"Long before it's in the papers"
January 22, 2012

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Comet dies on film, leaving trail of mystery

Jan. 22, 2012
Courtesy of NASA
and World Science staff

A com­et has been caught do­ing some­thing nev­er seen be­fore: die a scorch­ing death as it flies too close to the sun, sci­en­tists say.

That a com­et met this fate was no sur­prise, but the chance to watch it first-hand—in a vi­deo tak­en July 6—amazed even sea­soned com­et watch­ers, and left many scratch­ing their heads.

“Comets are usu­ally too dim to be seen in the glare of the sun’s light,” said Dean Pes­nell at NASA’s God­dard Space Flight Cen­ter in Green­belt, Md. “We’ve been tell­ing peo­ple we’d nev­er see one” from the agen­cy’s So­lar Dy­nam­ic Ob­serv­a­to­ry, which filmed the vi­deoand where he is proj­ect sci­ent­ist.

The gleam­ing com­et over­turned these as­sump­tions, but ex­actly how it man­aged this is un­clear, the in­ves­ti­ga­tors said. 

The ob­jec­t—from a group of com­ets known as the Kreutz com­ets—is seen mov­ing in over the right side of the sun, dis­ap­pear­ing 20 min­utes lat­er as it evap­o­rates. The mov­ie is more than just a nov­el­ty, as­tro­no­mers said. As de­tailed in a pa­pe­r in the jour­nal Sci­ence pub­lished Jan. 20, watch­ing the dis­inte­gra­tion pro­vides a new way to es­ti­mate the com­et’s size and mass. It turns out to be be­tween 150 to 300 feet (about 50 to 100 meters) long, and about the weight of an air­craft car­ri­er.

“It was mov­ing along... and was lit­er­ally be­ing evap­o­rat­ed away,” said Karel Schri­jver, a so­lar sci­ent­ist at Lock­heed Mar­tin in Palo Al­to, Calif., co-au­thor of the pa­pe­r and prin­ci­pal in­ves­ti­ga­tor of the At­mos­pher­ic Im­ag­ing As­sembly in­stru­ment at the ob­serv­a­to­ry. 

Typ­ic­ally, com­et-watch­ers see the Kreutz-group com­ets only through im­ages snapped by coro­n­a­graphs—special tele­scopes that view the Sun’s at­mos­phere, or co­ro­na, by us­ing a round ob­struc­tion to block the di­rect blind­ing sun­light. On av­er­age a new mem­ber of the Kreutz family turns up eve­ry three days, with some of the larg­er mem­bers be­ing seen for some 48 hours or more be­fore dis­ap­pear­ing be­hind the ob­struct­ing disk. They’re nev­er seen again. They ob­vi­ously dis­in­te­grate when they get close to theSun, ac­cord­ing to re­search­ers, but this had nev­er been wit­nessed di­rectly.

On July 6, Schri­jver spot­ted the bright com­et in a co­ro­nagraph pro­duced by the So­lar He­lio­spheric Ob­serv­a­to­ry, or SO­HO. He looked for it in im­ages from his So­lar Dy­nam­ic Ob­serv­a­to­ry and much to his sur­prise found it. Soon a mov­ie of the com­et cir­cu­lat­ed to com­et and so­lar sci­en­tists.

Karl Bat­tams, a sci­ent­ist with the Na­val Re­search Lab­o­r­a­to­ry in Wash­ing­ton, DC, who has studied many com­ets with SO­HO and is al­so an au­thor of the pa­pe­r, was skep­ti­cal when he first got the mov­ie. “But as soon as I watched it, there was ze­ro doubt,” he said. “I am so used to see­ing com­ets simply dis­ap­pear­ing in the SO­HO im­ages. It was breath­tak­ing to see one truly evap­o­rat­ing in the co­ro­na like that.”



