FOR millions of space rocks endlessly circling the sun, life is distinctly dull. Apart from the occasional jolt when a meteor hits, little has disturbed many of these asteroids since the solar system formed more than 4 billion years ago. But one unsuspecting rock is in for a big surprise.
It’s a tiny world roughly half a kilometre wide, and goes by the name of 25143 Itokawa. For the past five weeks, a pioneering Japanese spacecraft called Hayabusa, aka peregrine falcon, has had it under close surveillance. As New Scientist went to press, Hayabusa was preparing, despite some serious problems with its guidance system, to perform a daring set of manoeuvres. If the mission team can keep everything on track, the spacecraft will drop a space-hopper onto the asteroid and fire bullets at it to break up the surface. As well as beaming back detailed snapshots, Hayabusa will bring some of the surface grit back to Earth – the first ever sample from a known asteroid.
Not that there is anything unique about asteroid Itokawa – if anything, that is its appeal. Scientists are keen to learn more about common, small asteroids, the rocky debris that dominates the space between Mars and Jupiter. This interplanetary rubble is just the stuff to help clarify how the solar system formed. It could also help us come up with an emergency plan to deal with any menacing asteroid on collision course with the Earth.
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Like most asteroids, Itokawa is no oil painting. Images from Hayabusa confirm that it is extremely lumpy and asymmetric. Its longest axis measures about 600 metres. Radar observations in 2001 revealed the northern side of the asteroid to be a smooth ellipsoid that was devoid of any particular features or mountains. But further radar observations of the southern side in 2004 showed a much more rugged landscape, with dark, concave pits.
In September, Hayabusa entered an almost identical orbit to Itokawa. It initially surveyed the asteroid at a distance of 20 kilometres, then closed in until it was only about 7 kilometres away. Its instruments are mapping the asteroid as it rotates using an optical camera, a near-infrared spectrometer and an X-ray spectrometer. They will take images and measure the asteroid’s most common minerals and chemical constituents.
But the main event is scheduled for next month when mission controllers hope to begin the sampling manoeuvres. Hayabusa will drop a space-hopper onto the surface to gambol around taking snapshots and beam them back to Earth (see “Space-hopper”). It will also fire bullets at the asteroid to break the surface, then transport some of the grit back to Earth. This will now take place in the “middle to end of November”, says Jun’ichiro Kawaguchi, the mission manager. Although Hayabusa has lost two out of its three guidance wheels, the team is confident it can still do everything it needs to by adjusting the craft’s ion thruster, a control strategy that will be tested later this month.
Touch-and-go
It will be a nail-biting operation, and not just because of the guidance problems. Itokawa is so small that its surface gravity is less than 0.001 per cent of Earth’s, so anything jumping faster than about 20 centimetres per second will escape its pull. On Itokawa, even a hopping frog would launch itself into space.
This is problematic because any spacecraft attempting to drill into the asteroid would simply push itself away. Not only that but no one knows if the surface is rigid rock or fluffy soil. Instead the spacecraft will break the surface by firing a bullet at the asteroid and capturing the ejecta.
It will be touch-and-go, literally. Hayabusa will gently approach the asteroid until an onboard sampling cone touches the surface. That will trigger a gun to fire a 5-gram pellet into the asteroid at 300 metres per second. A sample catcher will trap the grit flying off the surface, then seal the 1-gram sample inside a capsule (see Diagram).
After collecting up to three surface samples, the spacecraft will pack its bags in early December and set off for Earth. In July 2007, it will drop the capsule into the Earth’s atmosphere, setting it on course for a soft landing by parachute in the Australian outback. Back in the lab at the Institute of Space and Astronautical Science (ISAS) in Sagamihara, near Tokyo, scientists will measure the sample’s minerals and elements. Then they’ll release the results and give fragments to other labs worldwide. NASA’s Johnson Space Center in Houston will receive a one-tenth share.
The sample should give scientists a wealth of information about Itokawa and its kin. Spacecraft such as NASA’s NEAR Shoemaker have visited asteroids before. But none was designed to return a sample. “In space, you really can’t do the detailed elemental composition measurements that you can do in Earth-based laboratories, where you’ve got enormous, sophisticated machines to do the analysis for you,” says Don Yeomans of NASA’s Jet Propulsion Laboratory in California and leader of the US science team for Hayabusa. “You can’t take these machines to the asteroid, they’re just too heavy, complex and expensive.”
“Itokawa’s gravity is so low, a frog hopping on its surface would launch itself into space”
With a chunk of Itokawa safely back in the lab, scientists will be able measure the ingredients of an asteroid accurately for the first time. They hope to match the composition to that of some meteorites that have landed on Earth. Tens of thousands of meteorites sit in museums round the world, and scientists assume that the most common ones, known as “ordinary chondrites”, were chipped off stony asteroids like Itokawa. That’s because the composition of these meteorites is similar to what scientists think stony asteroids are like from looking at the sunlight they reflect.
However, that link is tentative. Reflected sunlight only comes from a wafer-thin outer layer of an asteroid, and doesn’t tell you precisely what its composition is like. “We’d like to match the sample from Itokawa with certain types of meteorites in our collections,” says Yeomans. “It’s likely to be an ordinary chondrite, but we don’t know that for sure.” If the sample from Itokawa confirms the link once and for all, it should be possible to start building a map of the distribution of elements and compounds throughout the asteroid-packed regions of the solar system.
