Comet Champion of the Solar System

Last week, the ESA/NASA Solar and Heliospheric Observatory shattered its own record for comet discoveries when astronomers announced that the orbiting spacecraft had recorded its 200th sungrazing comet. Michael Oates, an amateur astronomer in Britain, spotted SOHO-200 in an online image captured by one of SOHO's coronagraphs. The picture showed the comet evaporating as it plunged through the superheated solar corona.

"With 200 discoveries, SOHO is way ahead [of any other comet hunter]," says Brian Marsden of Harvard University's Minor Planet Center. The automated asteroid and comet search program "LINEAR is a distant second with 50 comets. Among individuals, Carolyn Shoemaker is of course the leader, with 30-something discoveries."

Right: The SOHO C2 coronagraph captured this image of a sungrazing comet 0.9 degrees from the Sun on April 29, 2000. The solid brick-colored disk in the middle is the coronagraph's occulting disk; the light red graphic circle shows the true size of the Sun.

SOHO's primary mission is to investigate the solar interior, the solar atmosphere, and the solar wind. Space weather forecasters rely on the satellite, which enjoys an uninterrupted 24-hour-a-day view of the Sun, for advance warning of solar eruptions and geomagnetic storms. The comets are a bonus.

"No one expected to find all these comets when we launched SOHO nearly 5 years ago," says Doug Biesecker, a solar physicist at the Goddard Space Flight Center. "They came as a huge surprise, there's no doubt about it."

Only comets that pass perilously close to the Sun catch SOHO's attention. The vast majority, like SOHO-200, don't survive the encounter. They swoop so low over the blazing solar photosphere that their icy cores vaporize completely. Most of SOHO's 200 comets no longer exist -- they disintegrated hours after they were discovered!

The key(s) to spotting comets so close to the Sun are SOHO's extraordinary coronagraphs. A coronagraph is a device that blocks out the Sun's blinding glare with an occulting disk so that the faint corona is visible, as well as surrounding stars and planets. Coronagraph images at the SOHO web site are updated every 30 minutes or so. About once each week the photos include a faint comet that anyone can discover if they happen to be the first to look.

Above: In the left image Comet Hyakutake passes grandly through the SOHO C3 coronagraph's field of view at the beginning of May 1996. A more typical, sungrazing comet appears in the right image. The faint fragment was discovered by amateur astronomer Kazimieris Cernis of Vilnius, Lithuania, on 4 February. It marked SOHO's 100th comet discovery. (Editor's Note: Although SOHO-100 came close to the Sun, and thus qualifies as a "sungrazer," it was not a Kreutz sungrazer as described in the text. SOHO-100 did not share the same orbit as that family of comets possibly resulting from the breakup of a giant comet 2,000+ years ago.)

Remarkably, most sungrazing comets appear to be fragments of a single giant comet that broke apart near perihelion (closest approach to the Sun) long ago. Marsden speculates that the parent might have been a bright comet seen by the Greek astronomer Ephorus in 372 BC. Ephorus reported that the comet split in two. Splits may have occurred again and again, producing the Kreutz sungrazer family (named after the nineteenth-century German astronomer who studied them in some detail). The sungrazers share an elliptical orbit that brings some of the fragments less than 50,000 km from the Sun.

"The association of the Ephorus comet with the 'original' break-up is speculative," cautions Marsden. "Some may argue that the breakup took place much longer ago, while others might say that it happened in a somewhat different manner and quite a bit more recently. We really don't know much about what happened."

The nucleus of the original comet may have been as wide as 100 km, or 10 to 20 times larger than an ordinary comet. Most of the short-lived fragments seen nowadays by SOHO are tiny -- probably no more than a few tens of meters across. Occasionally, pieces come along that are large enough to survive the Sun's searing heat, and these can emerge from their close encounter as bright naked-eye comets. One such fragment in 1965, Comet Ikeya-Seki, was visible in broad daylight. Scientists estimate that Ikeya-Seki's icy nucleus was some kilometers wide.

"In the old days we thought these comets were a rarity because there were only a few like Ikeya-Seki visible from the ground," continued Marsden. "The smaller fragments are tremendously faint except for a few days before perihelion. By the time SOHO spots them they are in their death-throes."

Left: In August 2000 NASA's Hubble Space Telescope spotted fragments from the breakup of Comet LINEAR. A similar disruption of Ephorus's comet 2,000 years ago may have created the existing family of sungrazing comets seen by SOHO today.

SOHO's impressive spate of comet finds can be attributed in large part to the efforts of amateur astronomers. SOHO data are freely available to anyone with an internet connection and a computer. Both realtime and archival images are accessible at the SOHO web site, a popular destination for comet hunters.

"In late February, just after we announced our 100th comet, I mentioned in an email message to (amateur astronomer) Michael Oates that there might be undiscovered comets in images from our C2 coronagraph," says Biesecker. SOHO carries two coronagraphs. The C2 instrument monitors the corona from 2.5 to 6 solar radii from the center of the visible Sun. Its sister coronagraph, called "C3," has a wider field of view. It monitors activity at distances of 4 to 30 solar radii.

"Before this summer, most of SOHO's comets had been found in C3 images," said Biesecker. "We really didn't expect to find many in C2, because by the time the comets are close enough to the Sun to fall within C2's narrow field of view they are moving very quickly. We only record coronagraph images every half hour, so it's easy to miss fast-moving comets in the C2 data."

Nevertheless, Biesecker's February communique to Oates was the beginning of an avalanche. Nearly, two-thirds of the comets discovered since then were located by amateurs examining archived C2 images. SOHO-200 was found by Oates in a picture from April 1997.

"It turns out that C2 is a better instrument than C3 for finding comets during the months of May through July each year," explained Biesecker. "Sungrazing comets seem to reach a peak brightness at a certain distance from the Sun, between 10 and 12 solar radii. When we see a comet in a coronagraph image, we're not seeing its true distance from the Sun. What we see is its distance projected onto the flat plane of the image. In the May to July time frame the projected distance corresponding to 10-12 solar radii falls within the C3 instrument's vignetting. But, they show up nicely in C2 images. At that time of the year the comets appear to move more slowly, too, so we get a few additional images of the comet." All these factors have combined to make amateur searches through the archives so productive.

"Amateurs have even taken the lead on realtime discoveries," added Biesecker. "If a comet zooms through the coronagraph's field of view at 2 a.m. here at Goddard, someone in Europe is probably looking at the web site while we're asleep!"

Above: This 4-minute exposure of sungrazing comet Ikeya-Seki was captured by Roger Lynds at Kitt Peak, Arizona, on the morning of 1965 October 29. Unlike most of the sungrazing comets recorded by SOHO, Ikeya-Seki survived its close encounter with the Sun to become a bright naked-eye object.Copyright© Roger Lynds, all rights reserved.

By convention, comets discovered in SOHO data are named after the spacecraft rather than the astronomer. Nevertheless, say amateur comet hunters, being the first to spot a comet streaking past the Sun can be a real thrill.