The biggest surprise is that the 53-kilometre asteroid, which orbits between
Mars and Jupiter, has a giant crater about 25 kilometres across. “It’s all
crater—there’s hardly any asteroid,” says Joseph Veverka of Cornell
University in Ithaca, New York.

Another shock is that the rocky asteroid has a mass of just one-millionth
that of the Moon, about half what scientists expected from studies of other
asteroids. This implies that the density is only around 1.3 grams per cubic
centimetre, not much more than that of ice.

“If this thing was any lighter, it would float,” says Don Yeomans of the Jet
Propulsion Laboratory in Pasadena, California, leader of NEAR’s radio science
team. He says it is possible that instead of being a solid chunk of rock,
Mathilde is like a pile of rubble containing gaps.

ĐÓ°ÉÔ­´´s knew that the surface of Mathilde is very carbon-rich and dark,
reflecting only about 3 per cent of the Sun’s light. NEAR images show that the
deep craters are just as black, hinting that the composition of the asteroid is
the same throughout. Veverka says this suggests that carbon-rich asteroids are
unchanging worlds that preserve pristine samples of the material that formed the
large planets.

NEAR’s next encounter will be with the Earth in January 1998. The Earth’s
gravity will deflect the spacecraft towards its main target, the asteroid Eros,
where it will arrive in January 1999.

In February, astronauts fitted Hubble with the Near Infrared Camera Multi-Object Spectrometer (NICMOS), which contains three cameras sensitive to infrared wavelengths. To detect the emissions of distant galaxies, the cameras must be cooled to 58 kelvin. This is done by mounting them inside a vacuum flask containing 100 kilograms of frozen nitrogen.

To minimise thermal contact with the rest of the spacecraft, the vacuum flask is suspended on straps made from glass fibre and epoxy resin. But shortly after NICMOS began operating, ground controllers noticed that heat was leaking into the vacuum flask. This is making the nitrogen ice boil away faster than expected, which will cut NICMOS’s life from five years to less than three. The nitrogen has also expanded with the heat, throwing the most important of the three NICMOS cameras-the one that allows it to take spectra of many objects simultaneously-out of focus (This Week, 5 April, p 7).

NASA’s salvage plan to fix NICMOS takes advantage of a coolant loop that was filled with helium and attached to a mechanical refrigerator to freeze the nitrogen before launch. The idea is to bolt a similar refrigerator to the coolant loop, which will use neon at around 50 kelvin as its working gas. Though the nitrogen will have boiled away by 1999, the neon in the cooling loop will absorb heat from the vacuum flask and pump it into a radiator that will dump it in space.

Frank Cepollina of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who manages the Hubble telescope’s Flight Systems and Servicing Project, says that the technology to build the refrigerator is still being developed. “But we believe we can make that leap and install it in 1999.” Engineers hope to fly a prototype system on a space shuttle flight in 1998. If the refrigerator is successful, it will allow NICMOS to operate indefinitely.