Posted on by Nancy Atkinson

Integrating New Concepts for Entry, Descent and Landing for Future Human Missions to Mars

Editor’s note: This guest post was written by Andy Tomaswick, an electrical engineer who follows space science and technology.

One of the most technically difficult tasks of any future manned missions to Mars is to get the astronauts safely on the ground. The combination of the high speed needed for a short trip in space and the much lighter Martian atmosphere creates an aerodynamics problem that has been solved only for robotic spacecraft so far. If people will one day walk Mars’ dusty surface, we will need to develop better Entry Descent and Landing (EDL) technologies first.

Those technologies are part of a recent meeting of the Lunar Planetary Institute (LPI), The Concepts and Approaches for Mars Exploration conference, held June 12-14 in Houston, which concentrated on the latest advances in technologies that might solve the EDL problem.

Of the multitude of technologies that were presented at the meeting, most seemed to involve a multi-tiered system comprising several different strategies. The different technologies that will fill those tiers are partly mission-dependent and all still need more testing. Three of the most widely discussed were Hypersonic Inflatable Aerodynamic Decelerators (HIADs), Supersonic Retro Propulsion (SRP), and various forms of aerobraking.

HIADs are essentially large heat shields, commonly found many types of manned reentry capsule used in the last 50 years of spaceflight. They work by using a large surface area to create enough drag through the atmosphere of a planet to slow the traveling craft to a reasonable speed. Since this strategy has worked so well on Earth for years, it is natural to translate the technology to Mars. There is a problem with the translation though.

HIADs rely on air resistance for its ability to decelerate the craft. Since Mars has a much thinner atmosphere than Earth, that resistance is not nearly as effective at slowing reentry. Because of this drop in effectiveness, HIADs are only considered for use with other technologies. Since it is also used as a heat shield, it must be attached to the ship at the beginning of reentry, when the air friction causes massive heating on some surfaces. Once the vehicle has slowed to a speed where heating is no longer an issue, the HIAD is released in order to allow other technologies to take over the rest of the braking process.

One of those other technologies is SRP. In many schemes, after the HIAD is released, SRP becomes primarily responsible for slowing the craft down. SRP is the type of landing technology commonly found in science fiction. The general idea is very simple. The same types of engines that accelerate the spacecraft to escape velocity on Earth can be turned around and used to stop that velocity upon reaching a destination. To slow the ship down, either flip the original rocket boosters around upon reentry or design forward-facing rockets that will only be used during landing. The chemical rocket technology needed for this strategy is already well understood, but rocket engines work differently when they are traveling at supersonic speeds. More testing must be done to design engines that can deal with the stresses of such velocities. SRPs also use fuel, which the craft will be required to carry the entire distance to Mars, making its journey more costly. The SRPs of most strategies are also jettisoned at some point during the descent. The weight shed and the difficulty of a controlled descent while following a pillar of flame to a landing site help lead to that decision.

Once the SRP boosters fall away, in most designs an aerobraking technology would take over. A commonly discussed technology at the conference was the ballute, a combination balloon and parachute. The idea behind this technology is to capture the air that is rushing past the landing craft and use it to fill a ballute that is tethered to the craft. The compression of the air rushing into the ballute would cause the gas to heat up, in effect creating a hot air balloon that would have similar lifting properties to those used on Earth. Assuming enough air is rushed into the ballute, it could provide the final deceleration needed to gently drop the landing craft off on the Martian surface, with minimal stress on the payload. However, the total amount this technology would slow the craft down is dependent on the amount of air it could inject into its structure. With more air come larger ballute, and more stresses on the material the ballute is made out of. With those considerations, it is not being considered as a stand-alone EDL technology.

These strategies barely scratch the surface of proposed EDL methods that could be used by a human mission to Mars. Curiosity, the newest rover soon set to land on Mars, is using several, including a unique form of SRP known as the Sky Crane. The results of its systems will help scientists like those at the LPI conference determine what suite of EDL technologies will be the most effective for any future human missions to Mars.

Read our previous article about the difficulties of landing large payloads on Mars, an interview with JPL’s Rob Manning.


