A view from NASA's Mars Reconnaissance Orbiter's HiRISE camera catches sight of NASA's Curiosity Mars rover on April 8, 2015. Image credit: NASA/JPL-Caltech/Univ. of Arizona

May 1, 2015 The Martian fleet

Sen—Our space fleet is thinning. With MESSENGER dying on April 30, and Venus Express gone last fall, we are left with orbital missions operating at only five locations beyond Earth: one comet, one asteroid, our Moon, and two planets: Saturn and Mars. At Mars, though, our orbital fleet shows impressive numbers, with five spacecraft currently sending science data to Earth. Why so many, and what do we have to show for all this attention?

One reason for the high numbers is that Mars is the easiest destination to reach beyond the Moon (if anything in space exploration can be called "easy"). Of the five Mars orbiters, two represent space agencies' first attempts at orbiting a body beyond the Moon. Mars Express was the European Space Agency's first deep-space orbital mission, and Mars Orbiter Mission is a similar first for the Indian Space Research Organisation.

Navigating to Mars and Venus require only Hohmann transfer trajectories with no tricky gravity assists. At Mars, the power and thermal environments are benign for spacecraft traveling only a little bit farther from the Sun than Earth does, and the atmosphere is thin enough to allow clear views of the surface most of the time. Being farther from the Sun also mitigates solar storms, although this didn't help poor Nozomi, Japan's first attempt at a deep-space orbital mission to Mars, which was zapped by a solar flare while still in Earth's neighborhood.

The benign environment at Mars is a factor in another reason that there are so many current missions operating: Mars spacecraft last a really long time. Mars Odyssey arrived early in 2002, so has been operating there almost continuously for more than 13 years, closing in on the deep-space orbital mission record held by Pioneer Venus. Mars Express arrived in late 2003, and Mars Reconnaissance Orbiter in 2005. Two more, MAVEN and Mars Orbiter Mission, reached Mars in 2014. Will they last as long? MAVEN is a less expensive spacecraft than the other two NASA orbiters, which means it has fewer redundant systems to serve as backups, but it is expected to last into more than one mission extension. As for Mars Orbiter Mission, its lunar orbiter predecessor Chandrayaan-1 failed before completing its prime mission, so there's no track record of longevity; but the Mars spacecraft is already in its first mission extension and the head of ISRO is on record as saying the spacecraft "could last for years."

What are all these spacecraft doing at Mars? Although there's overlap in capability, each has a suite of instruments that fill slightly different niches. In particular, the three older orbiters are highly complementary to each other, and have built global data sets that are powerful in themselves but multiply in value when related to each other.

Odyssey is the mission that gave us global maps of Mars' near-surface hydrogen, hinting that there were vast deposits of water ice hidden beneath just a few centimeters of soil. Over the course of its extremely long mission Odyssey has also built up global maps of Mars in both visible light (which is good for studying the shape of the landscape) and infrared wavelengths (which show the distribution of rockier and dustier areas and have proven helpful in differentiating varied rock units from each other).

Mars Reconnaissance Orbiter has HiRISE, the most powerful camera ever sent beyond Earth, achieving 25 centimeter resolution on the surface. Such focused views come with a cost: Even after all this time, HiRISE has covered only about two per cent of the Martian surface. Some parts it has viewed multiple times, watching seasonal changes and building 3D data sets. The mission also has a high-resolution imaging spectrometer for mineral identification called CRISM. To compensate for HiRISE and CRISM's narrow views, Mars Reconnaissance Orbiter also carries the Context Camera, which has achieved near-global coverage at six meter resolution. Its MARCI camera has an even wider view, assembling daily maps of Martian weather.

Mars Express has given us beautiful visible-light photos of most of Mars, in color and in 3D, with its HRSC camera. But a more impressive feat is that its imaging spectrometer, OMEGA, has recently completed global maps of Martian mineralogy. OMEGA is to CRISM as CTX and THEMIS are to HiRISE, helping to place CRISM's finely detailed but spotty observations into global context. All of these missions' data sets can be related to each other thanks to the work of the aptly named Mars Global Surveyor, which arrived at Mars in 1997 with MOLA, a laser altimeter that established the topographic base maps to which everything else is georeferenced. Contact with Mars Global Surveyor was lost in 2006.

Together, these four orbiters have guided us to fantastic landing sites and have given us rich insight into the present and history of the Martian surface—a richer orbital data set than we have even for our nearer neighbor, the Moon. (From the Moon, of course, we have many kilograms of returned samples, something we lack for Mars except for Martian meteorites of unknown provenance.)

MAVEN will increase the altitude of our coverage of Mars to its upper atmosphere, telling us about its present and past.

These missions have lasted so long that it's hard to imagine work at Mars without them. Lander missions rely very heavily on NASA's two oldest orbiters, whose circular Sun-synchronous orbits carry them over the rovers on a reliable schedule, twice a day, once in the late afternoon and once in the wee hours of the morning. The regular relay schedule greatly simplifies rover operations. If we should lose one or both of the old orbiters, we'll still have MAVEN, which has demonstrated impressive relay capacity, but which would provide a much less regular relay schedule.

But they won't last forever. Odyssey is down a reaction wheel, and the loss of a second would put a limit on the mission's lifetime. Mars Reconnaissance Orbiter's computers are getting glitchy. Mars Express' operating system had to be substantially rewritten recently to work around a serious memory problem. And then there's the problem of funding: NASA's budget for Odyssey is zeroed out as of October 2015, and Mars Reconnaissance Orbiter in 2016.

Have we reached peak Mars? It's not clear. ESA plans to launch another orbiter in 2016, and there are rumors of a future Chinese mission. If the Fates are kind, we'll have at least five simultaneous orbiting missions at Mars for many years to come.

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Mars Orbiter Mission undergoing final assembly and testing in Bengaluru, September 2013. Image credit: PTI

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MOM's Mars Colour Camera (MCC) images Arsia Mons. Image credit: ISRO

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ESA's Mars Express's High Resolution Stereo Camera images Hellas Chaos on Jan 23, 2014. The resolution is about 18 meters per pixel. Image credit: ESA/DLR/FU Berlin,CC BY-SA 3.0 IGO

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Mosaic image of Valles Marineris, colored to resemble the martian surface, comes from the Thermal Emission Imaging System (THEMIS), a visible-light and infrared-sensing camera on NASA's Mars Odyssey orbiter. Image Credit: NASA/JPL-Caltech/Arizona State University

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Image was taken by Curiosity's left Navigation Camera on Oct 23, 2014 of the "Pahrump Hills" outcrop at the base of Mount Sharp. This was one of the first images relayed by NASA's MAVEN orbiter. Image Credit: NASA/JPL-Caltech

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