The joint mission between the European Space Agency (ESA) and NASA, the Solar and Heliospheric Observatory (SOHO), was not originally destined to find comets. Rather it was built to study the Sun, from the depths of its core to the outer corona. But nearly 25 years after its launch, SOHO has reached a momentous milestone – the discovery of its 4,000th comet. The “sungrazer,” aptly nicknamed SOHO-4000, was spotted amongst SOHO’s data by citizen scientist Trygve Prestgard on June 15.
“I feel very fortunate to have found SOHO’s 4,000th comet. Although I knew that SOHO was nearing its 4,000th comet discovery, I did not initially think that this sungrazer would be it,” Prestgard, said in a statement. “It was only after discussing with other SOHO comet hunters, and counting through the most recent sungrazer discoveries, that the idea sunk in. I am honored to be part of such an amazing collaborative effort.”
Residing a million miles towards the Sun from Earth, SOHO has a constant view of our star. In particular, to help focus in on the Sun’s faint outer atmosphere (corona), an onboard instrument called LASCO (a coronagraph) blocks out the Sun’s bright face with a solid metal disk. Not only does this enable astronomers to track explosions of material from the star (coronal mass ejections), but the instrument’s high sensitivity and wide field of view turned out to be the perfect tool to spot “sungrazing comets.”
Skimming so close to the Sun that they cannot be seen from Earth, these comets had been unknown until SOHO cast its eye on them. Even when SOHO spots these comets, nearly all of them are in the process of being destroyed by the Sun, offering up a peek into the “dirty snowballs’” final moments.
“Not only has SOHO rewritten the history books in terms of solar physics, but, unexpectedly, it’s rewritten the books in terms of comets as well,” Karl Battams, a space scientist at the US Naval Research Lab in Washington, D.C., who also manages SOHO’s comet-finding program, said in a statement.
Having contributed well over half of all known comets, SOHO has certainly established its place in cometary history. But its success cannot but attributed to its instruments alone; the hard work of numerous citizen scientists who identify the chunks of frozen gases, rock, and dust orbiting the Sun, is also to thank.
“I enjoy the feeling of discovering something previously unknown, whether this is a nice “real-time” comet or a “long-gone” overlooked one in the archives,” Prestgard said. His recent find, SOHO-4000, belongs to the Kreutz family, a group of roughly house-sized comets thought to be the fragments of a single giant comet, broken up when it flew too close to the Sun. In fact, 85 percent of SOHO’s discoveries belong to this family that continue to follow the path of the original Kreutz comet.
Other comets found in SOHO’s data rely on a different instrument to LASCO, the SWAN instrument. This camera, designed to look for interactions between the solar wind and hydrogen atoms, also picks up outgassing comets, which are spewing water (primarily made of hydrogen) as they approach the Sun. Indeed, the glorious Comet SWAN discovered earlier this year, was first detected by its namesake onboard SOHO.
Here’s to more SOHO cometary success.