Lead image caption: Artist’s concept of Hypersonic Inflatable Aerodynamic Decelerator slowing the atmospheric entry of a spacecraft. Credit: NASA

Second image caption: Supersonic jets are fired forward of a spacecraft in order to decelerate the vehicle during entry into the Martian atmosphere prior to parachute deployment. The image is of the Mars Science Lab at Mach 12 with 4 supersonic retropropulsion jets. Credit: NASA

Source: LPI Concept and Approaches for Mars Exploration

Posted on by Jason Major

New “Flying Tea Kettle” Could Get Us To Mars in Weeks, Not Months

At 54.6 million km away at its closest, the fastest travel to Mars from Earth using current technology (and no small bit of math) takes around 214 days — that’s about 30 weeks, or 7 months. A robotic explorer like Curiosity may not have any issues with that, but it’d be a tough journey for a human crew. Developing a quicker, more efficient method of propulsion for interplanetary voyages is essential for future human exploration missions… and right now a research team at the University of Alabama in Huntsville is doing just that.

This summer, UAHuntsville researchers, partnered with NASA’s Marshall Space Flight Center and Boeing, are laying the groundwork for a propulsion system that uses powerful pulses of nuclear fusion created within hollow 2-inch-wide “pucks” of lithium deuteride. And like hockey pucks, the plan is to “slapshot” them with plasma energy, fusing the lithium and hydrogen atoms inside and releasing enough force to ultimately propel a spacecraft — an effect known as “Z-pinch”.

“If this works,” said Dr. Jason Cassibry, an associate professor of engineering at UAH, “we could reach Mars in six to eight weeks instead of six to eight months.”

Read: How Long Does It Take To Get To Mars?

The key component to the UAH research is the Decade Module 2 — a massive device used by the Department of Defense for weapons testing in the 90s. Delivered last month to UAH (some assembly required) the DM2 will allow the team to test Z-pinch creation and confinement methods, and then utilize the data to hopefully get to the next step: fusion of lithium-deuterium pellets to create propulsion controlled via an electromagnetic field “nozzle”.

Although a rocket powered by Z-pinch fusion wouldn’t be used to actually leave Earth’s surface — it would run out of fuel within minutes — once in space it could be fired up to efficiently spiral out of orbit, coast at high speed and then slow down at the desired location, just like conventional rockets except… better.

“It’s equivalent to 20 percent of the world’s power output in a tiny bolt of lightning no bigger than your finger. It’s a tremendous amount of energy in a tiny period of time, just a hundred billionths of a second.”

– Dr. Jason Cassibry on the Z-pinch effect

In fact, according to a UAHuntsville news release, a pulsed fusion engine is pretty much the same thing as a regular rocket engine: a “flying tea kettle.” Cold material goes in, gets energized and hot gas pushes out. The difference is how much and what kind of cold material is used, and how forceful the push out is.

Everything else is just rocket science.

Read more on the University of Huntsville news site here and on al.com. Also, Paul Gilster at Centauri Dreams has a nice write-up about the research as well as a little history of Z-pinch fusion technology… check it out. Top image: Mars imaged with Hubble’s Wide-Field Planetary Camera 2 in March 1995.

Posted on by Nancy Atkinson

NASA Holding Big Events for Curiosity Rover Landing; Register for Chance to Attend

It’s a big rover, so nothing but a big event is appropriate. NASA is going to hold the first-ever multi-center “Social” (formerly known as Tweetups) in conjunction with the landing of the Mars Science Laboratory’s Curiosity rover on Aug. 6 EDT (Aug. 5 PDT). In case you’re not familiar, NASA Socials are in-person meetings with people who engage with the agency through Twitter, Facebook, Google+ and other social networks.

You can register to attend events at one of six NASA centers: Ames Research Center in Moffett Field, Calif.; Glenn Research Center in Cleveland; Goddard Space Flight Center in Greenbelt, Md.; Johnson Space Center in Houston; Langley Research Center in Hampton, Va.; and Jet Propulsion Laboratory (JPL) in Pasadena, Calif. JPL will be having the main event, and each of the other centers will be connected via a multi-center NASA Television simulcast with JPL, home of the Curiosity rover.

If you live close to any of these centers or are willing to travel, and are at all even remotely interested in space exploration, you should register for the chance to attend. Tweetups NASA Socials are incredible events (some attendees have called them life-altering) where you get behind-the-scenes looks at the centers, tours that aren’t normally given to the public, and presentations by scientists, engineers and managers. The events also will provide guests the opportunity to interact with fellow social media users, space enthusiasts and members of NASA’s social media team. Participants will learn about the Mars Science Laboratory mission and their respective NASA field center. They are encouraged to share their experience with others through their favorite social networks.

