The Space Reviewin association with SpaceNews

Mars astronaut illustration
Robotic rovers, working in conjunction with humans, could allow a manned expedition to explore a larger region of the planet than humans could alone. (credit: NASA)

Strategies for Martian exploration

There has been no shortage of ideas of how to send humans to Mars. From the “Battlestar Galactica” spacecraft of NASA’s Space Exploration Initiative 15 years ago to Robert Zubrin’s Mars Direct concept to high-speed alternatives powered by Franklin Chang-Diaz’s VASIMR engine, there have been plenty of proposals for mounting manned expeditions to the Red Planet, with varying flight times, crew sizes, and, of course, costs.

Far less has been said, however, about what exactly humans will do once on the surface. There are notions, of course, of base camps, hikes through the terrain, and rovers for longer-range expeditions from the landing site. All of these are roughly modeled on our experience from the Apollo missions. However, those journeys were relatively short: the longest Apollo expeditions, the science-heavy final missions, featured only three eight-hour EVAs, with only about three days between landing and takeoff. By contrast, “sprint” missions to Mars leave crews on the surface for at least a few weeks, while other mission profiles feature stays of 500 days or more.

For the short-duration Apollo landings, the limited mobility provided by walking and roving was sufficient to carry out the scientific objectives of those expeditions, regardless of landing location. For longer-duration Mars missions, though, landing site location becomes critical: if a mission lands in a safe but scientifically “bland” region, a crew could exhaust all of the key areas of interest there before the end of their stay. (The Mars Exploration Rovers have proven that there’s still plenty of good science to be done more than a year after landing, but a human crew would likely work at a far faster rate than these relatively-primitive robotic explorers.)

For long-duration Mars missions, landing site location becomes critical: if a mission lands in a safe but scientifically “bland” region, a crew could exhaust all of the key areas of interest there before the end of their stay.

Mobility will also likely be limited for at least the early Mars missions—Zubrin’s Mars Direct scheme, for example, includes a pressurized rover with a range of just a few hundred kilometers—putting more emphasis on picking a suitable landing site. That pressure will be increased by the fact that missions to Mars will be modulated, in most cases, by launch windows that open just once every 26 months: pick the “wrong” site and you may have to wait two years for the next mission to have better luck.

The limited history of space exploration to date has shown that serendipity—and ill-fortune—have played key roles in planetary landings and probes. Opportunity was fortunate enough to have landed in a crater with bedrock outcroppings that provided critical evidence for water in Mars’s past. Galileo’s probe entered Jupiter’s atmosphere in a rare dry hot spot: fortunate for some scientists but disappointing for others. And, while results are preliminary, some scientists think ESA’s Huygens probe landed in a relatively arid region that may not be characteristic of much of the rest of that moon. While any landing site for a manned Mars mission will likely be as well-scouted as any region beyond Earth, good luck and bad will play a role in what areas of interest Mars explorers will find.

Thus, any human mission to Mars will need to weigh tradeoffs between location and scienific viability. (Those issues also exist for human lunar expeditions, but are mitigated by the fact that lunar missions can be accomplished in the fraction of the time of a Mars mission, and have launch windows that are effectively open continuously.) Scientists will seek landing sites that have high scientific merit, but many of those—if planning for robotic landers holds true—may prove inaccessible or simply too risky in the eyes of mission planners. Those concerns can be lessened by giving crews the ability to travel wider distances beyond their landing site, but those options can prove costly in mass and dollars. While the first manned Mars mission might still be two decades or more in the future, strategists are already examining ways to give those human explorers the ability to explore far beyond any single landing site.

Four ways to explore

One such person is John Stevens, the Director of Space Exploration Architecture Studies at Lockheed Martin. Speaking at the SpaceVision2004 conference at the Massachusetts Institute of Technology in November, he said that any successful human mission to Mars has to be able to explore more than just one region of the planet.

“If you were given the challenge of going off to explore Mars, and I asked you to pick the one spot there that was the most representative of the geophysical history, the planetary evolution, the best spot to look for resources, the best spot to look for the presence of life, and possibly the most scenic spot, what single spot would you pick?” he asked. “If I asked you do that on the Earth, what single spot would you pick on the Earth? That’s the challenge before us, because obviously on the Earth there isn’t a single spot that’s truly representative of our whole planetary history or evolutionary history. The same thing is true on Mars.”

