AST 248, Lecture 17 - Stony Brook UniversityI Long-period come from Oort comet cloud, ˘ 1012...

AST 248, Lecture 17

James Lattimer

Department of Physics & Astronomy449 ESS Bldg.

Stony Brook University

April 8, 2020

The Search for Intelligent Life in the [email protected]

James Lattimer AST 248, Lecture 17

Facts Concerning the Solar System

I All the planets roughly orbit the Sun in a plane.

I This plane is roughly the same as the rotational equator ofthe Sun.

I Planetary orbits are, for the most part, circular.

I The planets all revolve in the same direction about theSun.

I The Sun and the planets, with the exception of Venus andUranus, rotate on their axis in the same direction. Withthe exception of Uranus, the tilt between the equator andthe orbital plane of the Sun and planets is small.

I The planets differ in composition: the planets nearest theSun tend to be small, dense and metal-rich, whereas theplanets farthest from the Sun tend to be large, light andhydrogen-rich.


I Meteorites differ in chemical and geologic properties fromplanetary and lunar rocks, but may be similar incomposition to asteroids and small moons.

I Planets and most asteroids rotate with similar periods,about 5 to 10 hours, unless obvious tidal forces slow them,as in the case of the Earth and Venus.

I The distances of the planets from the Sun obey theBode’s Law.

I Planet-satellite systems resemble miniature solar systems.

I Cometary orbits, as a group, define a large, almostspherical, cloud around the Solar System.

I The Sun contains 99% of the mass in the Solar System.

I The planets, not the Sun, contain nearly all the angularmomentum of the Solar System.


A Brief History of Solar System Discoveries300 BC Aristotle – Earth is round because of circular shadow during

lunar eclipses, some stars not visible from northern locations270 BC Aristarchus – Earth revolves around Sun and Moon around

Earth; estimated lunar size from eclipses and relative distancesand sizes of Sun & Moon; Sun much bigger than the Earth

250 BC Eratosthenes – Measured shape & size of Earth135 BC Hipparchus – Discovers precession of the equinoxes; better

estimate of the Moon’s distance; rejects Earth orbited Sun.100 AD Ptolemy – Proposed Earth as center of Universe (Aristotle)

1473–1542 Copernicus – Earth and planets orbit Sun1546–1604 Tycho Brahe – Measured detailed planetary positions1571–1630 Kepler – Analyzed Brahe data and deduced 3 Laws of Motion:

I Each planet moves in an ellipse with Sun at one focus(Wrong: focus is at center-of-masss)

I Line between Sun and planet sweeps out equal areas inequal times

I Orbital period2 proportional to semi-major axis3, led toNewton’s 1–2–3 Law: G(M� + mplanet)

1P2 = 4π2a3

1564–1642 Galileo – Laws of motion; telescopic observations provedCopernican theory

1642–1727 Newton – Law of gravity and derivation of Kepler’s 3 laws1656–1742 Halley – First good estimate of size of solar system from Venus

transits; Halley’s comet is periodic1644–1710 Roemer – Measured light speed using Jupiter’s moons


Debris in the Solar System: CometsI “Dirty snowballs”, a mixture of ices and dust,

ejected from solar system when planets formedI Mass mostly contained in a nucleus (solid dirty

snowball) with km-sized diameterI Coma, a cloud of H2O, CO2 and other gases va-

porized from nucleus during close passage to SunI Hydrogen cloud, huge but sparse neutral H cloudI Dust tail, 1–10 million km long, dust driven

off nucleus by escaping gases when close to SunI Ion tail, 100’s of millions of km long, plasma

driven off by solar windI Most comets reside far outside the orbit of Pluto,

a few occasionally perturbed into close-solarorbits; some are in elliptical orbits and reappear

I After a few hundred passages near the Sun,icy material is lost,leaves dead cometwhich can be mis-taken for asteroid

Hale-Bopp

Hally’s nucleusfrom Giotto

Shoemaker-Levy 9 before Jupiter impact


Comet Holmes, Halloween, 2007V. Peris and J. L. Lamadrid

Fragment GHST 7/18/1994

Shoemaker-Levy 9HST 7/7/1994


CometsHistory

I Records of comets exist at least to 1140 BCI Brahe observed comet of 1577,

proved they are extraterrestrialI Halley: some comets are periodic

(1531, 1607, 1682)