Registration for the five new NASA Socials opens at noon EDT, Friday, June 29, and closes at noon Tuesday, July 3. NASA randomly will select participants from online registrations. People may register for NASA Socials to be held at multiple locations, but selectees will be chosen for one event only. Each field center’s social and number of guests allowed varies. For more information on each center’s activities and rules pertaining to NASA Social registration, visit:
http://www.nasa.gov/social

Curiosity is currently scheduled to land at Mars’ Gale crater at approximately 1:31 a.m. EDT Aug. 6 (10:31 p.m. PDT Aug. 5), so be prepared to stay up late! But it should be worth it.

During the two-year prime mission, the rover will investigate whether the selected area of Mars offered environmental conditions favorable for microbial life or if evidence of it existed. Find out more about MSL at this website: http://www.nasa.gov/msl

Posted on by Jason Major

Mars Has Watery Insides, Just Like Earth

Researchers from the Carnegie Institution have found that water is present in surprisingly Earthlike amounts within Mars’ mantle, based on studies of meteorites that originate from the Red Planet. The findings offer insight as to how Martian water may have once made its way to the planet’s surface, as well as what may lie within other terrestrial worlds.

Earth has water on its surface (obviously) and also within its crust and mantle. The water content of Earth’s upper mantle — the layer just below the crust —  is between 50 and 300 ppm (parts per million). This number corresponds to what the research team has identified within the mantle of Mars, based on studies of two chunks of rock — called shergottites — that were blasted off Mars during an impact event 2.5 million years ago.

“We analyzed two meteorites that had very different processing histories,” said Erik Hauri, the analysis team’s lead investigator from the Carnegie Institute . “One had undergone considerable mixing with other elements during its formation, while the other had not. We analyzed the water content of the mineral apatite and found there was little difference between the two even though the chemistry of trace elements was markedly different. The results suggest that water was incorporated during the formation of Mars and that the planet was able to store water in its interior during the planet’s differentiation.”

The water stored within Mars’ mantle may have made its way to the surface through volcanic activity, the researchers suggest, creating environments that were conducive to the development of life.

Like Earth, Mars may have gotten its water from elements available in the neighborhood of the inner Solar System during its development. Although Earth has retained its surface water while that on Mars got lost or frozen, both planets appear to have about the same relative amounts tucked away inside… and this could also be the case for other rocky worlds.

“Not only does this study explain how Mars got its water, it provides a mechanism for hydrogen storage in all the terrestrial planets at the time of their formation,” said former Carnegie postdoctoral scientist Francis McCubbin, who led the study.

The team’s research is published in the July edition of the journal Geology. Read more on the Carnegie Institution for Science’s site here.

Image: The remains of what appears to be a river delta within Eberswalde crater on Mars, imaged by ESA’s Mars Express. Credit: ESA/DLR/FU Berlin (G. Neukum).

Posted on by Nancy Atkinson

Organics Found in Mars Meteorites, But Nothing Biological

Editor’s note: This guest post was written by Andy Tomaswick, an electrical engineer who follows space science and technology.

The search for biologically created organic molecules on Mars goes back at least to the 1970s with the Viking program. Those missions had famously mixed results, and so the search for carbon-based life on Mars continues to this day. Researchers keeping piling on more and more evidence to excite astrobiologists and new results published in a study by the Planetary Science Institute and the Carnegie Institute of Washington may heighten their enthusiasm.

The latest results come from a team led by Andrew Steele of the Carnegie Institution for Science who surveyed meteorites from Mars, which covered a 4.2 billion year time span of Martian geology. While it is no surprise that there are organics on Mars — that Martian meteorites contain carbon-based molecules has been known for years — the team confirmed those findings by detecting organics on ten of the eleven meteorites they examined. However, questions remained as to where exactly the meteorite-bound organic molecules came from and, if they were from Mars, what had created them?

The team set out to answer these questions and came to the conclusion that the molecules are indeed from Mars and not the result of some cross-contamination from Earth’s biosphere. However, they also found that the molecules were not created by any biological process. The organics actually formed in the chunks of rock that later became the meteorites that transported them to earth. Their formation was part of a volcanic process that traps carbon in crystal structures formed by cooling magma. Through a series of non-biological chemical reactions, the complex organics found in the meteorites are created using the carbon trapped in these crystals.

