SOHO Spacecraft Hits Milestone with Discovery of 5,000th Comet

The Solar and Heliospheric Observatory (SOHO), initially slated for a two-year mission, has far exceeded scientists’ expectations. Over 28 years since its launch, SOHO has not only become the longest-lived satellite observing our star but also an unexpected and prolific comet hunter. On Monday, March 25, the observatory marked a significant milestone with the discovery of its 5,000th comet.

SOHO’s instruments, designed to block the sun’s blinding glare, allow the study of the sun’s faint outer atmosphere, the corona, which is a billion times fainter than the sun’s body. This same technique enables the observatory to spot comets, including those that slingshot around the sun, passing so close to its sizzling surface that they are referred to as “sungrazer” comets. Before SOHO’s launch, only a few dozen sungrazers were on record.

“That’s all we knew existed,” said Karl Battams, a space scientist at the U.S. Naval Research Lab in Washington, D.C. SOHO has since discovered more than half of all known comets, including sungrazers, cataloged in the NASA-funded citizen science effort, the Sungrazer Project. For the past 20 years, volunteers worldwide have examined images from the observatory, discovering thousands of sungrazing comets.

“The fact that we’ve finally reached this milestone — 5,000 comets — is just unbelievable to me,” Battams, the principal investigator for the Sungrazer Project, expressed.

The 5,000th comet discovered by SOHO is a tiny, icy rock belonging to the Marsden group of comets, one of three major types known to skim past the sun. Scientists believe these comets are first-generation descendants of the comet 96P/Machholz, a 3.7-mile-wide rock that orbits the sun every 5.3 years, most recently in January 2023. Of the 5,000 comets spotted by SOHO, only 75 belong to the Marsden group.