History’s greatest comet hunter

From time to time a bright comet with long, trailing tails becomes visible with the naked eye. However, in recent years the ESA / NASA SOHO spacecraft has shown that these “dirty snowballs” are much more common than anyone could imagine.

SOHO was designed to stare at the Sun, not search for comets. One of its instruments, called LASCO, is used to observe the Sun’s outer atmosphere (the corona). But it has also enabled SOHO to become the greatest comet hunter in history.

LASCO studies the faint corona by blocking the brilliant light from the Sun. By chance, this also allows LASCO to detect small sungrazing comets. Most of these icy intruders sweep to within 800,000 km of the Sun’s surface, so close that they vaporise or fall into its fiery furnace.

Before SOHO was launched in December 1995, only 16 sungrazing comets had been discovered by spacecraft. By 6 July 2005, SOHO alone had found no fewer than 990. This means that SOHO has discovered almost half of all the comets whose orbits have been computed since history began. The staggering total is expected to pass 1,000 this summer and keep on rising.

Many of these discoveries have been made by amateurs, using SOHO images on the internet. SOHO comet hunters come from many countries, including the United States, United Kingdom, China, Japan, Taiwan, Russia, Ukraine, France, Germany and Lithuania.

Comets are chunks of ice and dust - ‘leftovers’ from the formation of the planets 4.5 billion years ago.