The team also casts doubt on another possible explanation: whether the organics might be caused by emissions from microbes that had migrated into the volcano via tectonic processes similar to those on Earth. They point out that Mars does not have the tectonic activity similar to Earth so there is very little likelihood that the molecules are created by microbial activity.

That might sound like a depressing result for the astrobiologists. But the important finding from this study is that Mars has been natively and naturally creating complex organic molecules for 4.2 billion years and may be still be doing so today. Since the creation of organic molecules on Earth was a precursor to life, scientists can still hold out hope that the same life-creating process might have already happened on the red planet.

Interestingly, one of the Martian meteorites that was studied was the famous ALH84001, the meteorite that some researchers claimed in 1996 might contain fossils from Mars. That claim was subsequently strongly challenged, and studies of the rock are ongoing. ALH84001 is a portion of a meteorite that was dislodged from Mars by a huge impact about 16 million years ago and that fell to Earth in Antarctica approximately 13,000 years ago. The meteorite was found in Allan Hills ice field in Antarctica.

Read the team’s abstract.

Lead image caption: ALH84001 is one of 10 rocks from Mars in which researchers have found organic carbon compounds that originated on Mars without involvement of life. Credit: NASA/JSC/Stanford University

Sources: Planetary Science Institute, LiveScience, NASA

Posted on by Nancy Atkinson

Dang, These Features on Mars are Groovy!

This computer-generated view of the yardangs in Danielson Crater on Mars was created using data obtained from the High- Resolution Stereo Camera (HRSC) on ESA’s Mars Express. Credit: ESA/DLR/FU Berlin (G. Neukum)

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The term ‘yardangs’ almost sounds like a fictional word from a Barsoomian tale of creatures living on Mars. However, this is a real word, a geologists’ term for narrow, wind-eroded ridges. These are common land features in the desert regions of Earth, eolian features created by wind and dust. With Mars’ dusty soil and frequent winds, these landforms are common on the Red Planet, too. The abrasive dust is blown by wind, impacting on the bedrock, slowly removing parts of the surface, like a sand-blaster. If the winds blow in the same direction for a long enough period, ‘wind-lanes’ are made. These features are called yardangs.

These latest images from the Mars Express mission show yardangs on the floor of Danielson crater, and scientists think this crater may provide evidence that the planet underwent significant periodic fluctuations in its climate due to changes in its rotation axis.

On June 19, 2011, Mars Express took a look at the region pictured here — Arabia Terra region of Mars — imaging Danielson and the smaller Kalocsa crater with its high-resolution stereo camera.

In the case of Danielson crater, scientists think the sediments were cemented in by water, possibly from an ancient deep groundwater reservoir, before being eroded by the wind.

Danielson and Kalocsa craters as seen by Mars Express. Credit: ESA/DLR/FU Berlin (G. Neukum)

The orientation of the yardangs leads scientists to theorize that strong north–northeasterly winds (from the lower right in the image) both deposited the original sediments and then caused their subsequent erosion in a later drier period of Martian history.

A 30 km-long field of darker dunes can be seen bisecting the yardangs and is thought to have formed at a later epoch.

Some scientists believe that this indicates periodic fluctuations in the climate of Mars, triggered by regular changes in the planet’s axis of rotation. The different layers would have been laid down during different epochs.

But Kalocsa crater shows a completely different topography, with no layered sediments. This is thought to be due to the higher altitude of its floor, with the crater not tapping in to the suspected underlying ancient water reservoir.

However, another hypothesis is that this crater is younger than its neighbor, created when water was not present anymore.

Dang.