I Halley’s Comet: 2467 BC (?),

240 BC (China), 1066 (Bayeux Tapestry)

I Motivated Newton to develop gravity theory

OrbitsI Hyperbolic: pass Sun once, depart foreverI Elliptical: periodic

I Peri-/Aphelion: closest/farthest solar approach

OriginI Short-period come from Kuiper Belt, 35,000

icy bodies larger than 100 km, 30-100 AUI Long-period come from Oort comet cloud,

∼ 1012 comets, 30,000 AU–1 lt. yr.I Most formed within inner solar system, then

pushed outwards by repeated near encountersJames Lattimer AST 248, Lecture 17

AsteroidsI Also called minor planets or planetoids, largely lying within the orbits

of Mars and Jupiter (main asteroid belt).I Objects lying outside Neptune are called trans-Neptunian objects,

centaurs or Kuiper-Belt objects.I The first asteroid to be discovered was Ceres on Jan 1, 1801.I Total asteroid mass is about 4% of the Moon’s mass. Ceres is about

32% of this. The first 4 asteroids have 51% of the total.I Can be roughly classified by spectral type:

C-type carbonaceous, 75%S-type silicaceous, 17%L-type metallic, 8%

These percentages do not reflect true proportions, but only observedproportions; some are easier to see than others.

I About a third of asteroids are members of families that have similarorbits. Probably they form as a result of collisions between asteroids.They last for about a billion years.

I Asteroids are slowly lost due to collisions with planets and togravitational encounters that eject them from solar system.

I Trojan asteroids collect in Lagrangian points (stable gravitationalminima) 60◦ ahead of and behind Jupiter in its orbit. Similarly forMars, Earth and Venus.

I Asteroids are given a number in order of their discovery and thediscoverer or the IAU may name them.


243 Ida

951 Gaspra

433 ErosJames Lattimer AST 248, Lecture 17

What is a Kuiper-Belt Object?The Kuiper Belt is a disk-shaped region past the orbitof Neptune extending roughlyfrom 30 to 50 AU from theSun containing many smallicy bodies. It is now consid-ered to be the source of theshort-period comets.About 1000 are known, andmany (like Pluto) are ina 3:2 orbital resonancewith Neptune.

Centaur objects, of which 9are known, orbit betweenJupiter and Neptune.Their orbits are unstable,and are probably displacedfrom the Kuiper Belt.

solarviews.com

solarviews.com


Kuiper Belt Objects

Name Diameter (km)2003 UB313 2400 ± 100Pluto 23202003 EL61 1200?2005 FY9 1250?Charon 1270Sedna < 1500?2004 DW ∼ 1500Quaoar 1200 ± 200Ixion 1065 ± 1652002 AW197 890 ± 120Varuna 900 ± 140

solarviews.com


They Have Moons!

solarviews.com


What are Near-Earth Objects?

I Comets and asteroids deflected into orbits with perihelion distanceq < 1.3 AU

I Made of water ice plus embedded dustI Originated in cold outer solar systemI Primordial, leftover building blocks of

planets

Classes of Near-Earth Asteroids:I Atens - Earth-crossing, semi-major

axis a < 1.0 AUI Apollos - Earth-crossing, a > 1.0 AUI Amors - Earth-approaching but

interior to Mars, a > 1.0 AU and1 AU < q < 1.3 AU,

I PHAs - Potentially Hazardous AsteroidsI Closest approach to Earth less than 0.05 AUI Absolute magnitude less than 22.0I Assuming albedo is 13%, this means diameter greater than 150 m

(500 ft)I There are currently 1885 known PHAs, 157 are larger than 1 km

NASA/JPL-Caltech


How Do We Find Near-Earth Objects?

