With Curiosity last year becoming the fourth rover to land on Mars, how long will it be before a human walks on the Red Planet?
It is easy to imagine that Nasa's engineers could send astronauts to Mars whenever it liked, if only the politicians would let them to get on with the job. In 1961, at a time when we barely knew how to fly into Earth orbit, John F Kennedy championed the idea of a lunar landing, and the task was accomplished in just eight years. Today, surely we know enough about space systems to make even faster progress across the solar system, so why can't a modern president give the green light to the Red Planet?
Actually, some presidents have tried. On 20 July 1989, George Bush Senior celebrated Apollo 11's 20th anniversary. With legendary astronauts Armstrong, Aldrin and Collins by his side, he talked of "a journey into tomorrow, a manned mission to Mars". A Nasa team set to work on the now infamous '90-Day Report', so named because that was how long it took to invent a plan guaranteed to make Bush wish he had never mentioned the idea.
A space station was to service a 1,000-tonne interplanetary craft assembled in Earth orbit. The Mars return trip would take 18 months, with two weeks spent on the surface: just enough time to plant a flag and snap some photos before heading for home. The price tag was $500bn.
At the Martin Marietta company, engineer Robert Zubrin was appalled by the costs. It seemed that building a giant ship had become more important than the mission itself. "I fired off a memo, saying it wasn't enough simply to reach our destination. We had to do something useful when we got there. I thought the plan was wrong, and too expensive. Many people at Nasa were upset when I spoke out of turn."
Industry wasn't keen on Zubrin either. "Aerospace companies usually tell Nasa exactly what they want to hear, because that's the way to make a sale. I was proposing to do the opposite and tell the truth, whether Nasa liked it or not. Theirs was the worst and most inefficient way to get to Mars."
Martin Marietta encouraged Zubrin to rewrite the company's official sales pitch to Nasa. By February 1990, his team had reduced the space station's role, cut the weight of the Mars craft in half and slashed the costs. But Zubrin was not yet satisfied. Time spent on the Martian surface was still only four weeks out of an 18-month round trip. The notion of linking modules in Earth orbit to assemble the ship piecemeal annoyed him because it cost money and wasted fuel. So he conceived Mars Direct, a scheme in which each small subcomponent lifts off from the Earth and heads straight for Mars without lingering in Earth orbit.
Like many an engineer before him, Zubrin began by discarding long-held assumptions. "Most Mars plans call for a huge mothership to circle the planet and send down landing teams, which then come up, rendezvous with the ship and fly home. I call it the 'Battlestar Galactica' approach. Why have the mothership at all? In Mars Direct, you fly items of hardware directly to Mars, and then a small Earth Return Vehicle (ERV) fires off the surface and immediately heads back home." Astronauts are confined to small cabins for the six-month outward trip and a similarly long slog home, "but we know from our space station experience that people can tolerate that if they're motivated. We don't have to build giant space cruisers."
Missions are launched only when the Earth and Mars swing close to each other in their orbits and are both on the same side of the sun. This happens roughly every two years, so Mars Direct employs a rolling schedule of missions to coincide with these planetary alignments.
The downside is that the crew has to wait 18 months on Mars, until the planet swings close by the Earth again, before they can set off on the homeward trip. "In theory this lengthens their exposure to cosmic radiation hazards," Zubrin acknowledges, "but the planet's bulk and its atmosphere are pretty good shields." Nasa's Curiosity rover has confirmed that the radiation levels on Mars could be acceptable for a human mission. It is indeed safer to stay down on the planet and to minimise the interplanetary journey times.
Nothing we don't know
Zubrin's original scheme took advantage of space shuttle components already available in Nasa's hangars. The big liquid-fuel tank from a space shuttle could have been attached to an upper stage with a payload cylinder. At the base, a pod with four shuttle engines was fed from the big tank. Two solid rocket boosters gave extra impetus, just as on a normal shuttle mission. The key point was to leave out the winged orbiter, and replace its weight with Mars hardware.