Source: ESA

Posted on by Nancy Atkinson

Mars Rover Drivers Inspired by Ray Bradbury

Two of the drivers of the Mars Exploration Rovers, Ashley Stroupe and Scott Maxwell, were fortunate to host a very special visitor to the Jet Propulsion Laboratory a couple of years ago. Science fiction author Ray Bradbury came, and even though he was wheel-chair-bound, he received the grand tour, and the two rover drivers showed him huge panoramic images taken by the rovers and even let Bradbury drive a 3-D computer model of the rovers. Stroupe tells the story of Bradbury’s visit in the video above, and Maxwell told his version of the story in a blog post back in 2009, which was re-published on the Planetary Blog. Of Bradbury seeing the panoramas, Maxwell wrote, “These just happen to be perfect for Bradbury: color panoramic images of Mars, taken from a human’s perspective, but mounted close to a wheelchair-bound man’s height. It must have seemed to him that he was, for the first time, really there. On the edge of a crater. On top of a mountain. On the plains, looking at the crashed remnants of the spaceship we rode there. On Mars.”
Continue reading “Mars Rover Drivers Inspired by Ray Bradbury”

Posted on by Nancy Atkinson

Humans on Mars by 2023?

Artist concept of the Mars One lander, a variant on the SpaceX Dragon. Credit: Mars One

Reality TV goes to Mars! Dutch entrepreneur Bas Lansdorp is leading a group visionaries and businesspeople who want to send four humans to Mars by 2023, and they say they can achieve their goal at an estimated cost of $6 billion USD. How can they do it? By building it into a global media spectacle. And oh, by the way, this will be a one-way trip.

“Who would be able to look away from an adventure such as this one?” asks Lansdorp in his bio on the Mars One website. “Who wouldn’t be compelled to watch, talk about, get involved in the biggest undertaking mankind has ever made? The entire world will be able to follow this giant leap from the start; from the very first astronaut selections to the established, independent village years later. The media focus that comes with the public’s attention opens pathways to sponsors and investors.”

As far as the one-way mission (a concept that Universe Today has written about extensively) the Mars One website notes, “this is no way excludes the possibility of a return flight at some point in the future.”

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The difference between this mission and the one proposed by Jim McLane back in 2008 is that McLane wanted to send just one person to Mars.

However, the Mars One group says that once the first trip is successful and Mars becomes developed, it will be “much easier to build the returning rocket there.”

In a Q&A on reddit, Lansdorp said the biggest challenge will be financing.

“We have estimated, and discussed with our suppliers that it will cost about 6 billion US$ to get the first crew of four people to Mars. We plan to organize the biggest media event ever around our mission. When we launch people to Mars and when they land, the whole world will watch. After that a lot of people will be very interested to see how ‘our people on Mars’ are doing.”

But the big challenge is that the biggest expenditures will be building the equipment before they send people to Mars. “This is why we are building a very strong technical case now. If we can convince sponsors and investors that this will really happen, then we believe that we can convince them to help us finance it,” Lansdorp said.

As far as technologies, Mars One expects to use a SpaceX Falcon 9 Heavy as a launch vehicle, a transit vehicle/space habitat built by Thales Alenia Space, a variant on the SpaceX Dragon as the lander, an inflatable habitat built by ILC Dover, a rover vehicle by MDA Space Missions, and Mars spacesuits made by Paragon.

The project website says “no new technologies” will be needed, but does any space agency or company really have a good handle on providing providing ample air, oxygen, energy, food and water for extended (lifetimes?) periods of time? Instead, the website provides more details on FAQ’s like, What will the astronauts do on Mars? Why should we go to Mars? Is it safe to live on Mars? How does the Mars base communicate with Earth? And the Mars One team emphasizes that this can be done with current technology. However, no one really knows how to land large payloads on Mars yet, so at least some development will be required there.

Who will go? Later this year they will begin to take applications and eventually 40 people will take part in a rigid, decade-long training program (which sounds very expensive) where the ‘contestants” will essentially be voted off the island to get to the final four astronauts. The selection and training process will be broadcast via television and online to public, with viewers voting on the final selected four.

It’s an intriguing proposition, but one filled with technological hurdles. I’ve just finished reading Ben Bova’s “Mars,” so I’m also thinking the Mars One folks will need to be on the lookout for micrometeorite swarms.

Mars One website.

Posted on by Nancy Atkinson

Meteorites Could Provide New Explanation for Mars Methane

A Murchison meteorite specimen at the National Museum of Natural History in Washington DC.

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Where does the methane on Mars come from? That has been one of the biggest unanswered questions in planetary science since the discovery of large plumes of methane gas in the Martian atmosphere. Scientists have been trying to figure out how the planet’s environment or geology can keep replenishing this short-lived gas, and of course, in the back of everyone’s mind is whether the methane has any connection to possible life on Mars.

