800 mi) ofPluto, with this closest approach on July 14, 2015 at11:50 UTC. New Horizons had a relative velocity of 13.78km/s (49,600 km/h; 30,800 mph) at its closest approach,and came as close as 28,800 km (17,900 mi) to Charon.Starting 3.2 days before the closest approach, long-rangeimaging included the mapping of Pluto and Charon to40 km (25 mi) resolution. This is half the rotation pe-riod of the PlutoCharon system and allowed imaging of

Icy mountains on Pluto, July 14, 2015

all sides of both bodies. Coverage will repeat twice perday, to search for changes due to snows or cryovolcanism.Because of Plutos tilt, a portion of the northern hemi-sphere will be in shadow at all times. During the yby,LORRI should be able to obtain select images with res-olution as high as 50 m/px (if closest distance is around12,500 km), and MVIC should obtain four-color globaldayside maps at 1.6 km resolution. LORRI and MVICwill attempt to overlap their respective coverage areas toform stereo pairs. LEISA will obtain hyperspectral near-infrared maps at 7 km/px globally and 0.6 km/px for se-lected areas.Meanwhile, Alice will characterize the atmosphere, bothby emissions of atmospheric molecules (airglow), andby dimming of background stars as they pass behindPluto (occultation). During and after closest approach,SWAP and PEPSSI will sample the high atmosphere andits eects on the solar wind. VBSDC will search fordust, inferring meteoroid collision rates and any invisi-ble rings. REX will perform active and passive radio sci-ence. Ground stations on Earth will transmit a powerfulradio signal as New Horizons passes behind Plutos disk,then emerges on the other side. The communicationsdish will measure the disappearance and reappearanceof the radio occultation signal. The results will resolvePlutos diameter (by their timing) and atmospheric den-sity and composition (by their weakening and strength-ening pattern). (Alice can perform similar occultations,using sunlight instead of radio beacons.) Previous mis-sions had the spacecraft transmit through the atmosphere,to Earth (downlink). Low power and extreme distancemeans New Horizons will be the rst such uplink mis-sion. Plutos mass and mass distribution will be evalu-ated by their tug on the spacecraft. As the spacecraftspeeds up and slows down, the radio signal will experi-ence a Doppler shift. The Doppler shift will be measuredby comparison with the ultrastable oscillator in the com-munications electronics.Reected sunlight from Charon will allow some imagingobservations of the nightside. Backlighting by the Sunwill highlight any rings or atmospheric hazes. REX willperform radiometry of the nightside.

17

Distribution of methane ice on Pluto(July 12, 2015).

6.3 Lesser satellite observations

New Horizons ' best spatial resolution of the small satel-lites is 330 m/pixel at Nix, 780 m/pixel at Hydra, andapproximately 1.8 km/pixel at Kerberos and Styx. Esti-mates for the diameters of these bodies are: Nix, 56 26km; Hydra, 58 34 km; Kerberos, 31 km; and Styx, 8to 28 km. This translates to a resolution of 170, 75, 20,and 5 to 15 pixels in width for Nix, Hydra, Kerberos, andStyx, respectively.

7 Current status

New Horizons ' current position[126]

The spacecraft ew by the PlutoCharon system onJuly 14, 2015, and has now completed the scienceof its closest approach phase. New Horizons has sig-naled the event by a phone home with telemetryreporting that the spacecraft was healthy, its ightpath was within the margins, and science data of thePlutoCharon system had been recorded.[14][127]

The spacecraft currently travels at 14.52 km/s (9.02mi/s; 3.06 AU/a; 0.00839 AU/d) relative to the Sunand at 13.77 km/s (8.56 mi/s; 2.90 AU/a; 0.00795AU/d) relative to Pluto.

Radio signals take four and a half hours to travel be-tween the spacecraft and Earth.[126]

At that distance, the spacecraft can only transmitdata at 1 to 2 Kbps, which means that the transmis-sion of all data will require sixteen months.[128]

The brightness of the Sun from the spacecraft ismagnitude 19.2.[129]

New Horizons is heading in the direction of the con-stellation Sagittarius.[129]

The latest news and photos from the probe areposted on NASAs mission website.[2]

8 Future mission objectives

8.1 Kuiper belt object (KBO) mission

8.1.1 Target background

MercuryVenus

EarthMars

Asteroids

Jupiter

InnerSolar System

Jupiter

Saturn

Uranus

Pluto

Kuiper BeltSedna

OuterSolar System

Orbit ofSedna

Inner extentof Oort Cloud

Neptune

Big picture: from the inner Solar System to the Oortcloud with the Kuiper belt in between.

