Phobos, seen above by the Mars Global Surveyor spacecraft several years ago, is not just an interesting world in its own right but could also speed up the human exploration of Mars. (credit: MSSS)

Going (almost) all the way to Mars

by Jeff Foust
Monday, November 21, 2005

The Earth’s own Moon will play a critical role in the future of human exploration of the solar system. While some might quibble whether the Moon should lie in the critical path for future human missions elsewhere in the solar system, it’s clear under the Vision for Space Exploration that, at least for the US, the Moon will be not just a major goal for exploration, but a place to test some of the systems and technologies needed for missions to Mars and beyond.

Less has been said, though, about the role of Mars’s own small moons, Phobos and Deimos, in any future exploration. The two moons are often an afterthought in exploration plans, which tend to focus on sending human crews to the surface of the planet, either directly from Earth or from spacecraft in orbit around the planet. Phobos and Deimos, unlike the Moon, are not seen as being in the critical path to Mars.

Not everyone agrees with that assessment, though. Some, like Pascal Lee, the chairman of the Mars Institute and the principal investigator of NASA’s Haughton-Mars Project, a Mars analog base in the Canadian Arctic, see Phobos in particular—the larger of the two moons—as not just an interesting world in its own right but as a key component for future human exploration of Mars itself. In fact, as Lee described in a speech at the Space Frontier Conference in Los Angeles last month, it makes sense to send human missions to Phobos alone—and not Mars—before any humans set foot on the surface of the Red Planet.

All about Phobos

At first glance, Phobos does not seem like a particularly interesting body. An irregularly-shaped “potato” 13.5 kilometers long on its longest axis, Phobos appears a lot like countless thousands of similarly-sized asteroids. Orbiting less than 10,000 km from Mars, Phobos completes one orbit in just seven hours and 39 minutes, far faster than the planet’s own rotational period—as a result, Phobos appears on the Martian surface to rise in the west and set in the east. Phobos is so close to Mars that it is gradually spiraling in, and will crash into the planet in about 45 million years.

Phobos’s rotation period is the same as its orbital period, tidally locked by Mars’s gravity in much the same way the Moon is tidally locked to the Earth. That means the same face—or same “pointed end of the potato”, as Lee described it—always faces Mars, creating what he described as valuable real estate. “So here’s a strategic location for you,” he said. “You don’t want to let the Russians or anyone else grab that one spot. The sub-Mars point is a strategic location on the surface of Phobos. That’s where you will have the best view from Phobos, the best opportunity to monitor Mars.”

Phobos’s small size and irregular surface results in some unusual properties. The moon’s escape velocity is only about 45 km/h, and it would be easy for people to put objects—or themselves—into orbit around the moon. “The gravity on Phobos is so weak that if you were to throw a baseball at a reasonable speed, it would come back to you in about an hour, having gone all the way around Phobos, as long as it didn’t hit any part of Phobos,” he said. In addition, there are places on the surface where the gravity vector goes against the slope, because the center of shape of the moon is not coincident with the center of mass. “There are places on Phobos where you might actually fall uphill.”

There are some interesting scientific questions about Phobos itself. Lee noted that there is no consensus about how the moon formed: did it form around Mars or is it an asteroid captured by the planet? These different origins have implications for the bulk density of the object, meaning that analysis of samples of the moon either returned to Earth or studied in situ would allow scientists to determine which scenario is accurate. Those samples would also allow scientists to determine the composition of the moon, which to date remains unknown. In addition, Phobos has a number of interesting surface features, including grooves that could be created by the gravity gradient of Mars—Phobos is within Mars’s Roche limit—trying to tear the moon apart.

Library, glovebox, and catalyst

The idea of sending human missions to Phobos is not new. For example, Fred Singer, a space scientist involved in the early years of the space program and now a research professor at George Mason University, has advocated for decades sending human missions to the moons of Mars as a prelude to landing on Mars itself. His “Ph-D Project” would send eight astronauts to Deimos, and later Phobos, with perhaps a brief two-day, two-person sortie to the Martian surface itself.

Proposals for human missions to the Martian moons usually have been based on five motivations, Lee noted. From a technical standpoint, a mission to Phobos requires minimal delta-v, because of its small mass and the ability to use aerobraking in the Martian atmosphere. “It’s actually cheaper to send one pound of dead weight to the surface of Phobos than it is to land one pound on the surface of the Moon softly,” Lee said. “Phobos and Deimos are the cheapest objects you can reach on a regular basis.” On top of that, he said, Phobos offers a stable platform for observations of Mars and is a place where astronauts can teleoperate robots on the Martian surface with little time delay. Phobos also allows an opportunity to advance the science of small bodies, and may harbor water ice deposits that could be used as a resource.

