A hummingbird could soon be visiting the heavens -- in a manner of speaking. NASA engineers want to build a space probe that behaves like a hummingbird approaching a flower. In other words it will use a touch-and-go landing technique to capture and analyse samples from a comet's central core for the first time.
The team, led by Glenn Carle of the NASA Ames Research Center in Moffett Field, California, is hoping to convince NASA chiefs to press ahead with its Hummingbird Comet Nucleus Analysis Mission this summer. The Ames team hopes to launch the probe sometime around 2005, to complement three other NASA comet missions.
Comets are at about 100 kelvin, and scientists believe they contain deep-frozen material from after the big bang but they would like to be certain. "What I think is the most interesting question to answer is the relationships of comets to material that ended up on the early Earth and took part in the origin of life," says Carle. "We just don't know what we started with."
The space probe would be powered by an ion engine. Once it reached the comet, it would orbit for up to a year taking samples of the dust, ice and gases in the comet's atmosphere and analysing their composition and isotope ratios. During that period, the craft would take detailed images to help the ground team choose a safe touchdown spot.
In the next phase of the mission, the craft would advance slowly toward the comet's solid core, or nucleus, stopping frequently for safety checks. But the craft would not land in the conventional sense: only two dangling tethers would make contact with the comet. One tether, equipped with temperature, hardness and contact sensors, would use electronics to sense certain conditions and trigger a new type of sampling mechanism attached to the second tether.
The sampler has two counter-rotating carbide wheels with sharpened blades that would grind up the surface of the comets and kick chunks of the material into collection funnels on the spacecraft. A prototype sampler is being built at Honeybee Robotics in New York, and will probably be finished by June.
The hummingbird sampling cycle takes less than two seconds, then gas thrusters would fire and send the craft back to analyse its samples as it orbits the comet. Ideally, the craft would repeat this hummingbird manoeuvre up to six times.
As well as taking samples at many points on a comet nucleus, the concept has several advantages. Comets have a tiny gravitational field because their nuclei are only tens of kilometres across, so a normal probe would have to latch onto the comet. The hummingbird probe's "bump sampling" gets round this problem. Cutting down the time at the surface is also safer, since it means the probe spends less time without sunlight and channels of communication.
"Almost all the questions we have from 20 years ago still exist," says William Boynton, an astronomer at the University of Arizona in Tucson. "We've flown by comets and taken a peek at them, but we really have not even scratched the surface. It sounds like this mission would scratch the surface literally as well as figuratively."
Author: Mark Schrope, Washington DC
New Scientist issue: 22nd April 2000
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