FIRST it was 1985. That was the year Wernher von Braun, the German engineer who led the Saturn V rocket programme that helped put men on the moon, believed humans would first reach Mars. Ronald Reagan thought it would be 2010. George Bush senior plumped for 2020. Now his son is setting his sights on the Red Planet too, though wisely he has avoided naming a date. NASA might never have had the chance to send a human to Mars, but it hasn’t been short of opportunities to plan.
The vivid images of the Martian surface beamed back by NASA’s Spirit rover have been both tantalising and frustrating. How much better it would be to have astronauts reach out and touch the surroundings, say supporters of a manned mission to Mars. Will they ever have the chance?
At its closest, Mars is 150 times farther away than the moon, a distance that takes six months to travel in hugely hazardous conditions. Even if scientists can find ways to protect astronauts from the health hazards of long-term space travel, propel and land the spacecraft, and enable the travellers to survive on the surface until the Earth-Mars alignment is ripe for a return journey, there will still be the challenge of getting them home. Every phase of the journey is fraught with life-threatening risks.
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Bush’s plan relies on technologies that will be developed to put a base on the moon. This makes a lot of sense. Both a mission to Mars and a moon base will require a launch vehicle capable of putting much heavier payloads into orbit than is possible today. With fuel, food, a landing and launch vehicle, life support and the like, even conservative estimates put the total weight of a Mars mission at 150 tonnes. By contrast, the biggest lifters today can manage only 25 tonnes into low-Earth orbit, let alone towards the moon or Mars.
With its weak gravitational field, the moon will almost certainly be an easier place than Earth to start from, says Steve Braham, vice-president of the Mars Institute, an organisation that promotes plans to visit Mars. Lots of small payloads could be assembled on the moon to create one giant payload. Even better than launching from the moon, he says, would be the point known as L1 some 80 per cent of the distance from the Earth to the moon, at which the gravitational fields of the two bodies balance out. “This is a kind of saddle where Mars can be reached with the lowest energy budget,” says Braham. Fuel is crucial: every kilo that can be saved could be valuable later in the mission. Assembling a mission at L1 from small payloads might just be possible if the moon were used as a base.
Central to the plan is a new spacecraft called the crew exploration vehicle, capable of ferrying astronauts and supplies to the moon and beyond (see “In Apollo’s footsteps”). But propelling the crew and equipment onward from the moon remains one of NASA’s biggest headaches. Dangerous radiation levels in interplanetary space mean it is essential to complete any journey as quickly as possible.
The choice of methods of propulsion is limited. Conventional rockets could provide enough thrust but are inefficient and so require unfeasible fuel loads. Solar-powered ion drives, which produce continuous thrust by accelerating particles in an electric field, are slow. Which means that NASA will almost certainly turn to nuclear power. “These days we can say the n-word,” says Stanley Borowski, advanced concepts manager at NASA’s Space Transportation Project office at the Glenn Research Center in Cleveland, Ohio.
Nuclear rocket engines fall into two categories. Thermal rockets use the energy released by the fission of uranium-235 to rapidly heat hydrogen, which is then expelled through a nozzle to provide thrust. Meanwhile nuclear electric rockets use a reactor to generate electricity that powers an ion drive.
NASA has substantial experience with both types. In the 1960s and 1970s, it built and tested 20 nuclear thermal rocket engines as part of a programme called the Nuclear Engine for Robust Vehicle Applications (NERVA). The engines were intended to expand NASA’s ability to travel to the moon and beyond, but the agency never had the chance to test them in space. When the Apollo programme was cancelled in 1972, NERVA too was quietly abandoned.
More recently, in the early 1990s, NASA and the Department of Defense jointly built and tested nuclear electric engines at Los Alamos National Laboratory in New Mexico. Borowski says this and other experiences with nuclear reactors could be an important factor in deciding whether to embark on a similar design. “We have decades of experience managing reactors safely in ships and submarines. We know how to do it.”
