Ready to go to Mars? Elon Musk is. So is Robert Zubrin. And so is NASA. But I wouldn’t book my flight quite yet. There are some significant challenges to be overcome before any human footprints are left on the Red Planet’s surface.
What are the problems involved in sending humans to Mars, landing them there, and returning them safely to Earth? After all, we sent 12 men to the surface of the moon and back more than 40 years ago. Why not Mars? A few comparisons will be helpful.
1) The moon is 240,000 miles (385,000 kilometers) away, and the longest Apollo mission (the final one, Apollo 17) lasted a little over 12 ½ days.
Mars is, at its closest, 34 million miles (55 million km) away from Earth. But the realities of interplanetary travel require a long and curving path from here to there and back again. How far it is and how long it takes depends on the relative positions of the two planets in their orbits around the sun. As an example, the Curiosity Rover currently exploring Gale Crater traveled 352 million miles (566 million km) in 253 days.
2) It takes an enormous amount of energy to lift something off the Earth, send it to another celestial body, land on that body, lift off from that body, and then return to the Earth. And any mass that isn’t fuel or payload is ultimately extraneous.
One solution to the problem of getting off the Earth is staging, where rockets burn all the fuel in a part of the rocket which is then discarded as dead weight. The fully fueled and lighter remainder requires less energy to continue its path upward.
But then you need an extra kick to leave the Earth. And more fuel to slow down into orbit once you reach your destination so you don’t go flying past it. More fuel is needed to lower yourself to the surface in a manner that doesn’t make a new crater, more to lift off from that surface and still more to kick out of orbit back to Earth. Whew!
The Apollo missions dealt with this by taking all the necessary fuel with them and throwing away everything except the small cabin holding the three astronauts. Of that enormous 363 foot tall Saturn V assembly, only the small cone at the top actually returned to eventually wind up in a museum.
In 1989, President George H.W. Bush announced the Space Exploration Initiative which would culminate in a crewed landing on Mars. The problem was that this rather grandiose plan would have required the largest single government expenditure since World War II. Needless to say, Congressional reactions were not kind, and the plan was scrapped less than two years later. This proposed Mars mission took a similar approach to that of Apollo. But taking all the necessary fuel with you to Mars would require either a massive launch vehicle or a fleet of smaller ones.
What’s the alternative?
This is where Bob Zubrin and his aerospace engineering colleague David Baker made an invaluable contribution to the future of solar system exploration. Lewis and Clark didn’t pack all the food they would consume on their two and a half year expedition when they left Saint Louis. They lived off the land. Similarly, Zubrin and Baker proposed sending a chemical factory to Mars to manufacture fuel from the available resources there.
Mars’ atmosphere is 95% carbon dioxide, CO2. Their initial plan called for an Earth Return Vehicle to be sent to Mars along with a chemical plant, a relatively small supply of hydrogen, and a nuclear reactor for power. Once landed, the chemical plant would use the CO2 in the atmosphere to carry out the following simple reactions, ones routinely used by industry since the 19th century.
CO2 + 4H2 → CH4 + 2H2O
2H2O → 2H2 + O2
Methane (CH4), the main component of natural gas, is a perfectly acceptable rocket fuel. The oxygen produced in the second reaction is necessary for actual combustion in a rocket engine, and the hydrogen can be recycled back to the first reaction. In order to have enough oxygen to make use of all the methane, we would need to produce more oxygen by another reaction. There are several possibilities, all using proven chemical processes and the carbon dioxide that Mars provides free of charge. Any extra oxygen would be used for breathing atmosphere once the crew arrived, and some water could be saved for all the uses it can be put to.
So—send a spacecraft to Mars that a crew will take back to Earth, let it sit on Mars for a while producing fuel for the journey back home. Don’t launch the crew until you know there is a fully fueled craft waiting for them on Mars. It’s obviously important to land the crew close to that return vehicle! Which brings us to another issue…
3) When Neil Armstrong flew the Eagle lunar module to the first landing on the moon, he did so using rocket power alone. There was no atmosphere to either slow the craft down (parachutes would have been useless) or to provide lift (wings would be equally useless.) The ungainly and unstreamlined appearance of Eagle was perfectly functional.
We’ve already said that Mars has an atmosphere, one that is quite useful for manufacturing rocket fuel. It does pose some problems, however, for someone wanting to land on its surface. It is substantial enough to heat by friction any spacecraft descending through it either from orbit around Mars or directly from an interplanetary trajectory, but not substantial enough to allow descent by parachute as with spacecraft returning to Earth, or by gliding as with the retired space shuttle.
Some combination of heat shielding, parachutes, and rocket power will be needed to get any craft safely to the Martian surface. It ain’t easy!
4) So how long would such a mission be? Unless you want your crew to spend inordinate amounts of time between planets (and there are good reasons why you don’t), you want to wait until the two planets are lined up to allow the quickest transit time. This happens roughly every 26 months, and gives you a minimum 6-month journey to the red planet from Earth. After so much effort to get there, you’d want to spend some time exploring, and waiting for the next favorable planetary alignment would allow a year and a half to explore Mars both from orbit and on the surface. The return to Earth would also take 6 months. Total mission time: two and a half years.
5) Two and a half years is a very long time to ask people to live in an artificial environment. The spacious craft of science fiction movies are unlikely to exist in reality for quite some time.
There are three potential problems with a Mars mission of this length:
- the long-term effects of weightlessness
- the radiation exposure
- the psychological hardships
The first is at least potentially the easiest to solve. No fancy mechanism is needed, just an empty fuel tank and a sturdy tether. Set the two ends of the tether spinning with rocket motors, and you have an artificial gravity field that can be adjusted to Mars normal—38% that of Earth—as the red planet approaches.
Radiation exposure is a different issue. The people who spend months in the International Space Station do so within the protective shield of the Earth’s magnetic field. This protects them from the two sources of space radiation: solar radiation and cosmic rays.
But once you venture beyond, that protection is gone. The only real way to shield yourself is to put something in the way. The more of that something there is, the more radiation you stop from reaching you. The problem with this is of course that extra shielding adds to the mass you are carrying, and every bit of extra mass requires extra fuel to propel it.
Could we somehow generate our own magnetic shield to mimic that of the Earth and thus gain protection? Yes, but doing so would require a good bit of power. It’s not a crazy idea, though, and is discussed here, with a link to a more detailed article.
There is also the problem of radiation on the surface of Mars itself. Neither Mars’ non-existent global magnetic field nor its thin atmosphere provides any real protection from space radiation. You do have the advantage of the planet itself blocking any radiation coming from below, unlike in space where you are assaulted from all sides. One of the first tasks of astronauts once they land might be filling sandbags to provide shielding around their habitat. Surface explorations would still expose them to pretty hefty doses. Matt Damon’s Martian hiking across Mars protected only by his spacesuit would have been cooked before he ever made it back to Earth.
So who do you send that could deal with a dangerous and unprecedented lengthy mission to Mars? The proper mix of crew members and the personality types needed are probably the biggest unknowns. Apollo crew members trained together for years but only had to share cramped living quarters for two weeks at the most. (The International Space Station is far more spacious than any near-future Mars craft will be.) Some have called for sending only married couples. Anyone who thinks this is an optimum solution is probably an engineer instead of a psychologist, or has not been married for decades, whether to one person or more.
So do I think I’ll see a human land on Mars in my lifetime? I’m 66 years old and in good health. I’d say the odds were about even. I would love to see it, but won’t be surprised if I don’t. I do believe that the first humans on Mars are probably alive now. Perhaps one or more of these cuties will be the ones.