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Alex Wormuth

Bio: Alex Wormuth is a junior at USC majoring in Computer Science and specializing in Web Development and Cyber Security. In his spare time, he interns at NASA’s Jet Propulsion Laboratory and is working on his own startup for the ancient board game, Go.

Keywords: Mars, landing, rover, NASA, Jet Propulsion Laboratory, JPL, Curiosity, exploration, space, robotics, mechanical engineering, aerospace

Multimedia Suggestions: http://www.jpl.nasa.gov/video/index.php?id=1090 http://www.jpl.nasa.gov/video/index.php?id=1103 http://mars.nasa.gov/msl/mission/timeline/edl/

Abstract: Many people have grown up with the fascination of outer space: the darkness, the mystery, the unknown. For many years, scientists have been searching for life apart from that on earth. Because of its similarities to earth, they have chosen Mars as a good candidate for potential life. However, many people don’t know what it takes to land on Mars. The past, present, and future techniques of landing systems used on Mars pave the way for space exploration and discovery.

Landing on Mars: The Seven Minutes of Terror

Introduction

“Parachute has deployed; we are decelerating. Heatshield has separated.

Standing by for backshell separation. We are in powered flight at an altitude of 1

kilometer, descending. Standing by for sky-crane. Sky-crane has started. Touchdown

confirmed. We’re safe on Mars” [1]. Space exploration is an essential part of humanity.

Who are we? Where did we come from? Are we alone? These are unanswered questions

that have been asked for centuries. Exploring Mars is one of the largest steps we have

taken in finding answers to these questions. Because of its proximity and similarity to

Earth, scientists believe that Mars could have once supported life. So how do we get

there to explore this theory? Unfortunately, there is a lot of radiation between Earth

and Mars, and an astronaut would very likely die from radiation poisoning after

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returning to Earth. However, sending a rover into space is much safer and cheaper than

sending a human into space. Also, rovers can live in space much longer than a human

can because they aren’t affected by radiation, temperature changes, and oxygen levels

like a human is. So how does the rover get there? After launch and many days of guided

flight, the rover finally breaks into the Martian atmosphere. The phase from this point

until the rover lands on Mars is called the EDL (Entry, Descent, and Landing) phase, also

known as the 7 minutes of terror. By exploring the past, present, and future of EDL

technologies, one can better understand what it takes to land on Mars.

History of EDL

Although current EDL techniques have proven successful, the history of Mars

EDL is a sad reality. Since 1971, when the Russian Mars 2 rover crash-landed on Mars,

over half of the rovers sent to Mars have either failed to launch, missed Mars, or crash-

landed [2]. Most of the failures were due to design flaws and computer malfunctions

that prevented the landing sequence from functioning properly.

During the beginning phases of EDL, the rover needs to deal with the Martian

atmosphere. As Figure 1 shows, Mars’ atmosphere is very weak compared to Earth’s.

Figure 1: Graph

showing the difference

between the density of

Earth’s and Mars’

atmosphere (Source:

Braun [3]).

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This presents a problem for rovers landing on Mars. The atmosphere is strong enough

such that the rover would burn up if the heat weren’t accounted for; however, the

atmosphere isn’t strong enough to slow the rover down. To mitigate the heat taken by

the rover, engineers install a heat shield that can reach up to 2500 degrees Fahrenheit.

This is released after the rover breaks through the atmosphere. Once inside the

atmosphere, the rover is traveling about 10,000 miles per hour; therefore, mechanisms

must be in place to drastically reduce the velocity of the rover to prevent it from

crashing into the surface.

