As I was walking up the stair,
I saw a man who wasn't there.
He wasn't there again today,
I wish that man would go away.
An Irish space scientist has turned Hugh Mearnes' well-known rhyme on its head with the announcement that she has discovered a new comet that nobody has yet seen. Prof Susan McKenna-Lawlor of NUI Maynooth made the discovery, even though she is the first to admit that she has never actually seen this object in the way that astronomers normally do.
Instead, the presence of the comet was revealed by data captured by an Irish-built particle detector aboard the European Giotto spacecraft. The European Space Agency, which launched the Giotto probe in 1985, said the discovery was unique in that it was the first to be claimed by scientists using a particle detector.
Giotto's primary mission was to fly past Halley's comet during its 1986 passage through the inner solar system, but the satellite's mission was later extended to allow it to investigate a comet called Grigg-Skjellerup in 1992.
Passing within 200 km of the comet's icy nucleus, Giotto radioed back a stream of measurements from the Irish instrument, which is named Epona after the Celtic goddess. "We got tremendous data during the fly-by," said Prof McKenna-Lawlor. "The information allowed us to compare the results from two comets, Halley and Grigg-Skjellerup, but we never imagined there would be a third."
The data were recorded eight years ago, but indications of a third comet emerged only recently when she and her Russian collaborator, Dr Valeri Afonin, looked more closely at measurements taken after the Grigg-Skjellerup fly-by.
What they saw was an unmistakable surge in the number of charged particles entering the detector as Giotto swept away from Grigg-Skjellerup, 90,000 km from the comet's core. Prof McKenna-Lawlor said a number of possible explanations were considered, but her conclusion was that the changes could be explained only by the presence of a companion comet flying in formation ahead of Grigg-Skjellerup. It is not uncommon for lumps of material to break off comets, and that is the likeliest explanation for this comet, which may be three to four times smaller than Grigg-Skjellerup.
Aastronomers world-wide have been notified of the discovery. They will have an opportunity to search for the new comet when Grigg-Skjellerup returns to the inner solar system in March 2008.