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Science: Tailing a Comet

3 minute read

A rendezvous in space

When Halley’s Comet last streaked across the skies in 1910, it was for many an unwelcome visitor. Fearful that the earth would be enveloped by deadly gases in its glowing tail, people bought comet pills to ward off its effects, and held end-of-the-world gatherings. In 1985, when the comet returns—as it does every three-quarters of a century —it should get a friendlier reception. In fact, NASA is planning a scientific welcoming party in space.

Last week the space agency announced the award of $1.15 million contracts to Boeing and Lockheed for preliminary studies of a new rocket. Its purpose: to power an unmanned spacecraft that will intercept Halley’s Comet as it sweeps around the sun. Known as the solar electric propulsion system, the engine could become the workhorse of deep space, carrying probes on far-flung missions across the solar system.

Until now all space probes have been powered entirely by chemical rockets. Though they can develop enormous thrust, they are voracious consumers of fuel. In only nine minutes, the Saturn 5 moon rockets burned up 3,000 tons of liquid fuel. With such propellants, even larger rockets and exorbitant amounts of fuel would be needed to rendezvous with fast-moving objects like comets, which travel at 198,000 km (124,000 miles) per hour in the vicinity of the sun.

The ion engine is far more economical. With electricity generated by solar panels, it strips electrons off the atoms of vaporized mercury passing through a coffee-can—like chamber, converting them to ions. Expelled at high speeds in a focused beam, the charged particles act like a rocket exhaust, propelling the craft forward. Though its thrust is minuscule and far too feeble to lift payloads from the earth, the ion engine performs efficiently in the vacuum of space. It can function for years because it draws on solar energy and uses fuel sparingly. It can be stopped and restarted countless times and accelerate spacecraft to extremely high speeds.

If Congress provides the needed funding, the probe will be carried into earth orbit by the space shuttle in the summer of 1985. Boosted by a conventional rocket, it will fly off toward the comet, gradually accelerated by its cluster of six or eight small ion engines, during the four-month journey. On command from earth, it will drop a small instrument-packed probe provided by the European Space Agency directly into the comet’s head, which scientists believe is made up of icy debris and a smattering of organic molecules. Because comets have probably changed little since they were formed, data from the probe may reveal much about the early days of the solar system. Three years later, while swinging around the sun, the mother ship will rendezvous with a second comet called Tempel 2 and follow it for a year. During that time, it will continually observe all the changes the comet undergoes as it makes its fiery hairpin turn around the sun and heads off into space again. Then the craft will maneuver toward Tempers head and perhaps give it a parting nudge to see how solid it is.

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