There still is a chance for a US mission to study Halley's comet, when that luminous wanderer returns in 1985. An elaborate mission with specially designed spacecraft has been ruled out by federal budget cuts, but a research satellite already in space could join Soviet , European, and Japanese probes that will rendezvous with the famous comet when it passes by Earth.
This satellite is ISEE, the International Sun-Earth Explorer. It is now pointed toward the sun, monitoring the stream of electrically charged solar particles that affect utility networks and terrestrial communications.
Although ISEE wouldn't be able to send back beautiful pictures of the comet, its instruments could make important scientific observations, argues Frederick Scarf of TRW Inc., one of the ISEE researchers who advocates sending the satellite comet-seeking. And, best of all to the budget-conscious, the costs would be negligible, he says.
Not all the ISEE scientists favor such an interplanetary foray, however. Some argue that the satellite is performing a valuable role where it is. But Dr. Scarf and a number of other scientists are pushing for the more adventurous course. This is currently the subject of a sometimes passionate internal debate at the US National Aeronautics and Space Administration (NASA).
NASA's Deep Space Network, which communicates and controls US spacecraft, has determined that it can communicate with and control ISEE during a comet chase. As a result, there are a number of possible missions.
One mission would be to fly ISEE through Halley's glowing tail. Instruments on board the satellite-turned-spacecraft would allow scientists to profile the tail's density and other characteristics. Even more intriguing, however, would be the possibility of exposing a protected portion of the craft to the comet's gases, steering it back into an orbit around the Moon, picking it up with the space shuttle, and bringing it back to Earth for analysis. If such an analysis were properly done, it would tell scientists a great deal about the conditions when and where Halley formed, says Dr. Scarf.
Comets are of great interest to scientists who study the formation of the sun and planets. Generally, comets are thought to be left-over building blocks from which the planets were formed. They are also thought to have been part of a tremendous bombardment more than 3 billion years ago that left the Moon, Mercury , Mars, and a number of other satellites pockmarked with craters.
Another possible ISEE mission would take it to another comet, not Halley's. This is an older and less dramatic comet called Giacobini-Zinner.
This alternative has a number of planetary scientists intrigued because it would give them information about a second comet that they could compare with Halley. In addition to the possible mission by the spacecraft, an ambitious schedule of scientific observations of Halley's are scheduled from Earth and the Shuttle-based Space Laboratory. If ISEE went to Giacobini-Zinner, these observations could be duplicated for the second comet. There is even discussion about an ISEE flyby of a third comet.