52 | NewScientist | 26 November 2011

Landing on an asteroid could be our biggest space challenge yet, says Stephen Battersby

Giant leap, tiny rock

ON BOARD the spacecraft Little Prince, three intrepid astronauts are preparing to set foot on a new world. They are the first humans ever to have travelled this far from Earth, and their home planet has shrunk to a pinprick of faintly bluish light behind them. By contrast, their goal looms in the viewports ahead, flooding the cabin with dazzling light reflected from a wild landscape of canyons, deserts and hills. This great conquest stretches almost 60 metres across.

Since NASA’s Mars programme was cancelled last year, the agency’s human space flight efforts have been directed at

sending explorers to a passing asteroid. Such a target, announced by President Obama in April 2010, may be small in size, but the rewards could be great. Asteroid-nauts will try to discover what these planetary scraps are made of, whether they helped to start life on Earth and how they can be prevented from smashing into our delicate planet. Fortunately, the objects that present the biggest hazard are also easiest to reach, says Paul Abell, who studies near-Earth objects from NASA’s Johnson Space Center in Houston, Texas. “They can come and get us, but we can get to them too.”

Setting up camp on a planetoid smaller than a football stadium, with almost zero gravity, will not be easy. Not only might the little world be spinning rapidly, it may not even be solid. Instead it could be a loose ball of dust with a surface more fragile than any earthly object.

The first steps towards this distant goal have already been taken underwater. In April, at the Aquarius laboratory on the seabed off Key

Largo in the Florida Keys, NASA engineers prepared an asteroid mock-up with rock walls, rubble and sandy beds to mimic the different surfaces that explorers might encounter.

Then in October, in ersatz weightlessness 19 metres under the waves, aquanauts tested all sorts of methods to move around these fake terrains and take rock samples. “There’s a real challenge in how to get from point A to point B. You’re in zero gravity, there’s no ground force, so you can’t walk,” says Bill Todd of the Johnson Space Center, who ran the tests.

Drifting off into deep space is only the most obvious hazard. Others include small satellite rocks, which might be lurking nearby, waiting to smash into the spacecraft. And any fine dust on the surface would be easily disturbed, puffing up into a cloud that could last days, obscuring vision and penetrating the joints of

spacesuits and other machinery to gum up the works. “This is more complex and difficult than operating on the moon,” says Todd.

Yet in one respect the mission should be easier. When visiting a world with substantial surface gravity such as the moon or Mars, you must burn lots of fuel to slow your descent and avoid hitting the surface faster than a rifle bullet. Then you must burn more to take off again and escape. Every drop of the stuff >

” You don’t want to go to something smaller than your spacecraft. That would be embarrassing”

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“space weathering” – the relentless rain of cosmic rays and micrometeoroids.

Only by visiting a few near-Earth asteroids and grabbing samples can we be sure what those spectra mean. That should allow us to reinterpret observations of more distant asteroids, most of which inhabit a belt between Mars and Jupiter. “It would be like making a geological map of the solar system,” says Abell – a map that would help test models of how the planets formed.

Earth-bound asteroids may have brought in a supply of organic chemicals that eventually helped life to begin. The Murchison meteorite, which fell in Australia in 1969, holds thousands of complex organic compounds including amino acids. To see what else might

be out there, Abell is keen to visit one of the “C-type” asteroids, known from their spectra to be rich in organics. “It could help us understand the role asteroids played in the emergence of life on Earth,” he says.

There is even a slim chance that our ancestors were immigrant microbes, perhaps from Mars, that travelled here in a dormant state on wandering rocks. We are unlikely to find dried-out alien bugs on a near-Earth asteroid, but it is not out of the question.

As asteroids giveth, so they taketh away. Their explosive impacts have scarred our planet many times – most recently in 1908 when an object exploded above the Tunguska river in Siberia, flattening 2000 square kilometres of forest. Although the chance of a big impact in your lifetime is slim, the consequences would be dire. An asteroid 1 kilometre across could devastate a continent and would probably put an end to civilisation. A more common 100-metre cannonball could flatten a large city or even a small country. We need not be defenceless, however. With time, money and data on asteroid structure gathered from sample-return missions, it should be possible to devise a scheme to protect our planet (see “Deflect and Survive”, left).