These “classical” motivations, though, don’t put Phobos on the critical path to Mars, Lee said. “If anything, Phobos has often been considered by proponents of Mars exploration as something that was a distraction. If you want to go to Mars, go to Mars, and forget about Phobos, forget about the Moon.”

However, Lee and colleagues have come up with three new motivations that he believes will put Phobos in that critical path to Mars. One is that the surface of Phobos likely has material that was blasted off the surface of Mars. Just as a handful of Martian meteorites have been discovered on Earth, Phobos could be a “Library of Alexandria” for Mars, preserving samples of the Martian crust from across the planet and throughout the planet’s history, in better condition than if they were on the planet itself. “The regolith of Phobos might hold a wonderful long time-based record of Martian crustal rocks,” Lee said. That might make it possible, for example, to look for “biosignatures” in the samples to see if and when the planet once supported life.

The potential for life on Mars is key to the second new motivation for going to Phobos: that the moon could be a “glovebox” for Martian samples. If there was any question about the danger of contamination of the Earth from Martian life, Lee said, it makes more sense to quarantine and study those samples on Phobos than anywhere else, including a space station in Earth orbit. “There’s a great risk that when you launch something from Mars it might not end up in the right place when it arrives at the Earth and just enters the Earth’s atmosphere,” he said. In addition, with a “decent” facility on Phobos researchers could study much of the material there rather than sending it all back to Earth.

The final new motivation for going to Phobos is as a catalyst for eventual human missions to Mars. One issue Lee has with people who believe we can effectively go to Mars right now is that considerable technology development, ranging from advanced spacesuits to pressurized rovers, is required before humans can effectively work on the Martian surface. “Going all the way to Mars is super expensive, and if we are planning to do this in the immediate wake of returning to the Moon,” he warned, “either we have to have a huge surge in our budget, or we have to postpone landing humans on Mars long after their return to the Moon.”

A way around that, he believes, is to go to Phobos first, testing the technologies needed to work on Mars while building up experience and maintaining interest in Mars exploration. “We can go to Phobos shortly after returning to the Moon, if we plan it right,” Lee said. “That will get humans on their way to Mars much earlier than if we somehow waited for all the hardware that enables the surface exploration of Mars to be lined up.” Otherwise, he cautioned, “if we want to learn all our lessons on the Moon, then we’re going to spend a long time doing that on the Moon, and we still won’t be going to Mars.”

“Basically, we need to view Phobos as a beautiful opportunity for Apollo 8-style missions, that is, before you land on Mars you first want to go to Mars, and do that several times. Using Phobos as a target before going to Mars is a reasonable suggestion.”

Exploration plans

Before there can be any serious talk of sending humans to Phobos, though, there needs to be extensive robotic exploration of the moon. To date Phobos and Deimos have received scant attention from spacecraft that have focused their attention on Mars. The most significant effort to explore Phobos, the Soviet Union’s Phobos 1 and 2 missions in the late 1980s, both ended in failure before arriving at the moon.

“We need to successfully rendezvous with Phobos, which has never been done before, land on Phobos, roam on Phobos, and return some samples from Phobos if we can,” Lee said. The Mars Institute is investigating a mission called PRIME, for Phobos Reconnaissance In preparation for Mars Exploration. This would be an international mission, with a potential for involvement from the private sector. He said the institute soon plans to start a “pre-Phase A” study to determine how much PRIME would cost and who the partners should be.

While the Mars Institute is studying the PRIME mission, Russia is planning another mission to Phobos, called Phobos-Grunt. That mission has been on the drawing boards for some time, and was included in the ten-year plan for the Russian space program announced in recent weeks for launch as early as 2009. “The Russians actually have a laudable record for persistent support of Phobos exploration,” Lee said. However, he was skeptical the project would actually ever get funded, although he added that the PRIME mission has already notionally baselined some Russian contributions.

It should be possible, he said, to have humans on Phobos as early as 2020, provided humans return to the Moon in 2015—a few years earlier than what NASA is now contemplating. That is particularly true if NASA makes lunar exploration goals “Phobos-consistent” on areas like mobility systems. “It would take sitting down and really looking carefully at what the requirements of Phobos exploration are, and from there feed that into the lunar architecture design effort,” he said.

Still, though, there’s that nagging doubt that stopping just shy of the Martian surface would be a disappointment. Lee doesn’t think so. “Occasionally I get hit with the question, ‘How could you possibly go all the way to Mars and not land on Mars? That would be psychologically so damaging to the crew.’ Well, I’d go, and I don’t think it would be psychologically damaging. I think it would be a spectacular mission.” And, he adds, Phobos “would be a lot of fun to explore.”

Jeff Foust (jeff@thespacereview.com) is the editor and publisher of The Space Review. He also operates the Spacetoday.net web site and the Space Politics weblog. Views and opinions expressed in this article are those of the author alone, and do not represent the official positions of any organization or company, including the Futron Corporation, the author’s employer.

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