Artists concept of a Kuiper belt object (KBO), targetedby New Horizons[130]

18 8 FUTURE MISSION OBJECTIVES

The KBO 2014 MU69, a potential target for the Kuiperbelt object mission

After passing Pluto, New Horizons will continue onthrough the Kuiper belt, making a yby of a Kuiper beltobject (KBO) the next reasonable objective of an ex-tended mission. Mission planners searched for one ormore additional Kuiper belt objects (KBOs) of the orderof 50100 km (3162 mi) in diameter as targets for y-bys similar to the spacecrafts Plutonian encounter, but,despite the large population of KBOs, many factors limitthe number of possible targets. Because the ight path isdetermined by the Pluto yby, and the probe only has 33kilograms of hydrazine remaining, whatever object is vis-ited will need to be within a cone, extending from Pluto,of less than a degrees width. This ruled out any possi-bility for a yby of Eris, a trans-Neptunian object com-parable in size to Pluto.[131] It will also need to be within55 AU, because beyond 55 AU, the communications linkwill become too weak, and the RTG wattage will havedecayed signicantly enough to hinder observations. De-sirable KBOs would be well over 50 km (31 mi) in diam-eter, neutral in color (to compare with the reddish Pluto),and, if possible, have a moon that imparts a wobble. Af-ter a search along New Horizons ight path using theHubble Space Telescope, only three KBOs were found inrange, and one of those objects was later dropped fromconsideration.[132]

8.1.2 Search

In 2011 a dedicated search for suitable KBOs usingground telescopes was started by mission scientists.Large ground telescopes with wide-eld cameras, no-tably the twin 6.5-meter Magellan Telescopes in Chile,the 8.2-meter Subaru Observatory in Hawaii, and theCanadaFranceHawaii Telescope[101][133] were used tosearch for potential targets. Through the citizen-scienceproject, the public helped to scan telescopic images forpossible suitable mission candidates by participating inthe Ice Hunters project.[134][135][136][137][138] The ground-based search resulted in the discovery of about 143 KBOsof potential interest,[139] but none of these were close

enough to the ight path of New Horizons.[133] Only theHubble Space Telescope was deemed likely to nd a suit-able target in time for a successful KBO mission.[140] OnJune 16, 2014, time on Hubble was granted.[141] Hub-ble has a much greater ability to nd suitable KBOs thanground telescopes. The probability that a target for NewHorizons would be found was estimated beforehand atabout 95%.[142]

8.1.3 Suitable KBOs

On October 15, 2014, it was revealed thatHubbles search had uncovered three potentialtargets,[130][143][144][145][146] provisionally designatedPT1 (potential target 1), PT2 and PT3 by the NewHorizons team. All are objects with estimated diametersin the 3055 km range, too small to be seen by groundtelescopes, at distances from the Sun of 4344 AU, whichwould put the encounters in the 20182019 period.[143]The initial estimated probabilities that these objects arereachable within New Horizons ' fuel budget are 100%,7%, and 97%, respectively.[143] All are members ofthe cold (low-inclination, low-eccentricity) classicalKuiper belt, and thus very dierent from Pluto. PT1(designated 1110113Y on the HST web site[147]), themost favorably situated object, is magnitude 26.8, 3045km in diameter, and would be encountered around Jan-uary 2019.[148] A course to reach it would require about35% of New Horizons ' available trajectory-adjustmentfuel supply. A mission to PT3 may be preferable, becauseit is brighter and therefore probably larger than PT1.[143]Once sucient orbital information was provided, theMinor Planet Center gave ocial designations to thethree target KBOs: 2014 MU69 (PT1), 2014 OS393(PT2), and 2014 PN70 (PT3). (By the fall of 2014, apossible fourth target, 2014 MT69, had been eliminatedby follow-up observations.)New Horizons principal investigator Alan Stern willmake a recommendation to NASA in August 2015 onwhich of the two KBOs the spacecraft should visit. Thepreferred yby target is PT1, a 4070 km object, butPT3, a slightly bigger object, could also be targeted fora yby, with the decision to be made in August 2015.PT2 is no longer under consideration.[149][150] Once thetarget is chosen, a series of trajectory correction maneu-vers will be performed in October and November 2015.In order to complete the mission, funding will need to beobligated following a senior review of planetary sciencemissions in 2016, with the results of that review to be an-nounced in August or September 2016. If funded, theKBO yby would occur in 2019.[132]

8.2 Post-KBO mission

Even though it was launched far faster than any outwardprobe before it, New Horizons will never overtake either

19

Voyager 1 or Voyager 2 as the most distant human-madeobject from Earth. Close ybys of Saturn and Titan gaveVoyager 1 an advantage with their gravity assist. WhenNew Horizons reaches the distance of 100 AU, it will betravelling at about 13 km/s (29,000 mph), around 4 km/s(8,900 mph) slower than Voyager 1 at that distance.[151]

8.3 Public-dened TNO missionIn August 2013, it was proposed[152] to use the radio-tracking data from New Horizons to discover con-straints on the position of a hypothesized trans-Neptunianplanetary-sized body.[153]

9 Gallery

9.1 Images of the launch

9.2 Images of Jupiter yby

9.3 Images of Jupiters moons

Media related to New Horizons Jupiter systemencounter at Wikimedia Commons

9.4 Images of Pluto yby

Media related to New Horizons Pluto encounter atWikimedia Commons

9.5 Images of Plutos moons Pluto moons, Nix and Hydra, as viewed by New Horizons

on 14 July 2015.

Media related to New Horizons Charon encounter atWikimedia Commons

10 Timeline

11 See also List of articial objects escaping from the Solar Sys-

tem List of spaceights (2006) New Horizons 2a proposed, but scrapped, twin of

New Horizons

Pioneer 10 Pioneer 11 Voyager 1 Voyager 2 Robotic spacecraft Space exploration Timeline of Solar System exploration

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