The primary landing system used by earlier rovers was a propulsion system,

parachute, and legs for the touchdown. This landing technique was possible because

many of the early rovers were very small and didn’t carry a lot of extra equipment. Also,

the legs could easily be added to the outer structure without disturbing any of the

subsystems containing more sensitive hardware. However, there were many challenges

that legged landers had to face. First, the legs had to deal with large rocks and rough

terrain. The legs had to be tall enough so that the rover wouldn’t hit the rocks, and they

had to be strong enough to withstand a hit from a rock. Another problem is maintaining

stability on the touchdown. Often times, the rover will be off course and could be

headed toward uneven terrain. The legs must have the ability to stabilize in order to

prevent the rover from tipping over. This problem becomes even more challenging

when rovers become larger and there is more mass to support. Finally, when the rover

successfully touches down on the surface, it still needs to be lowered to the ground,

more commonly known as rover egress. This was typically done using ramps (Fig. 2)

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Figure 2: Illustration of a legged lander with multiple ramps. (Source: BBC News).

that could bring the rover down in multiple directions [4]. Overall, legged landers are

simple and reliable, but there is very small margin for error and there are many

problems they have to overcome.

Early rovers weren’t very constrained by budget because of the Space Race.

However, after the hype died down, scientists started receiving less funding and had to

think of better, more cost effective ways to land a rover. Also, as rovers became more

capable and could carry more instruments, they became much larger. To accommodate

for the increase in mass, some rovers made use of the airbag landing system (Fig. 3).

Figure 3: Pre-flight

testing of the

airbags (left) and

internal view of the

rover (right)

(Source: Braun [3]).

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However, this technique has many complexities to deal with. First, the airbags must be

strong enough to hit the Martian surface without exploding, as there are many rocks

and rough terrain. To alleviate this problem, scientists use a material called Vectran that

has almost twice the strength of Kevlar and can withstand cold temperatures [5]. Also,

the airbags must be deployed seconds before landing; one wrong calculation and it’s too

late. At the height of about four stories, the rover is released from the parachute and

free falls to the surface. The half-ton rover can bounce up to five stories high and travel

a distance of over half a mile before coming to a complete stop [6]. Interestingly enough,

the direction in which the rover lands isn’t a problem. The system has the capability to

right itself from any orientation, which eliminates the challenge of stability presented

by the legged landers [4]. Airbags are very effective for landing small rovers; however,

larger rovers increase the chance of hitting sharp rocks at higher velocity, thereby

increasing the chance of destroying the airbags. There needs to be a more reliable

lander for future space exploration.

Current EDL (Curiosity)

Tommaso Rivellini, lead mechanical engineer for EDL at the NASA Jet Propulsion

Laboratory, states that “missions are also being designed to access larger areas of the

planet and, therefore, will require more robust landing systems that are tolerant to

slope and rock combinations that were previously considered too hazardous to land or

drive on” [4]. Unfortunately, the legged and airbag landing systems already have

enough problems to deal with on relatively safe landing zones. That being the case,

engineers at NASA have developed an elegant solution to the problem: the sky-crane

landing system [7].

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NASA received a lot of skepticism from the public when they were considering

the sky-crane for a landing technique; they were even a little skeptical themselves. It is

a safer, more reliable landing technique if executed correctly, but the software and

sensor readings had to be on point and it was something that hadn’t been attempted

before. When the rover reaches about 200 miles per hour, it breaks away from the

parachute and goes into powered descent. A rocket-powered system decelerates the

rover to about 66 feet above the ground while maintaining stability, and prepares to

release the rover using the sky-crane. The sky-crane extends the rover to the ground

using three nylon tethers and an umbilical that provides power and communication to

the rover, as seen in Figure 4. Meanwhile, four of the eight rocket engines stabilize the

Figure 4: Diagram of the entry, descent, and landing configuration for Curiosity’s landing

on Mars. (Source: AeroSpaceGuide).

rover until touchdown and the propulsion system flies away after touchdown

confirmation [8]. Lowering the rover wheels first eliminates the stability and egress

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problems presented by the previous landers, and the sensors can guide the rover to

avoid rocks, steep hills, and rough terrain [9]. Because the propulsion systems and

sensors are placed above the rover, their operation is uninterrupted throughout the

landing sequence [4]. This means that the velocity is more controlled and the rover can

have a “soft landing” that reaches near zero velocity. Unlike previous landers, the sky-

crane proves a reliable solution for larger rovers; however, this isn’t a long-term

solution.