“Observations of Hayabusa will give us vital clues about another enigma – the structure of small asteroids”
It could also clarify the chemical make-up of dust and gas that built the solar system. The Earth’s composition tells us little about the nature of the building blocks of the solar system, because its gravity and internal heat drive restless change through geological processes like volcanism and plate tectonics. But the chemical make-up of small asteroids has hardly changed at all since the solar system formed 4.5 billion years ago. “They’re likely to be the chemical mix from which the inner planets formed,” says Yeomans.
Hayabusa’s observations will give vital clues about another enigma – the structure of small asteroids. NASA’s NEAR Shoemaker spacecraft revealed that large asteroids have diverse structures. The 33-kilometre-long asteroid Eros turned out to be a solid rock with a similar density to the Earth’s crust, while another called Mathilde has a low, uneven density, not much greater than that of water. This hints that Mathilde is a bit like a giant beanbag – a pile of rubble held loosely together by its own gravity.
However, we know almost nothing about the structures of small asteroids like Itokawa. And it is vital that we find out, probably sooner rather than later, because small asteroids hit the Earth fairly regularly, with those 50 or 60 metres across striking once every couple of centuries. If one that size hits land, it would devastate an area of up to 3000 square kilometres – an area the size of a large city.
Itokawa itself might one day smash into the Earth, judging by the computer models run by Patrick Michel of the Côte d’Azur Observatory in France, and Makoto Yoshikawa of the Japan Aerospace Exploration Agency in Kanagawa. The asteroid’s orbit is chaotic and hard to predict. But by projecting the orbits of 39 “clones” of the asteroid 100 million years into the future, Michel and Yoshikawa found that four of them crashed into the Earth, all within the next 13 million years. That means an impact within a million years or so should be considered “a serious possibility”, they will report in the journal Icarus.
And you need to know the anatomy of a menacing asteroid before you can knock it off course with a missile. Simulations suggest an asteroid made of a single solid rock would be deflected most easily, while a rubble pile would tend to absorb the impact energy and would be hard to shift. “Itokawa will give us the first useful information for guessing the general properties of the most populous and most dangerous asteroids,” says the mission’s head scientist, Akira Fujiwara at ISAS.
Not that the mission is by any means done and dusted. In fact Hayabusa has some difficult days ahead. Fujiwara says a catalogue of failed missions designed to return samples shows just how difficult the task is. Between 1969 and 1976, the Soviet Union tried to get rocks back from the moon 11 times with unmanned craft. Only three attempts succeeded. Three spacecraft either crashed or failed on the moon’s surface (two of them, rather appropriately, in the Sea of Crises) while five failed to reach the moon at all.
Since then, no one has even tried to get a sample back from the surface of a planet or moon. Landing a sample safe and sound is no mean feat, as NASA’s Genesis mission proved last year. Genesis soaked up solar particles for more than two years before returning them to Earth in a canister. But then disaster struck: the capsule’s parachute failed to open and the delicate samples smashed on the ground. Another NASA mission called Stardust is bringing a smidgen of comet dust back to Earth, but the sample won’t get here until January next year.
In the meantime Fujiwara is worrying most about Hayabusa’s sampling phase going wrong next month. Itokawa is several light minutes away from Earth, so mission controllers can’t control the spacecraft in real time by sending radio signals. Instead designers have equipped the craft with intelligent software that allows it to figure out each move on its own.
“It is a risky business, because it has to be done autonomously,” agrees Yeomans. “The spacecraft has to sense the surface, fire the pellet, collect the sample then go back to the home position all on its own. You can’t control it with a joystick from the ground.”
Yeomans adds that the return to Earth in 2007 will be just as tense: “The return is always a nail-biter, because the targeting has to be precise and the chute has to open,” he says. “But I’ve been very impressed with the engineering team and I think that they’ll pull this mission off.”
Space-hopper
As well as sampling the surface, Hayabusa will drop a robot called Minerva onto Itokawa’s surface. Minerva is an octagonal cylinder just 10 centimetres high and 12 centimetres across – smaller and lighter than a bag of sugar. It will be the first ever robot to explore an asteroid. Designing it has required some ingenious thinking, because Itokawa’s surface gravity is puny, less than 0.001 per cent of Earth’s.
Minerva can’t have wheels because they wouldn’t grip the surface tightly enough to move around in such low gravity, especially if the terrain proves troublesome. “We don’t know the roughness of the surface,” says the mission’s head scientist, Akira Fujiwara of the Institute of Space and Astronautical Science in Sagamihara, near Tokyo. Early images from Hayabusa suggest Minerva might have to contend with some loose soil.
To propel Minerva, a weight inside the robot rotates on the edge of a turntable, powered by solar panels on the robot’s surface. The reaction to the weight rising and turning will be enough to send Minerva hopping gently across the surface at up to 9 centimetres per second for a couple of days. It carries three cameras, two of which will take stereo images of the surface right next to it, while a third will snap pictures of the more distant landscape. It also has thermometers to measure the surface temperature.
