Japan鈥檚 Hayabusa spacecraft is set to perform the first of three daring rendezvous with a small asteroid on Friday, as planetary scientists puzzle over close-up images of the space rock.
The probe has been hovering above the 600-metre-long asteroid Itokawa since 12 September 2005, mapping most of its surface to a resolution of less than 1 metre. Now, the spacecraft is preparing to carry out its dramatic scientific mission, which involves swooping down to the surface three times in order to return the first asteroid samples to Earth in 2007.
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It will make the first approach 鈥� to a spot near the asteroid鈥檚 spin axis 鈥� on Friday at 0500 GMT. Considered a 鈥渞ehearsal鈥� descent, Hayabusa will test a laser range finder to judge its distance from the asteroid and drop a shiny 鈥渢arget marker鈥� to the surface that it will try to illuminate and photograph.
鈥淭he primary rehearsal goal is to see how accurately we can predict the spacecraft鈥檚 landing spot,鈥� says Don Yeomans, leader of the mission鈥檚 US science team at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. He expects the accuracy to be better than 60 metres.
Robot rover
During Friday鈥檚 run-through, the spacecraft will also release a 10-centimetre-tall robot called MINERVA. The robot carries thermometers and three cameras to take measurements from the surface, where it will gently hop around by reacting to a rotating weight within its body.
Hayabusa will then attempt its first sample collection on 12 November from an expanse of fine dust in the middle of the asteroid called MUSES-Sea. The second sampling will take place on 25 November from a broad, flat region on the asteroid鈥檚 tip dubbed the Woomera Desert.
These flat targets were chosen 鈥渁s the safest points to attempt a landing鈥�, says Yeomans. In each case, when a fabric cone protruding from the spacecraft touches the asteroid, it will fire a 5-gram pellet into the surface and attempt suck up the resulting debris.
Failed components
The landings will be controlled autonomously by the spacecraft鈥檚 software but will be even more challenging than originally expected because two of Hayabusa鈥檚 three stabilising reaction wheels have failed. But Yeomans says the one remaining wheel, along with onboard hydrazine thrusters 鈥� which will fire with as little thrust as possible to conserve fuel 鈥� can keep the spacecraft correctly oriented. 鈥淭he mission is going amazingly well so far,鈥� he says.
Asteroid researcher Bill Bottke of the Southwest Research Institute in Boulder, Colorado, US, agrees. 鈥淭he pictures are really remarkable 鈥� and very different from any asteroid images taken to date,鈥� he told New 杏吧原创.
While most asteroids appear to be covered largely by regolith 鈥� powdery material created when small meteorites crash into a rocky body 鈥� Itokawa鈥檚 surface shows only small amounts of the pulverised stuff. Instead, it appears that 鈥渓ittle blocky things鈥� make up most of the surface, says Bottke.
Mysterious surface
This, along with Hayabusa鈥檚 discovery that the asteroid鈥檚 density is less than expected, leads Bottke to believe it may be composed of smaller rocks that are held together by gravity.
鈥淭here鈥檚 always been a big debate about whether asteroids should be considered solid pieces of rock or rubble piles,鈥� says Bottke. 鈥淚f some asteroids are rubble piles and I had to guess what one might look like, I would think it looks like this.鈥�
The dearth of craters on its surface is also puzzling, he adds. 鈥淚t鈥檚 unlikely this body has completely avoided impacts in the past,鈥� he says. He speculates the asteroid may have been gravitationally deformed 鈥� and had its surface rejigged 鈥� on previous close passes by Earth.
Alternatively, the asteroid鈥檚 small size may be to blame. Its surface gravity 鈥� just 0.001% of Earth鈥檚 鈥� may allow regolith particles that become charged by the Sun to briefly lift from the surface. 鈥淚f the Sun could cause small particles to move a little bit, they might start to fill in the depressions,鈥� Bottke says.
Hayabusa is scheduled to leave Itokawa in December 2005 and should drop its sample-return capsule down to Australia in July 2007.
