Designed to observe the sun, SOHO became a prolific comet finder with the discovery of the 5,000th comet marking a major milestone and demonstrating the contribution of volunteer comet hunters.

On March 25, 2024, a public scientist from the Czech Republic detected a comet in an image from the Solar Heliospheric Observatory (SOHO) spacecraft; This is the 5,000th confirmed comet discovered using SOHO data. Although SOHO was never designed to be a comet chaser, it achieved this milestone in its 28 years in space.

A comet is a small mass of ice and rock that takes only a few years to orbit the Sun. It belongs to the “Marsden group of comets”. This cluster is believed to be associated with Comet 96P/Machholz (observed by SOHO as Machholz passes close to the Sun every 5.3 years) and is named after the late scientist Brian Marsden, who first identified the cluster using SOHO observations. Of the 5,000 comets discovered by SOHO, only 75 belong to the Marsden group.

SOHO’s unexpected role

The joint ESA (European Space Agency) and NASA SOHO mission was launched in December 1995 to study the Sun and the dynamics of its outer atmosphere, called the corona. A science instrument at SOHO called the Large Angle and Spectrometric Coronagraph (LASCO) uses an artificial disk to block the Sun’s blinding light so scientists can study the corona and the environment directly around the Sun.

It also allows SOHO to do something many other spacecraft cannot: see comets flying close to the Sun, known as sun-grazing comets or sun-grazing comets. Many of these comets only get brighter when they are too close to the Sun to be seen by other observatories, otherwise they would disappear unnoticed into our star’s bright glare. Although scientists expected SOHO to find some comets by chance during its mission, the spacecraft’s ability to detect them made it the most successful comet finder in history, discovering more than half of the comets known today.

Community involvement and success

In fact, shortly after SOHO launched, people around the world began noticing so many comets in its images that scientists needed a way to keep track of them all. In the early 2000s, they launched the NASA-funded Sungrazer project, which allowed anyone to report comets they found in SOHO images.

Comet 5000 SOHO was discovered by Hanji Tan, a member of the Sungrazer project, who is originally from Guangzhou, China, and is currently pursuing her PhD in astronomy in Prague, Czech Republic. Tan has been involved with the Sungrazer project since the age of 13 and is one of the youngest comet explorers.

“I have discovered more than 200 comets since 2009,” Tan said. “I joined the Sungrazer project because I love looking for comets. “After traveling through space for thousands of years, it’s truly exciting to be the first to see comets shining near the Sun.”

Most of the 5,000 comets detected by SOHO were found by an international team of volunteer comet hunters, many of whom had no formal scientific training, participating in the Sungrazer project.

“Only a few dozen solar comets had been reported before the launch of the SOHO mission and the Sungrazer project; these were the only ones we knew existed,” said Carl Battams, a space scientist at the U.S. Naval Research Laboratory in Washington, D.C., and Sungrazer’s principal investigator on the project. “It’s incredible to me that we’ve finally reached this milestone of 5,000 comets.”

The large number of comets discovered by SOHO allowed scientists to learn more about sun-grazing comets and groups of comets orbiting the sun. The comets discovered by the Sungrazer project have also helped scientists learn more about the Sun by observing comets diving into our star’s atmosphere like small solar probes.

“The statistics of 5,000 comets and looking at their orbits and trajectories in space is an extremely unique data set – it’s really valuable science,” Battams said. “This is a testament to the countless hours project participants have put into this. “We would certainly never have reached this milestone without the work of the project volunteers.”