Before we set off to explore one of these little worlds, astronomers must decide which one. For easy access, the asteroid should be on an orbit very similar to that of Earth, so it comes close and moves past slowly. But that makes the thing difficult to spot from the ground, because it will be in the daytime sky most of the time, only emerging clearly as we overtake it or it overtakes us. By the time we spot such an asteroid, it will be too late to launch a mission – we would have to wait decades for it to pass by again. So a new space-based telescope is vital to track down these elusive Earth-shadows.

Miniature conquestAlan Harris of the Space Science Institute in Boulder, Colorado has calculated how many near-Earth objects of different sizes are likely to be in such convenient orbits, based on the rate at which we are discovering them. Small asteroids are much more common than big ones, so we will probably have to settle for a miniature conquest. “It is unlikely we will find anything much larger than 100 metres in diameter that is easy to get to,” says Harris.

Most of these small asteroids are super-fast rotators –their equators move faster than the velocity of an object in orbit around them. Place a stone on the surface of one of these

We could visit small asteroids such as Itokawa – if they are not spinning rapidly

Schemes for deflecting an asteroid come with varying degrees of hare-braininess. Blowing the thing apart with nuclear warheads is an option but only a last-ditch one, as the shrapnel could prove as dangerous as the intact object. Nukes could be deployed more carefully, exploding far from the object so the flash of heat will evaporate some of its surface, generating a modest shove. Or we could fix rocket engines to the surface, turning the asteroid into a ponderous spacecraft. The

gentlest method would be to use a “gravity tractor”, stationing a heavy spacecraft (or rocket-powered rock) near the looming asteroid. The tractor’s mass will tug the asteroid aside, but this would be a slow process, only effective if we had many years’ warning.

While solid rocks will cope with violent remedies, dustballs might need more careful handling. So before we choose our weapon, we will need to find out a lot more about the structure of asteroids. As well as

gathering samples to test the strength of asteroid rocks, astronauts could plant seismometers and use radar to scan the innards of an asteroid.

Not everyone agrees that this will strengthen our defences. “The ‘one with your name on it’ may be utterly different from the ones you visit,” says Alan Harris of the Space Science Institute in Boulder, Colorado. “As a scientist I want to learn about asteroids, but I don’t think it has a lot to do with the impact hazard.”

DEFLECT AND SurvIvE

” You don’t want to go to something smaller than your spacecraft. That would be embarrassing.”

must be hauled from Earth at enormous expense. Head instead for a small asteroid, where the surface gravity may be one-millionth of Earth’s, and you can coast relatively slowly to your destination, landing gently. And when the mission is over, lift-off takes almost no effort. “It’ll be cheaper than a Mars mission for certain,” says Abell.

That is the main reason why asteroids are now on the agenda, at a time of tight budgets. But there is more to it than money. “They hold the most unaltered ancient materials,” says Abell. “Learning about them tells us about the state of the early solar system.”

We already have a rough idea of their make-up from studying meteorites – bits of asteroid that have fallen to Earth – and from analysing the spectrum of light reflected by different types of asteroid. A few are metallic, forged inside ancient protoplanets that were smashed to pieces long ago, their molten metal cores scattered and frozen into slabs of iron. Some are stony, made of silicate minerals similar to Earth’s rocks. Finally, the most common and primitive meteorites are rich in carbon and complex organic chemicals. Both the stony and carbonaceous types are often “rubble piles”, loose aggregates that have been smashed apart and re-formed repeatedly.

Yet there are some discrepancies between spectral data and the meteorites we find. “We have meteorites that don’t seem to match anything we see in the sky,” says Abell. Conversely, there are asteroids with ambiguous spectral signatures that don’t clearly fit known meteorites. It may be that the surface substances have been mutated by

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whirling worldlets and it will just lift off and fly away, so tethering people and vehicles will be extra difficult. Choosing a slow rotator will reduce the choice of target further, perhaps meaning we have to aim well below 100 metres on our first outing. “You don’t want to go to something smaller than your spacecraft,” says Abell. “That would be embarrassing.”

Abell would be happy with anything more than 50 metres across. “It would be a meaningful target, with lots to explore. That is also around the size that things become a hazard for Earth.”