In 2001, he and colleagues published a detailed plan that could send a six-person crew to Mars loaded with about 80 tonnes of hydrogen fuel, heated by nuclear thermal rockets to generate both thrust and power for the journey. And in February 2003, NASA unveiled Project Prometheus, a $3 billion effort to develop nuclear propulsion for missions to the outer planets. This project will now be taken under the wing of the Office of Exploration Systems, set up after Bush’s announcement to develop technologies for the proposed missions.
But building and testing these engines on Earth is not easy because of the safety precautions that have to be taken in case of radiation leaks. And launching a reactor on top of an all-too-fallible rocket is bound to be controversial.
Whatever the means of propulsion, the radiation that pervades space will be another hazard for anyone on a journey to Mars (see “Medicine’s final frontier”). Borowski reckons that during a year in space – the approximate time of a return trip to Mars – an astronaut in a shielded spacecraft might receive a radiation dose of 500 millisieverts, of which 50 millisieverts would come from the reactor. On Earth, the average annual background dose is just 3 millisieverts.
One of the toughest challenges will be finding a way to shield astronauts. A dense shielding material such as tungsten cannot be used in large quantities because of its weight. Each kilogram of payload costs as much as $10,000 to launch into Earth orbit. For many years, NASA has looked for alternatives. Materials rich in hydrogen look the most promising. Hydrogen atoms absorb impacts from high-energy particles by jumping to higher energy levels rather than splitting into smaller particles. By contrast aluminium, commonly used in aerospace alloys, splits to form harmful by-products and is thus likely to be used sparingly in interplanetary spacecraft.
Spacecraft designers don’t have to look far for hydrogen-rich materials: the hydrogen fuel and the water supply could both be used to protect the crew. NASA is also studying hydrogen-rich polymers such as polyethylene as potential construction materials to replace aluminium.
Humans are not the only radiation-sensitive components of space missions. Computers are vulnerable too. “Our experience on the space station is that the computers are constantly affected by radiation,” Braham says. These are mostly off-the-shelf computers, but the US military has expertise in developing “hardened” computers that are immune to most radiation strikes. One of the features of Bush’s plan is an increase in cooperation between civilian and military space programmes, and this is one area where this may be vital. “The importance of reliability cannot be overstated,” Braham says.
NASA and its European counterpart ESA have both lost vehicles in attempts to land on Mars. That’s why the first astronauts to visit the Red Planet are unlikely to try it. Like the Apollo 8 mission, which orbited the moon and returned to Earth in a dry run for Apollo 11, the first human trip will almost certainly orbit Mars and then return to Earth. “That makes sense,” Braham says. “It tests the spacecraft systems and gives them the opportunity to get back if anything goes wrong.”
When engineers are satisfied that the spacecraft’s systems are reliable, they will give the go-ahead for a landing. Borowski’s plan is to send an unpiloted craft ahead of the manned mission to set up a base on the surface and begin the lengthy process of scouring the Martian atmosphere for the raw materials that will be needed to make fuel for the return journey. The craft would carry several tonnes of liquid hydrogen that, when combined with carbon dioxide from the Martian atmosphere, would make enough methane and oxygen to launch the astronauts into orbit around Mars for the return journey. The crew need not set out for Mars until the advanced mission has landed safely and the fuel for the return journey is ready.
Once on the surface of Mars, there is no chance of return for the astronauts until the Earth and Mars are again favourably aligned – which occurs only every 500 days. Equipped with skis and Skidoos for travelling across the Martian terrain, the astronauts would be able to explore an area within about 500 kilometres of the landing site. Then, if all has gone to plan, they will return to Earth, arriving almost three years after they left.
But what of NASA’s international partners? The US is unlikely to be able to undertake such a mission alone. It may find a willing partner in ESA which already has a plan called Aurora to put humans on Mars by 2030. Franco Ongaro, who heads the programme, says ESA has had discussions about the mission with the US, Russia, Japan and China over the past two years. “We take it for granted that a mission to Mars would require international cooperation,” Ongaro says.