“…obviously on the Earth there isn’t a single spot that’s truly representative of our whole planetary history or evolutionary history,” said Stevens. “The same thing is true on Mars.”

In his presentation Stevens examined four different approaches to human Martian exploration that are largely independent of how a crew gets to the planet. The first approach would use an expendable infrastructure, with no reuse of equipment between missions. This approach, identical to the Apollo missions, would also allow for no more mobility than any rovers or other modes of transportation included with the landers. Stevens calls this, when used on Apollo, “trash heaps on the Moon”, and made it clear it wasn’t his preferred mode of exploration. “I much prefer the notion of incrementally building up the site and using the equipment more than once.”

Stevens’ second approach is to not go directly to the Martian surface but instead establish a base on the moons of Phobos or Deimos. Human crews would stay on the base while robotic landers scouted out several locations on the surface, returning samples for study by the crew on the moon. Only once an ideal landing site has been selected would the crew actually land on the surface, and even then for a relatively brief period before returning to their moon base. “This kind of sounds like a space station around Mars,” he admitted, “and I really want to go to Mars, not to Phobos or Deimos.”

Both of these approaches, Stevens noted, ignore one development in Martian exploration in recent years: the success of the twin Mars Exploration Rovers. These rovers, as well as future rover missions like the Mars Science Laboratory, open the possibility of integrating their capabilities into manned missions. One way Stevens described is to make the entire manned base a rover: putting it on wheels or treads and moving it robotically from one site to another between visits by human crews. “You get to use the same base over and over again, so all you have to do is bring the consumables for each mission,” he said.

A manned base on a Martian moon “kind of sounds like a space station around Mars,” Stevens admitted, “and I really want to go to Mars, not to Phobos or Deimos.”

This approach would even allow the base to do science during those transit periods when no humans are on board. “You can put little robots on it, and as it’s traveling from one spot to another these little rovers can take photographs and pick up rocks,” he explained. “So, when the astronauts come, all the samples will be waiting for them the next time around.” The problem with this architecture, though, is that a mobile base would be heavy and difficult to transport to Mars.

There may be a compromise, though, between a single static landing site and robotic mobility. “The aspect of that [the mobile base] which really appealed to me was the notion of these little guide robots picking up samples and bringing them along,” Stevens said. “So, why don’t we focus on these side rovers and see what we can do with these?”

The result, Stevens’ fourth architecture, integrates robotic and human explorers. Three rovers would land two years ahead of a human mission in different parts of the planet—one in each of the polar regions and the third in the heavily-cratered highlands, in the strawman example he described at the conference. The rovers would then travel towards the planned landing site of a human expedition, collecting samples (and marking the locations where the samples are collected) along the way. When that human mission arrives, “they’ve got a whole bunch of samples waiting for them,” he said. The crew would then have samples from a far wider array of locations than a single human mission could visit. The crew would also be able to analyze the samples and determine which are worthwhile enough to return to Earth for additional study, as well as for use planning future human missions. The rovers can also go and collect additional samples from other regions between human missions.

“Fundamentally, as a nation we have a reluctance to do sample-return missions because there’s some kind of urge to do that ourselves,” Stevens said.

This approach does have one significant drawback. Because robots will collect most of the samples, the expedition won’t benefit as much from the on-site expertise that has long been a major selling point for sending humans to Mars. Moreover, while a human mission would likely be able to perform far more sophisticated analyses than any robotic mission, its capabilities will still be far less than many terrestrial labs. In this architecture humans almost become superfluous: one could instead send the robotically-collected samples directly to Earth for analysis there, using those studies to determine the best spot to send a human mission.

Stevens acknowledged this issue, but believes that a mixed human-robotic approach is better than a robotic-only sample return mission for both financial and intangible reasons. “NASA has been trying to do a sample-return mission to Mars for a long time and has not been very successful. It costs about $2 billion to return two kilograms of rocks from Mars; a billion dollars a kilogram,” he said “That’s a pretty expensive kilogram of rocks. If I’m going to pay that price I’d rather send humans there and let them look at rocks and perform exploration. Fundamentally, as a nation we have a reluctance to do sample-return missions because there’s some kind of urge to do that ourselves.”

While these architectures are still in the early conceptual stages, they show the potential for combining the best abilities of humans and robots to get the most out of future expeditions to Mars. These approaches may answer John Stevens’ two biggest questions: “What do we do when we get there? How do we explore?”