Future EDL

As technology continues to advance, rovers will be able to carry more advanced

technology and will, in turn, increase in mass. Systems until now have dealt with the

increased mass in intuitive ways, but it is up to engineers of today to find new ways to

land larger rovers on Mars. Also, NASA is aiming to increase the precision of landing. As

Figure 5 shows, the landing ellipses have become increasingly smaller over the past few

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Figure 5: Map that displays how spacecraft landings have become more and more

precise over the past few decades. (Source: NASA).

decades. This is due to better technology and better landing techniques, but there is still

room for improvement. At a higher altitude, the parachute has a greater probability of

carrying the rover off course, thereby increasing the probability of a failed landing or

wasting days of travel. By letting the parachute open later in the descent, the rover can

reach lower altitudes before starting the landing sequence. According to Van Kane, this

could reduce the current landing ellipse by 25%. In fact, NASA’s Science Definition

Team (SDT) has recommended that this feature be included in the Mars 2020 rover,

which is a follow up to Curiosity [10]. These improvements, along with the passion of

scientists and engineers, will pave the way for future space exploration.

As the search for life continues, so does the search for answers pertaining to

humanity. Who are we? Where did we come from? Are we alone? Technology is always

changing, and many things will happen before we find answers to these questions.

NASA’s Jet Propulsion Laboratory has selected over 15 missions for the next decade

that will require more sophisticated landers [11]. It is imperative that engineers of

today create stronger, more reliable landing systems that can withstand larger masses,

rougher terrain, and unstable temperatures. However, as Rivellini points out, one thing

has and will remain the same in the years to come: “As Mars explorers have learned the

hard way, its not typically the fall that kills you, it’s the landing” [4].

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References

[1] NASA Jet Propulsion Laboratory. (2012). Curiosity Has Landed [Online]. Available: http://www.jpl.nasa.gov/video/index.php?id=1103

[2] E. Hines. (2008). Historical Perspectives: Evolution of Recent Mars EDL Systems Development [Online]. Available: http://smartech.gatech.edu/bitstream/1853/26435/2/115-201-1-PB.ppt

[3] R. Braun; R. Manning. (2007). Mars Exploration Entry, Descent, and Landing Challenges [Online]. Available: http://arc.aiaa.org/doi/pdf/10.2514/1.25116

[4] T. Rivellini. (2004). The Challenges of Landing on Mars [Online]. Available: http://www.engineeringchallenges.org/cms/7126/7622.aspx

[5] NASA Jet Propulsion Laboratory. Spacecraft: Airbags [Online]. Available: http://marsrover.nasa.gov/mission/spacecraft_edl_airbags.html

[6] NASA Jet Propulsion Laboratory. Step-by-Step Guide to Entry, Descent, and Landing [Online]. Available: http://marsrover.nasa.gov/mission/spacecraft_edl_airbags.html

[7] M. Ivanov. (2011). Entry, Descent and Landing Systems Analysis Study: Phase 2 Report on [MSL] Improvement [Online]. Available: http://permanent.access.gpo.gov/gpo20449/20110015739_2011016739.pdf

[8] D. Chow. (2012). How Mars Rover Curiosity’s Nail-Bing Landing Works [Online]. Available: http://www.space.com/16503-photos-mars-science-laboratory- curiosity-landing-guide.html

[9] M. Adler; M. Wright. (2010). Entry, Descent, and Landing Roadmap. [Online]. Available: http://www.nasa.gov/pdf/501326main_TA09-EDL-DRAFT- Nov2010-A.pdf

[10] V. Kane. (2013). The Mars 2020 Rover In-Depth [Online]. Available: http://www.planetary.org/blogs/guest-blogs/van-kane/20130727-mars-rover- 2020-in-depth.html

[11] NASA Jet Propulsion Laboratory. Mission Control [Online]. Available: http://www.jpl.nasa.gov/missions/