I Early efforts relied on comparison photographs takenseveral minutes apart

I Vast majority of objects recorded were stars and galaxies,which don’t move

I Special stereo viewing microscopes pick out NEOsI Current efforts use CCD camerasI Computer-aided comparisons of digital imagesI NASA’s goal: Find > 90% of NEAs > 1 km within 10

yearsI Total population estimated to be about 1000

I Lincon Near-Earth Asteroid Research (LINEAR)I Near-Earth Asteroid Tracking (NEAT)I SpacewatchI Lowell Observatory Near-Earth Object Search (LONEOS)I Catalina Sky SurveyI Japanese Spaceguard Association (JSGA)I Asiago DLR Asteroid Survey (ADAS)


Studying Near-Earth Objects in Space

Typical Instruments:I Imager: CamcorderI IR and UV spectrometers to infer mineralocgical and

gaseous compositionsI Lidar: Optical equivalent of radar, useful to define shape

of target and aids in navigationI X-ray, γ-ray and αX-ray spectrometers to infer

chemical/elemental composition of surfae. X-rays comefrom Sun, γ-rays from cosmic rays, α-particles from aspacecraft Curium source

I Dust mass spectrometer detects high-velocity dust whoseimpacts produce ions

I Magnetometer to infer magnetic field of targetI Package to measure temperatures/densities of plasma

clouds created as sun’s radiation ionizes cometaryatmosphere. Also used to monitor DS1 spacecraft ionengine drive


Spacecraft MissionsI Ice (8/12/1978 - 5/1985)

I 21P/Giacobini-Zinner

I Giotto (7/2/85 - 7/10/92)I Halley’s Comet 1985 and Comet 26P/GriggSkjellerup 1992

I Galileo (10/18/89 - 9/21/03)I Flybys of 931 Gaspra and 243 Ida, discovered Dactyl

I Near-Earth Asteroid Rendezvous (NEAR 2/17/96 - 2/12/01)I Flyby of 243 Mathilde and 433 Eros, touchdown on 433 Eros

I Deep Impact (11/1/99 - 7/4/05)I Orbited and impacted on Comet Tempel 1, excavating a crater

I Deep Space 1 (DS1) (10/25/98 - 9/22/01)I Flybys of 9969 Braille and Comet BorrellyI Tested new technology – ion drive rocket, concentrating solar panel,

auto navigation system using asteroids

I Stardust (2/7/99 - 1/15/06)I 2 interstellar dust collections, flyby of 5535 Annefrank, encounter

with Comet Wild 2I Discovery of olivine and other minerals containing Ca, Al and Ti, all

hi-T condensates, in comet dust


More Spacecraft MissionsI Hayabusa (5/9/03 - 9/12/05)

I landed on 25143 Itokawa (diameter = 600 m), planned samplereturn to Earth in summer 2007

I Suffered fuel leak after successful second landing, preventscommunications to direct rocket’s return

I New Horizons (1/19/06 - 1/1/19)I Pluto and moons 7/14/15I 486958 (2014 MU69) 1/1/19

I Dawn (9/27/07 - present)I Vesta 7/16/11 - 9/5/12I Ceres 2015 - present

I Change’e-2 (1/10/10 - 13/12/12)I 4179 Toutatis

I Rosetta (3/2/04 - 2015)I ESA probe to Comet 67 Churyumov-Gerasimenko, utilizing 3 Earth

and 1 Mars gravity assist mvrs.I Orbited comet toward its perihelion for 17 monthsI Included lander named Philae, island in River Nile containing obelisk

with bilingual inscriptions providing Champollion with final cluesneeded to decipher hieroglyphs of Rosetta Stone

I Flybys of 2867 Steins (5/9/08) and 21 Lutetia (7/10/2010)


Future Missions

I Hayabusa 2 (12/3/14 - present)

I 162173 Ryugu, orbiterDCAM-3, landers MINERVA II-1 (Rovers-1Aand B), MINERA II-2 and MASCOT, and impactor SCI

I OSIRIS-REx (9/8/16 - )

I 101955 Bennu, sample return

I Lucy (planned for 2021)

I 52246 Donaldjohanson 4/20/2025I 3548 Eurybates 8/12/2027I 15094 Polymele 9/15/2027I 11351 Leucus 4/18/2028I 21900 Orus 11/11/2028I 617 Patrocius 3/2/2033

I Psyche (planned for 2022)

I 16 Psyche 2026


AST 248, Lecture 17 - Stony Brook UniversityI Long-period come from Oort comet cloud, ˘ 1012...

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