快猫短视频

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鈥檚 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 快猫短视频 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 鈥� the first ever sample from a known asteroid.

Not that there is anything unique about asteroid Itokawa 鈥� if anything, that is its appeal. 快猫短视频s 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.

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鈥檚 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 鈥淪pace-hopper鈥�). 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 鈥渕iddle to end of November鈥�, says Jun鈥檌chiro 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鈥檚 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鈥檚, 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鈥檚 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鈥檚 minerals and elements. Then they鈥檒l release the results and give fragments to other labs worldwide. NASA鈥檚 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鈥檚 NEAR Shoemaker have visited asteroids before. But none was designed to return a sample. 鈥淚n space, you really can鈥檛 do the detailed elemental composition measurements that you can do in Earth-based laboratories, where you鈥檝e got enormous, sophisticated machines to do the analysis for you,鈥� says Don Yeomans of NASA鈥檚 Jet Propulsion Laboratory in California and leader of the US science team for Hayabusa. 鈥淵ou can鈥檛 take these machines to the asteroid, they鈥檙e just too heavy, complex and expensive.鈥�

鈥淚tokawa鈥檚 gravity is so low, a frog hopping on its surface would launch itself into space鈥�

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 鈥渙rdinary chondrites鈥�, were chipped off stony asteroids like Itokawa. That鈥檚 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鈥檛 tell you precisely what its composition is like. 鈥淲e鈥檇 like to match the sample from Itokawa with certain types of meteorites in our collections,鈥� says Yeomans. 鈥淚t鈥檚 likely to be an ordinary chondrite, but we don鈥檛 know that for sure.鈥� 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.

鈥淥bservations of Hayabusa will give us vital clues about another enigma 鈥� the structure of small asteroids鈥�

It could also clarify the chemical make-up of dust and gas that built the solar system. The Earth鈥檚 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. 鈥淭hey鈥檙e likely to be the chemical mix from which the inner planets formed,鈥� says Yeomans.

Hayabusa鈥檚 observations will give vital clues about another enigma 鈥� the structure of small asteroids. NASA鈥檚 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鈥檚 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 鈥� 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 鈥� 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 C么te d鈥橝zur Observatory in France, and Makoto Yoshikawa of the Japan Aerospace Exploration Agency in Kanagawa. The asteroid鈥檚 orbit is chaotic and hard to predict. But by projecting the orbits of 39 鈥渃lones鈥� 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 鈥渁 serious possibility鈥�, 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. 鈥淚tokawa will give us the first useful information for guessing the general properties of the most populous and most dangerous asteroids,鈥� says the mission鈥檚 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鈥檚 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鈥檚 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鈥檚 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鈥檛 get here until January next year.

In the meantime Fujiwara is worrying most about Hayabusa鈥檚 sampling phase going wrong next month. Itokawa is several light minutes away from Earth, so mission controllers can鈥檛 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.

鈥淚t is a risky business, because it has to be done autonomously,鈥� agrees Yeomans. 鈥淭he 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鈥檛 control it with a joystick from the ground.鈥�

Yeomans adds that the return to Earth in 2007 will be just as tense: 鈥淭he return is always a nail-biter, because the targeting has to be precise and the chute has to open,鈥� he says. 鈥淏ut I鈥檝e been very impressed with the engineering team and I think that they鈥檒l pull this mission off.鈥