Was it a comet, or merely the solar wind?

Was it a comet, or merely the solar wind?

For nearly two years, Christopher Russell of the University of California at Los Angeles has been pursuing the possibility that a Venus-orbiting spacecraft in 1982 detected the invisible signs of the passage of an otherwise unknown comet. The case got even stranger last year, when a study of similar events suggested that at least some of them might indeed be due to a known object--but the object, named 2201 Oljato, was one that had previously shown almost no signs of being a comet. Perhaps, it was inferred, Oljato might be an old cometary nucleus that retained only enough of its ices to "outgas' a little in the sun's heat and produce the traces recorded by spacecraft, but not enough to form a more active comet's familiar, fuzzy "coma' and tail.

Now Devri S. Intriligator of the Carmel Research Center in Santa Monica, Calif., has concluded that the "original' event, seen in data from the Pioneer Venus orbiter, was not due to a comet at all, and was simply an example of the well-known variable behavior of the solar wind.

Intriligator notes that around the time of the original event, the orbiter showed a marked increase in the number of helium ions, compared with the number of hydrogen ions, in the interplanetary medium. And helium ions almost surely come from the sun, not from comets. In addition, she reports in the April GEOPHYSICAL RESEARCH LETTERS, there was a disturbance in the interplanetary magnetic field of just the sort that would be produced by a trailing "filament' of the solar wind blowing past the spacecraft. Furthermore, she says, both the increased helium and the magnetic-field disturbance were also detected by another spacecraft (ISEE-3), which was about 40 percent farther from the sun.

But the case is far from closed. Russell agrees that helium ions are not likely to come from comets, and that the fickle behavior of the solar wind is capable of producing magnetic-field disturbances that will show up at widely separated spacecraft. However, he says, a detailed examination of the magnetic-field data from the Pioneer Venus orbiter shows a "signature' that would be difficult to explain without putting some sort of obstacle in the way-- like a comet or a cloud of ionized atoms that have "outgassed' from it. The magnetic-field lines, he says, appear bunched up around the center of the disturbance, becoming more separated off to the sides, as though the field lines borne by the solar wind had run into something and were on their way to wrapping around it. The same "wraparound' effect is what yields the magnetic "tails' of planets such as earth and Jupiter. The solar wind, he says, could have accounted for the helium, and magnetic-field disturbances are far from rare. But no matter what the solar wind was doing at the time, the particular bunching-up or "compression' of the field lines looks too much as though something else was also present--even if no earthbased astronomers saw it.

Still, says Intriligator, compressions do happen in the solar-wind field, triggered, for example, by blobs of escaping solar "plasma' or by the interaction of separate streams of the solar wind.

So the possibility of the unseen comet remains a mystery. More insight may come, however, when the ISEE-3 spacecraft (now renamed ICE) flies through the tail of Comet Giacobini-Zinner this September, and six months later, when several probes pass Comet Halley.