Amateur Astronomers Caught Sungrazing Comet during Solar Eclipse

Observers who photographed the April 8th total solar eclipse received an unexpected bonus when totality revealed a comet diving toward the Sun, known as a Kreutz sungrazer. But spotting the death-diving comet required special tricks in observing and image processing.

In anticipation of the eclipse, Thai amateur Worachate Boonplod searched recent coronagraph images in the online archives of the Solar and Heliospheric Observatory (SOHO). Just hours before the start of totality, he spotted an unusually bright comet approaching the Sun in a Kreutz orbit. As he realized that the new comet, designated SOHO 5008, might be bright enough for ground-based observers to photograph it during totality, Boonplod posted the discovery on the Sungrazer Project's website.

So far, at least four ground-based observers have reported success at sites on the arc of totality from Mexico to the US-Canadian border. Chinese amateur Lin Zixuan spotted it from the Third Connecticut Lake area of New Hampshire, close to the Canadian border in the state's uppermost region. Ye Quanzhi (University of Maryland and Boston University) captured the comet using a 28-millimeter camera connected to a Raspberry Pi-compatible lens in Newport Center, Vermont, also near the Canadian border. Qicheng Zhang (Lowell Observatory), a colleague of Ye’s, also photographed the comet during totality in Mountain Home, Arkansas. And Petr Horálek (Institute of Physics, Opava, Czech Republic) photographed the comet from Durango, Mexico, near the start of the eclipse. Boonplod also observed the comet using SOHO's cameras in space. 

Sungrazing Comets

Astronomers have known comets can pass close to the Sun since they observed the Great Comet of 1680, which was the first comet discovered using a telescope. Other discoveries followed, and in May 1882 a group that had come to Egypt to watch a solar eclipse were surprised to see a bright streak of light near the Sun during totality. They noticed the streak moved visibly during less than two minutes of totality. It proved to be a comet moving at about 500 kilometers (300 miles) per second, which had gone otherwise unreported because it was close to the Sun and visible only in the daytime sky.

German astronomer Heinrich Kreutz later found that the Egyptian eclipse comet and many (but not all) other sungrazing objects had similar orbits. Astronomers now believe all Kreutz comets are fragments from the break-up of a gigantic comet seen by Aristotle in 371 BC.

Bright Kreutz sungrazers can be spectacular. The most recent was Comet Ikeya-Seki in 1965, which exceeded a visible magnitude of -10, making it the brightest comet of the 20th century. But such giants are few and far between, and small ones are difficult to observe when close to the Sun.

SOHO was designed primarily to study the Sun and its heliosphere, but the spacecraft includes a coronagraph that has revealed a multitude of small Kreutz sungrazers. Since it began operations in 1996, SOHO has discovered more than 5,000 comets near the Sun, most of them small objects in Kreutz orbits. The intense solar radiation soon destroys them. SOHO 5008 lasted a little more than 12 hours as seen through SOHO’s cameras.

The spacecraft’s cameras operate day and night, allowing them to discover an average of one comet every two to three days at sizes down to a few meters across. Yet because SOHO was designed to study the Sun and heliosphere, it can't tell us much about the comets. “We can't really [use SOHO to] observe a lot of features of comets because of its low spatial resolution,” says Ye. His little 28-mm camera on the ground can see more detail, and larger cameras can offer even higher resolution.

NASA's Sungrazing Comets Project looked for sungrazing comets during the 2017 North American eclipse but failed to find any. This year's success comes in part because SOHO 5008 is much brighter than average; Ye says a comet that bright only comes along about every couple of months.

Shooting Techniques

“I got especially lucky with this shot with the comet SOHO-5008, which was discovered just before the eclipse,” Horálek told Space Weather. But he also credited the quality of his image showing the comet's tail to his use of high dynamic range. Totality lasted 3 minutes and 25 seconds from his location, the air was windy, and the sky was partly cloudy, but he managed to obtain 83 usable shots, with exposures lasting from 1/4000 to 2 seconds.

Lin also used a 70-mm lens with a 350-mm focus to collect 15 frames during totality, which he processed to produce the image he distributed. “Even in the original image, [the comet] presents a distinct yellow-orange color,” he says. Horálek's image showed the same color.

NASA’s SOHO observations show that comets near the Sun often glow brightly at yellow wavelengths of sodium’s D line (at 588.9950 and 589.5924 nanometers), due to the fluorescence of sodium atoms excited by intense solar radiation.

The results show that “capturing a good image of such comet during a total solar eclipse is actually not very hard,” says Ye. “The technology has gotten good enough; we mostly just need a comet that is bright enough.” However, he notes that only deep wide-field imaging can capture comets beyond the corona.  

Photographers using cameras with fields wider than 10 degrees might even gotten shots of the comet without realizing it, he adds. “The comet was so bright that it might even be visible in single frames!”