STEREO, SOHO spacecraft catch comet diving into the Sun

Using instruments aboard NASA’s twin Solar TErrestrial RElations Observatory (STEREO) spacecraft, four postdoctoral fellows at UC Berkeley’s Space Sciences Laboratory were able to track the comet as it approached the Sun and estimate an approximate time and place of impact. STEREO, launched in 2006, consists of identical spacecraft orbiting the Sun, one ahead of Earth and one behind the Earth, providing a stereo view of the Sun.

The postdoctoral fellows then looked at data from the ground-based Mauna Loa Solar Observatory in Hawaii and found images in the predicted spot of what appears to be a comet approaching the limb of the Sun from behind the solar disk.

“We believe this is the first time a comet has been tracked in 3-D space this low down in the solar corona,” said Claire Raftery, a postdoctoral fellow newly arrived from Dublin’s Trinity College.

Sun-grazing comets, composed of dust, rock and ice, are seldom tracked close to the Sun because their brightness is overwhelmed by the solar disk. This comet apparently survived the heat of the corona and disappeared in the chromosphere, evaporating in the 100,000 kelvin heat.

Raftery and her colleagues, Juan Carlos Martinez-Oliveros, Samuel Krucker, and Pascal Saint-Hilaire, concluded that the comet was probably one of the Kreutz family of comets, a swarm of Trojan or Greek comets ejected from their orbit in 2004 by Jupiter, and making its first and only loop by the Sun. The swarm probably resulted from the disintegration of a larger comet.

The team presented its data and images Monday at the Miami, Florida, meeting of the American Astronomical Society.

Martinez-Oliveros’ attention was first drawn to the comet after seeing it mentioned in a summary of March 12, 2010, observations by STEREO and by the Solar and Heliospheric Observatory (SOHO). The comet’s long, bright tail of dust and ions tagged it as a Sun-grazing comet seen often by solar astronomers and observatories such as STEREO.

Assuming it was a going to loop around the Sun, they decided to see whether the STEREO data were good enough to let them calculate its trajectory.

In fact, the data were good enough to chart the comet’s approach for 2 days before impact.

With an estimate of the impact zone within a circle about 620 miles (1,000 kilometers) in diameter, they searched online data from the Mauna Loa Solar Observatory in Hawaii to see if they could see the comet next to the Sun’s edge in the ultraviolet region of the spectrum.

They found a short track, lasting about 6 minutes, just a few thousand kilometers above the Sun’s surface in the million-degree corona and 100,000-degree chromosphere.

Based on the comet’s relatively short tail, about 2 million miles (3 million km) in length, they believe that it contained heavier elements that do not evaporate readily. This would also explain how it penetrated so deeply into the chromosphere, surviving the strong solar wind as well as the extreme temperatures, before evaporating.

For their study, the team used the two coronagraphs on STEREO A and B and multiple instruments on SOHO, “demonstrat[ing] the importance of multi-view observations of non-solar phenomena,” they wrote in their poster.

All members of the team study explosive events on the Sun, such as coronal mass ejections, and the hot ionized plasmas they throw into space. Their detour into cometary physics was purely accidental, they said.

“It was supposed to be an exercise, but it took over our lives,” Raftery said.