Even with such a careful choice of target, astronauts will face a long, perilous journey. The asteroid will probably be several million kilometres away, much further than the moon, which means astronauts will be exposed to the harsh radiation of deep space for three to six months. Background levels will be enough to raise their cancer risk by about 1 per cent over a six-month voyage. More disturbing is the prospect that a large solar flare could send a blast of radiation their way. So their spacecraft will need a well-shielded “storm shelter”

where the crew can huddle if a flare goes off. Asteroid-nauts will also be far more isolated

than their moon-bound predecessors, as signals from Earth will be slow to arrive. “We are simulating operations with a 50-second delay,” says Abell. That makes ordinary conversations impossible. In any case, if you are suddenly cast adrift 7.5 million kilometres from Earth, there may be little point telling Houston you have a problem: at that distance there’s not much chance of rescue.

A robot probe might seem more suited to the rigours of deep space. Yet humans make more effective explorers, and even if machines could perform the science as well as trained astronauts, says Abell, there is another motivation for sending people. “Our long-term goal is to try to expand humanity into the solar system,” he says. Asteroid missions could extend our ability to make long voyages.

Asteroids might even serve as literal stepping stones in space. Locked within them are precious metals and water, which could be extracted by grinding the rocks and heating the powder. Water is vital for life support and

can also be turned into fuel using electricity from solar panels to split it into hydrogen and oxygen. Topping up with fuel en route would make an interplanetary voyage more feasible.

But before they go hunting for water in space, NASA personnel are diving in the Caribbean. At the Aquarius lab, planetary scientist Steven Squyres of Cornell University in Ithaca, New York, joined three astronauts in a serious game of make-believe. Despite having to evacuate five days into the planned 13-day mission as hurricane Rina approached, they still had time to do most of the experiments, and have some fun.

“We had back-mounted thruster packs, which are wonderful for travelling long distances,” says Squyres. “But they are not good for stabilisation. When you hit a rock with a hammer, you go flying.” Fixed cables proved to be less useful than the team had hoped. “You get a bit of tension, but not enough to make you really stable,” says Squyres. Instead, astronauts might tie themselves to the end of a telescopic boom, as the Aquarius team tried underwater. “Being moved was very slow and tedious, but once you got where you wanted to be it was fantastic for stabilisation.” They will need a combination of these techniques, he says.

Meanwhile space hardware is already in the works. NASA is designing a new heavy launch rocket based on space-shuttle technology, and is building the spacecraft: an Apollo-esque capsule called the Multi-Purpose Crew Vehicle (MPCV), with solar panels and a sturdy heat shield to withstand high-speed re-entry from deep space. It might be augmented with an inflatable living area for the voyage, and even small craft, called space exploration vehicles, for the final approach to an asteroid.

In a time of tightening budgets, will this adventure actually be bankrolled? “I’m an optimist,” says Abell. “It is such a good idea for humanity to try to do this.” If his optimism proves correct, then the first asteroid explorers might set off sometime after 2020.

After months stuck in a metal tube, the astronauts of MPCV Little Prince survey their new kingdom. The peanut-shaped planetoid known as 2019 XS52 has two little pools of dust and a weird greenish boulder at one end. There to greet the explorers is the probe Iron Chicken, which scouted the new world four years before. The crew ready their grappling hooks, for now comes the hard part: touching down on a world where down has no meaning. n

Stephen Battersby is a consultant for New Scientist based in London

It’s a small worldThe target for any future human asteroid mission is likely to be far smaller than even the diminutive Itokawa

IDA

53.6 x 24 x 15.2 kmVisited by NASA’s Galileo

1993

ŠTEINS 6.67 x 5.81 x 4.47 km

Visited by The European Space Agency’s Rosetta

2008

EROS

34.4 x 11.2 x 11.2 kmVisited by NASA’s NEAR-Shoemaker

2000

ITOKAWA 0.5 x 0.3 x 0.2 km

Visited by Japan’s Hayabusa 2005

MATHILDE

66 x 48 x 46 kmVisited by NASA’s NEAR Shoemaker

1997

LUTETIA121 x 101 x 75 km

Visited by ESA’s Rosetta 2010

VESTA 530 km diameter

Currently being studied by NASA’s Dawn probe

2011

CERES950km diameter

The largest asteroid known, Ceres orbits in the main asteriod belt and will be

visited by NASA’s Dawn probe

2015

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NAS

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