Comets, and Meteors /lIB lions of miles long, it contains less matter than the best vacuum that can be artificially made on Earth. The tail is longest when the comet is near the sun and quickly shrinks as the comet moves away from the sun. Up to 6 m of ice and dust can vaporize from the surface of a comet nucleus with each pass of the sun. Eventually, the surface of the nucleus cools below the freezing point of the ices, and the tail disappears until the next return to the sun. One comet that astronomers have studied closely is comet Halley. Although its 1986 return 6-10 The parts of a comet showing the two types of was disappointing for many because it was more difficult to see than tails it can develop expected, Halley's comet did provide a wealth of information when space probes passed near it (see the Facet, below). Halley's nucleus

In March of 1986, Halley's comet, long, delicate booms used in other probes which passes the earth once every to detect plasma waves and electric seventy-six years, provided the world's fields. Instead, its sensors studied the young space program with its first plasma's composition, energy, and con• opportunity to study a comet up close. centration. The leading nations of the world planned The Vega and Giotto probes provided a years ahead to send a small fleet of wealth of information about comet Hal• space probes t6 unlock its secrets. ley's core. Dramatic pictures unveiled a In the 1970s the United States pro• peanut-shaped black core about 15 km posed a bold plan for a probe to ren• (9 mi) long~ and 10 km (6 mi) wide. dezvous with the comet, to stay with it, Expecting it to be like a hard snowball, and to make continuous observations. Sakigake and Suisei were very similar. scientists instead found a fluffy mass But such a probe would need a new "ion with a density as little as one-tenth that comet. As they entered the coma, the drive" propulsion system, and the funds of frozen water. The nucleus surface was abrasive dust damaged several instru• were not granted in time. So, U.S. scien• cold, ranging from -27 to -127 O( (-17 ments, but the magnetometer on Vega 1 tists did the best they could-they to -197 OF). Ices could be seen vaporiz• and a plasma-wave sensor on Vega 2 sur• organized the International Halley Watch ing out of cracks in the hard, protective vived. to coordinate earth-based scientific ~rust. Many gases appeared in the coma, Two probes from Japan, Sakigake and studies. caused by complex processes that sci• Suisei, did not come very close (within Three other nations proposed to send entists cannot fully explain. The comet 150,000 km [93,000 miD. Suisei focused probes on quick flyby missions, each dust proved to have silicates, carbon, on the solar wind in front of the comet; with a slightly different but overlapping and organic compounds. Armed witn' and Sakigake, on the trailing coma. Sui• aim. The Soviets sent Vega 1 and 2 to many new questions, astronomers are sei confirmed that the coma stops for a within 8000 km (5000 mi) of the comet, taking new opportunities to study the moment every fifty-three hours, appar• three days apart. While still 172 million mystery of comets. ently while the nucleus rotates. km (107 million mi) from the nucleus, The European Space Agency probe the probes found dust particles from the Giotto was the most exciting. It entered ~ the coma and came to within 600 km (370 mi) of the nucleus, taki ng over sixty pictures. About fourteen seconds before its closest encounter, a dense stream of dust-knocked it into a tumble and shut down its camera, but it sur• vived and continued to transmit data. Anticipating this rough encounter, its makers had stripped the probe of the

Vega 7and 2 were identical. Giatta