A new potential explanation squelches both the life and environment prospect and offers a unique answer. A group of researchers found that meteorites, which continually bombard the surface of Mars, may contain enough carbon compounds to generate methane when they are exposed to strong UV sunlight.

“Whether or not Mars is able to sustain life is not yet known, but future studies should take into account the role of sunlight and debris from meteorites in shaping the planet’s atmosphere,” said Dr. Andrew McLeod, of the University of Edinburgh, co-author of a new study published in Nature this week.

The group of European researchers looked at the famous Murchison meteorite, a carbonaceous chondrite meteorites that fell in Australia more than 40 years ago. Carbonaceous chondrites are very common meteorites, so they likely will be falling on Mars. The team exposed particles of the Murchison meteorite to levels of ultraviolet radiation equivalent to sunlight on Mars.

When the meteorite pieces were exposed to ample amounts of UV light the meteor fragments rapidly released methane. After the UV exposure was reduced, the amount of methane produced would lessen, but if there were other activities, such as heating, shaking or lowering the pressure on the meteorite, the amount of methane released would rise again.

With Mars thin atmosphere, UV light easily gets to the surface of the planet. The thin atmosphere also allows more meteorites to hit Mars than on Earth (estimates range from just a few thousands of metric tons to as much as 60,000 metric tons.) The team said that temperature changes on Mars, especially during the summertime when it gets warm, could account for a boost of methane release from meteorites, and seasonal dust storms could shake or move the meteorites.

However, while only small amounts of methane are present in the Martian atmosphere, it seems to be coming from very specific, localized sources. Meteorites would likely be falling across the planet.

Top: Map of methane concentrations in Autumn (first martian year observed). Peak emissions fall over Tharsis (home to the Solar System's largest volcano, Olympus Mons), the Arabia Terrae plains and the Elysium region, also the site of volcanos. Bottom: True colour map of Mars. Credit: NASA/Università del Salento

Additionally, levels of methane vary in the seasons, and are highest in autumn in the northern hemisphere, with localized peaks of 70 parts per billion. There is a sharp decrease in winter, with only a faint band of methane appearing in the atmosphere between 40-50 degrees north.

Methane was first detected in the Martian atmosphere by ground based telescopes in 2003 and confirmed a year later by ESA’s Mars Express spacecraft. In 2009, observations using ground based telescopes showed the first evidence of a seasonal cycle.

Other research has said that the methane in the Martian atmosphere lasts less than a year, making it a flitting – and difficult – feature to study.

Another issue is that the estimates for the amount of meteorites hitting Mars’ surface would likely not bring enough carbon to explain the amount of methane seen in the atmosphere.

The researchers said, however, that their findings give valuable insights into the planet’s atmosphere and these findings would be helpful for future robotic missions to Mars so scientists could fine-tune their experiments, potentially making their trips more valuable.

Read the team’s paper in Nature.

Posted on by Jason Major

Opportunity Gets a View From The Edge

Opportunity's shadow aims eastward to the rim of Endeavour crater

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The rover Opportunity captured a view into Endeavour crater as a low Sun cast a long shadow in this image, acquired back on March 9.

Endeavour is a large crater — 14 miles (22 km) wide, it’s about the same area as the city of Seattle. Opportunity arrived at its edge in August of 2011 after several years of driving across the Meridiani Plains.

Opportunity is currently the only operational manmade object on the surface of Mars… or any other planet besides Earth, for that matter. It’s a distinction it will hold until the arrival of Mars Science Laboratory at Gale Crater this August.

From the NASA news release by JPL’s Guy Webster:

The scene is presented in false color to emphasize differences in materials such as dark dunes on the crater floor. This gives portions of the image an aqua tint.

Opportunity took most of the component images on March 9, 2012, while the solar-powered rover was spending several weeks at one location to preserve energy during the Martian winter. It has since resumed driving and is currently investigating a patch of windblown Martian dust near its winter haven.

Opportunity and its rover twin, Spirit, completed their three-month prime missions on Mars in April 2004. Both rovers continued for years of bonus, extended missions. Both have made important discoveries about wet environments on ancient Mars that may have been favorable for supporting microbial life. Spirit stopped communicating in 2010. Since landing in the Meridiani region of Mars in January 2004, Opportunity has driven 21.4 miles (34.4 kilometers).

Image credit: NASA/JPL-Caltech/Cornell/Arizona State University