As dramatic exits go, it's on par with Major T. J. "King" Kong riding a falling nuclear bomb like a rodeo bull at the end of Dr. Strangelove. A NASA spacecraft has documented a comet's demise as it plunged toward the sun at 600 kilometers per second, broke apart and vaporized inside the solar atmosphere.
The comet, known as C/2011 N3 (SOHO), met its fiery fate on July 6. The object's official name designates that it was discovered in early July 2011 by the Solar and Heliospheric Observatory (SOHO) spacecraft. Many comets meet a similar end, but astronomers and solar physicists have never been able to track a comet's trajectory all the way into the depths of the solar corona, the outermost layer of the sun's atmosphere.
With the help of another spacecraft—NASA's Solar Dynamics Observatory (SDO), which was launched in 2010—a group of scientists were able to witness the final minutes of the comet's existence. The observations of C/2011 N3 as it broke apart allowed the researchers to estimate the comet's mass and the size of its nucleus; similar events in the future may provide clues about the origins of comets as well as probe conditions near the sun that are otherwise difficult to explore. The team of researchers published their findings in the January 20 issue of Science.
SOHO has discovered more than 2,000 comets near the sun, most of them thanks to the help of unpaid amateur astronomers who comb through imagery from the spacecraft. Most of the sun-grazing comets, like C/2011 N3, belong to the Kreutz family, which is thought to have originated from a single progenitor that broke apart within the past few thousand years. The smallest of these comets are destroyed by the sun before they draw too close, so C/2011 N3 was rather sizable for a Kreutz-family comet, with a nucleus 10 to 50 meters across.
"It must have been on the large side," says lead study author Carolus Schrijver, a solar physicist at the Lockheed Martin Advanced Technology Center in Palo Alto, Calif. The comet's size contributed not only to its survival deep into the solar atmosphere but also to its receiving close scrutiny during the sunward plunge. "This was noted as a particularly bright one," Schrijver says. "That morning as it was approaching the sun I said, 'Well, let's see if we can see it.'"
An atmospheric imaging camera on SDO was indeed able to track the inbound comet, watching it bear down on the sun in an ultraviolet streak that lasted about 20 minutes before it disappeared. By that time the comet was only about 100,000 kilometers above the solar surface and had broken into a number of fragments, further hastening its vaporization.
"The temperatures [at that point] are so high that things are evaporating," says astronomer Matthew Knight of Lowell Observatory and the Johns Hopkins University Applied Physics Laboratory, who did not contribute to the new study. "Not just gases and ices, but heavy elements."
The comet's total obliteration in the solar atmosphere let Schrijver and his colleagues estimate how much material was lost in the process. "Because it vanished, we could actually measure its mass," Schrijver says. The researchers estimate that the comet may have shed as much as 60 million kilograms of material in its plunge—about the mass of the Titanic. But the comet's composition is less clear. "We're still trying to understand what was glowing," he says. The imager used to track C/2011 N3 is most sensitive to iron, but Schrijver notes that the glow could also have been produced by carbon or oxygen.
Comets preserve some of the remnant materials from the solar system's birth, so their composition can provide important clues to planet formation. What is more, a comet's infall provides a natural experiment that can determine not only its properties but the conditions within the solar corona, much of which is not routinely visible. "It's a really neat thing to do, to have this comet come in and tell us about the oldest material in the solar system, and to explore this region that you can't otherwise see," Schrijver says.
A more recent cometary passage could shed further light on the nature of Kreutz-family bodies and their interactions with the solar atmosphere. In December a comet called C/2011 W3 (Lovejoy) actually survived its encounter with the sun and continued along its orbit, although it may have been partially disrupted in the process. It was named after its discoverer, Australian amateur astronomer Terry Lovejoy.
"That's the most spectacular of these sun-grazers that we've seen since 1965," Knight says. "How long it's survived tells us that it's larger than the other ones." As with C/2011 N3 a few months prior, SDO's cameras documented the solar rendezvous. And Knight adds that he and his colleagues are still following Comet Lovejoy with whatever telescopes they can muster to try to learn about its nature before drawing any major conclusions about the encounter. "We're all just trying to wrap our heads around how much data we got," he says.