The Solar Dynamics Observatory's AIA instrument captured the first image of a comet passing directly in front of the sun in the early morning of July 6. The comet comes in from the right and appears very faint. It's best to watch it in the full-screen version by clicking on the arrows icon at lower right. The homepage image from SOHO's LASCO C2 camera (Credit: ESA & NASA)

Af­ter the ex­cite­ment, the sci­en­tists got down to work. Hu­mans have been watch­ing and re­cord­ing com­ets for thou­sands of years, but find­ing their di­men­sions has typ­ic­ally re­quired a di­rect vis­it from a probe fly­ing near­by. This mov­ie of­fered the first chance to meas­ure such things from afar. The very fact that the com­et evap­o­rat­ed in a cer­tain amount of time over a cer­tain amount of space means one can work back­ward to de­ter­mine how big it must have been be­fore hit­ting the sun’s at­mos­phere, re­search­ers rea­son.

The pa­pe­r de­scribes the com­et and its last mo­ments as fol­lows: it was trav­el­ing some 400 miles (640 km) per sec­ond and made it to with­in 62,000 miles of the sun’s sur­face be­fore evap­o­rat­ing. Be­fore its fi­nal death throes, in the last 20 min­utes of its ex­ist­ence when it was vis­i­ble to the cam­era, the com­et weighed an es­ti­mated 100 mil­lion pounds (50 mil­lion kg). It had al­so bro­ken up in­to a doz­en or so large chunks with sizes es­ti­mated be­tween 30 to 150 feet (10 to 50 me­ters). These were em­bed­ded in a “co­ma”—the fuzzy cloud sur­round­ing the com­et—a­bout 800 miles wide, and fol­lowed by a glow­ing tail some 10,000 miles long.

It is ac­tu­ally the co­ma and tail seen in the vid­e­o, not the com­et’s co­re. Close ex­amina­t­ion shows that the light in the tail pulses, get­ting dim­mer and brighter al­ter­nate­ly. The team suspects these fluc­tua­tions result from suc­ces­sive breakups of each of the in­di­vid­ual chunks that made up the com­et ma­te­ri­al as it fell apart in the Sun’s in­tense heat.

“I think this is one of the most in­ter­est­ing things we can see here,” said Schri­jver. “The com­et’s tail gets brighter by as much as four times eve­ry min­ute or two. The com­et seems first to put a lot of ma­te­ri­al in­to that tail, then less, and then the pat­tern re­peats.” Fig­ur­ing out ex­actly why this hap­pens is one of the mys­ter­ies re­main­ing. 

High on the list is to an­swer the not-so-simple ques­tion of why we can see the com­et at all, sci­en­tists said. Cer­tain­ly, a few bas­ic char­ac­ter­is­tics of this situa­t­ion help. For one, this com­et was big enough to sur­vive long enough to be seen, and its or­bit took it right across the face of the Sun. It was al­so, said Bat­tams, probably one of the top 15 bright­est com­ets seen by SO­HO, which has ob­served over 2,100 sun-grazing com­ets to date. The film­ing cam­era al­so con­tri­but­ed by tak­ing a pic­ture eve­ry 12 sec­onds, where­as most oth­er si­m­i­lar in­stru­ments cap­ture im­ages only eve­ry few min­utes.

But ul­ti­mate­ly, the fact that one can see this com­et against the back­ground of the sun means there is some phys­i­cal pro­cess not yet un­der­stood, Pes­nell said. “Normal­ly,” he ex­plained, “a com­et pass­ing in front of the sun ab­sorbs the light from the sun. We would have ex­pected a black spot against the sun, not a bright one.” Fig­ur­ing out this ques­tion should of­fer in­forma­t­ion not only about ma­te­ri­al in the com­et, but al­so about the sun’s at­mos­phere – and so this opens up the door to a new niche of stu­dy, ac­cord­ing to as­tro­no­mers. As­sum­ing, of course, that one can spot some more com­ets.