In theory, this legacy kit could have lifted 130 tonnes of payload into space: just 10'tonnes less than the giant Saturn V boosters of the Apollo era. But would this have been enough to enable a Mars mission? "We found we could launch a habitation module and surface equipment, or else an ERV, but we couldn't lift everything at once." Zubrin's machinery could fly people out, or bring them home, but it couldn't do both.
Far from dooming the plan, this 'problem' turned out to be a brilliant solution, not just in terms of fuel efficiency but also for safety. Zubrin had a flash of inspiration. "For the first mission, don't send the crew. Send the ship out empty. You save a huge amount of weight on oxygen, water, food and other life support, and you lose the weight of the people." It seems crazy, but this approach is what makes Mars Direct so clever. The first component to reach Mars is an empty landing vehicle, with its ERV earth return rocket and capsule installed. Only when this is sitting safely on the surface are humans risked for the outward voyage.
With an ERV safely waiting on Mars, there is no need to launch that hardware again. A second rocket carries inflatable habitats and an additional ERV for backup. When the planetary alignments are once again appropriate, a third rocket delivers the crew in a small ship capable of landing on Mars, but not of taking off again. Each flight carries only what it must.
Zubrin continues to promote this scheme today. A recent Nasa proposal calls for a new space station orbiting at L2, a 'Lagrange' point, between the Earth and the Moon, where the influences of both worlds combine to create a gravitationally neutral zone from where it is easy to escape. This platform supposedly will be a staging post for future Mars missions and asteroid survey trips. Zubrin insists, "We don't need an L2 space station to go to Mars. We don't need an L2 space station for anything."
Nasa is no longer the only game in town. Paypal entrepreneur Elon Musk, the founder of SpaceX, has developed Falcon 9, a powerful rocket that has already delivered two Dragon cargo supply capsules to the International Space Station, and should be capable of carrying humans soon. Development is under way on Falcon Heavy, which will use an additional pair of first stages as liquid strap-on boosters. This beast, the most powerful rocket since the Apollo era, will lift a payload of 50 tonnes.
Musk wants to reach Mars. His Falcons are not quite large enough for Zubrin's idea to work. However, a compromise scheme, Mars Semi-Direct, could allow a smaller and lighter version of the ERVs to blast off Mars and rendezvous with small interplanetary propulsion modules parked in Martian orbit ready for the homeward trip. This does involve some space docking and assembly, but it fits within the Falcon 9's payload capacity.
Spinning on a line
Whatever the exact details turn out to be, it looks as if the technology for an assault on Mars will be available soon, especially if we adopt Zubrin's most audacious idea. Send just two explorers, riding a Dragon-style landing capsule, with perhaps a lightweight inflatable module deployed in space to provide extra leg room. The word 'cramped' barely describes such a voyage, but it could be done.
On coming home in their stuffy cabin, at least the crew would know that help would be available when they reached Earth. However, there wouldn't be any such assistance waiting on Mars, so they would have to arrive as fit as possible. Zubrin proposes that artificial gravity could be supplied for the outward trip. After the upper stage of the launch vehicle has finished firing, its empty tanks would be hurtling through space still attached to the crew module. By disconnecting the rocket stage and swinging the module on the end of a long tether, the two components could be made to spin around each other in a huge circle, creating centrifugal gravity inside the crew compartment.
Musk favours something more comfortable than just his Dragon capsules. His key innovation is, of course, to try and privatise the mission. Once an initial Mars base is established, "you can get the cost down to half a million dollars for someone to go there. I think there are enough people who would buy that to have it be a reasonable business case." But for the economics to work, he needs a larger crew than Zubrin proposes.
In the meantime, the originator of Mars Direct would like Nasa to bite the bullet and purchase Falcon hardware for a minimum mission, just to set the interplanetary ball rolling."Mars Direct is a poor man's space project," he insists, "and America is not a poor country. We should be able to afford this."