What might make cooperation easier is that some of NASA’s plans bear a striking resemblance to Aurora. For example, ESA emphasises that human and robotic exploration will have to develop hand in hand. Till now, NASA has kept its human and robotic exploration programmes separate, but Bush called for this to change. For instance, the Mars landing site would be thoroughly explored by robots before a human landing party even sets out for the planet, and human trips over the Martian surface would always be explored in advance by a robotic rover to check for pitfalls.
NASA already has ambitious projects in the pipeline to explore the Red Planet from afar. Next year it will launch the Mars Reconnaissance Orbiter, which will carry the most powerful camera system yet to journey to another planet. It will be used to scour possible landing sites for obstacles that could damage future landers. In 2007, NASA plans to send the first in a series of “scouts”, small rovers, planes or balloons that will drive, fly or float around Mars. And in 2009, it will send the first of a new generation of smart landers capable of avoiding hazards during touchdown, which will allow NASA to explore otherwise hard-to-reach sites. Beyond that there are no firm plans, although this is likely to change given Bush’s sudden new interest.
So is the US about to take a bigger step, perhaps even a giant leap, towards Mars? NASA and its potential partners are all too aware that previous plans for manned missions to Mars have evaporated as quickly as carbon dioxide frost in a Martian summer.
Ongaro is cautious about the prospects. “I’ll get excited if the interest in Mars is still there after the US presidential elections in November,” Ongaro says. The winner of that election, whoever it is, could make all the difference.
Medicine’s final frontier
Long space missions will expose astronauts to health risks on a scale that no human has ever endured. NASA has identified no less than 55 potential problems, some worse than others. It now intends to direct all its research efforts on the International Space Station to investigating the health impacts of space travel and ways to minimise them.
One major focus is certain to be the effect of cosmic rays – high-energy particles from the sun and outer space. Earth’s magnetic field normally shields us from most of them, and offers some protection on the space station, but astronauts staying on the moon or on the long trip to Mars would be exposed to a high cumulative dose.
So far, attention has focused on the damaging effects of these rays on DNA as a possible trigger for cancer. But new research has revealed another possibility: cosmic rays could cause brain damage. The brain is particularly vulnerable, owing to its extremely complex and dense structure, and limited ability to repair itself. As well as directly injuring cells, cosmic rays could interfere with their ability to generate the structural and chemical changes that underpin learning and memory.
Researchers at the Brookhaven National Laboratory in New York are trying to establish how much of a problem this will be. They are using an accelerator to generate particles like those of cosmic rays, and testing the effects on mice and on human neurons grown in culture. Both the mice and the neurons are showing signs of damage. “It’s too early to say what exactly it means for humans,” says radiobiologist Marcelo Vazquez at Brookhaven.
The other major risk comes from chronic exposure to reduced gravity. Bones rely on bearing physical loads to maintain their strength and soon thin in low gravity. So astronauts would return to Earth crippled by osteoporosis unless they took steps to prevent it. Other tissues are also affected: muscles shrink and blood pressure falls, which can weaken the heart; and the constant leaching of calcium from the skeleton can lead to kidney stones.
To prevent these problems, the current generation of astronauts must exercise every day. But the regimes they follow, such as running on a treadmill while strapped down by bungee cords, are awkward and uncomfortable, so researchers are developing alternatives.
One approach is to encase the lower body in a chamber with a partial vacuum, sucking the astronaut against the floor to create an artificial body weight while exercising. Marco Cardinale, an adviser to the European Space Agency is investigating another: applying vibrations to the body. This seems to benefit bone, muscle and hormone function, he says.
Physical health is not the only thing affected by space travel. Spending long periods in cramped quarters can result in psychological problems. The happiness of a crew can make or break a mission: in 1982 two Russian cosmonauts spent a seven-month flight on Salyut 7 in near silence because they couldn’t stand each other. Clare Wilson