* * *

A comet has been caught doing something never seen before: die a scorching death as it flies too close to the sun, scientists say. That a comet met this fate was no surprise, but the chance to watch it first-hand—in a video taken July 6—amazed even seasoned comet watchers, and left many scratching their heads. “Comets are usually too dim to be seen in the glare of the sun’s light,” said Dean Pesnell at NASA’s Goddard Space Flight Center in Greenbelt, Md. “We’ve been telling people we’d never see one” from the agency’s Solar Dynamic Observatory, which filmed the video and where he is project scientist. The gleaming comet overturned these assumptions, but exactly how it managed this is unclear, the investigators said. The object—from a group of comets known as the Kreutz comets—is seen moving in over the right side of the sun, disappearing 20 minutes later as it evaporates. The movie is more than just a novelty, astronomers said. As detailed in a paper in the journal Science appearing Jan. 20, watching the meltdown provides a new way to estimate the comet’s size and mass. It turns out to be between 150 to 300 feet long, and about the weight of an aircraft carrier. “It was moving along at almost 400 miles per second through the intense heat of the sun—and was literally being evaporated away,” said Karel Schrijver, a solar scientist at Lockheed Martin in Palo Alto, Calif., co-author of the Science paper and principal investigator of the Atmospheric Imaging Assembly instrument at the observatory. Typically, comet-watchers see the Kreutz-group comets only through images taken by coronagraphs—specialized telescopes that view the Sun’s atmosphere, or corona, by using a round obstruction to block the direct blinding sunlight. On average a new member of the Kreutz family is found every three days, with some of the larger members being seen for some 48 hours or more before disappearing behind the obstructing disk. They’re never seen again. They obviously disintegrate when they get close to the sun, according to researchers, but this had never been witnessed directly. On July 6, Schrijver spotted the bright comet in a coronagraph produced by the Solar Heliospheric Observatory, or SOHO. He looked for it in images from his Solar Dynamic Observatory and much to his surprise found it. Soon a movie of the comet circulated to comet and solar scientists. Karl Battams, a scientist with the Naval Research Laboratory in Washington, DC, who has extensively observed comets with SOHO and is also an author of the paper, was skeptical when he first got the movie. “But as soon as I watched it, there was zero doubt,” he said. “I am so used to seeing comets simply disappearing in the SOHO images. It was breathtaking to see one truly evaporating in the corona like that.” After the excitement, the scientists got down to work. Humans have been watching and recording comets for thousands of years, but finding their dimensions has typically required a direct visit from a probe flying nearby. This movie offered the first chance to measure such things from afar. The very fact that the comet evaporated in a certain amount of time over a certain amount of space means one can work backward to determine how big it must have been before hitting the sun’s atmosphere, researchers reason. The paper describes the comet and its last moments as follows: it was traveling some 400 miles (640 km) per second and made it to within 62,000 miles of the sun’s surface before evaporating. Before its final death throes, in the last 20 minutes of its existence when it was visible to the camera, the comet weighed an estimated 100 million pounds (50 million kg). It had also broken up into a dozen or so large chunks with sizes estimated between 30 to 150 feet (10 to 50 meters), embedded in a “coma”—the fuzzy cloud surrounding the comet—about 800 miles wide, and followed by a glowing tail some 10,000 miles long. It is actually the coma and tail seen in the video, not the comet’s core. Close examination shows that the light in the tail pulses, getting dimmer and brighter alternately. The team speculates that the pulsing variations are caused by successive breakups of each of the individual chunks that made up the comet material as it fell apart in the Sun’s intense heat. “I think this is one of the most interesting things we can see here,” said Schrijver. “The comet’s tail gets brighter by as much as four times every minute or two. The comet seems first to put a lot of material into that tail, then less, and then the pattern repeats.” Figuring out exactly why this happens is one of the mysteries remaining. High on the list is to answer the not-so-simple question of why we can see the comet at all, scientists said. Certainly, a few basic characteristics of this situation help. For one, this comet was big enough to survive long enough to be seen, and its orbit took it right across the face of the Sun. It was also, said Battams, probably one of the top 15 brightest comets seen by SOHO, which has observed over 2,100 sun-grazing comets to date. The filming camera also contributed by taking a picture every 12 seconds, whereas most other similar instruments capture images only every few minutes. But ultimately, the fact that one can see this comet against the background of the sun means there is some physical process not yet understood, Pesnell said. “Normally,” he explained, “a comet passing in front of the sun absorbs the light from the sun. We would have expected a black spot against the sun, not a bright one. And there’s not enough stuff in the corona to make it glow, the way a meteor does when it goes into Earth’s atmosphere. So one of the really big questions is why do we see it at all?” Figuring out this question should offer information not only about material in the comet, but also about the sun’s atmosphere – and so this opens up the door to a new niche of study, according to astronomers. Assuming, of course, that one can spot some more comets.