Remember Comet ISON? Last year began with a blizzard of hype, with stories repeating the mantra that this mysterious celestial visitor could become the "comet of the century." This year begins with Comet ISON obliterated, an invisible cloud of debris expanding and traveling outward from the sun.
Death of Comet ISON: It entered the frame from lower right and fled the sun toward the upper right, falling apart as it went, in this time-lapse SOHO image from November 28, 2013.(Credit: ESA/NASA/SOHO/SDO/GSFC) For the millions of enthusiasts hoping to see a glowing dagger of light hanging in the night sky, the premature demise of Comet ISON was a crushing disappointment. But for the astronomers who had pinned great hopes on the comet as an object of study, Comet ISON fully lived up to its billing (see my preview article, The Life and Death of Comet ISON). It is already the most closely observed comet in history. It inspired the Comet ISON Observing Campaign, which coordinated studies not only from around the world but from across the solar system. Some of the first scientific papers will be coming out this week at a meeting of the American Astronomical Society; expect the flow to keep going for a long time. Amateurs will get to participate as well, since much of the data on the comet will be released openly to the public. But why wait? We've already learned some illuminating lessons from the late, great Comet ISON.
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1. You can find amazing things while looking for junk. The two observers who found Comet ISON--Vitali Nevski and Artyom Novichonok--made their discovery largely by accident. They were working for the International Scientific Optical Network (hence ISON), a network of observatories that collaborate to study debris in orbit around the Earth and to monitor asteroids. Nevski has written a short explanation of how he and his partner happened on a comet instead. ISON continues on its primary mission of tracking space junk; Nevski modestly describes himself as an "engineer observer, not a professional in cometary astronomy."
2. Comet ISON really was a comet of the century, and then some.
Wait--what? Oh sure, it was a total dud for backyard observers, since it never even reached easy naked-eye visibility. As an astronomical object, however, Comet ISON really was something extraordinary: a fresh Oort Cloud comet that was also a sungrazer. Let me decode the terms. The Oort Cloud is a huge swarm of inert comets orbiting in the icy depths, hundreds or thousands of times farther from the sun than Pluto. Most bright comets are "periodic," meaning that they have flown past the sun repeatedly, getting cooked and altered in the process. Comet ISON was different. Its orbit shows that it came straight from the Oort Cloud. It had been in deep freeze since the formation of the solar system, so what astronomers were observing was an intact time capsule from 4.5 billion years ago.
Comet ISON in happier times. All that glowing gas and dust, stretching more than 8 million kilometers long, came from a solid body scarcely larger than a skyscraper. (Credit: Damian Peach) Sungrazing comets are ones that pass extremely close to the sun, where the intense heat boils off their innards the fierce solar radiation makes the chemicals in the comet intensely visible. That makes sungrazing comets great scientific subjects...but most of them are too small and dim to study readily, and very few of them are fresh arrivals from the Oort Cloud. Comet ISON checked all the boxes: fresh, hot, and (relatively) bright. I asked Matthew Knight of Lowell Observatory: When is the last time astronomers saw a comet like that? His answer: "never," at least not in the 200 years that astronomers have been able to track cometary orbits. By that measure, Comet ISON was at least the comet of the bicentennial. 3. The early solar system was a wildly unstable place. How did Comet ISON get out to the Oort Cloud in the first place? According to the latest theories, it was exiled during an era of extreme chaos in the solar system, starting 4.5 billion years ago. Early on, before Earth had even formed, Jupiter plunged in to less than 1/3 its present distance from then sun and then zoomed back out, flinging Comet ISON and trillions of others like it out beyond Neptune. A second convulsion, about 600 million years later, exiled Comet ISON completely to the Oort Cloud, perhaps 1,000 times farther from the sun. At the same time, many other comets and asteroids came raining down on the young planets, battering the surface of Earth just as life was beginning to gain a foothold. I describe these ideas in much greater detail in my new article The Madness of the Planets. Astronomers are now scrutinizing Comet ISON's composition to uncover evidence of its amazing journey. 4. Comet ISON's cousins may have seeded life on Earth. This old idea gets a new spin from the latest theories. It may not be coincidence that the solar system's second period of chaos, known as the Late Heavy Bombardment, roughly coincided with the first appearance of living things on Earth. Comets like ISON contain a lot of organic material--carbon-rich molecules that might have primed the early Earth with key ingredients for life. So while comets were blasting our planet's surface, they might also have been setting the stage for today's vibrant world. Unfortunately it is very hard to know what was happening on Earth at that early stage. Almost all traces of Earth's surface from that time have been thoroughly eroded and erased, though Bruce Simonson at Oberlin College is making a dogged effort to sift through the traces of evidence from the attack of Comet ISON's cousins. 5. It took a star (or a whole galaxy) to bring Comet ISON home. Once the comet found its way to the Oort Cloud, it remained largely inert for the next 3.9 billion years. It was in deep freeze, at temperatures just a few degrees above absolute zero (-273 degrees C, or -459 degrees F). It moved in a lazy orbit, taking 100,000 years or more to circle the sun just once; from out there, the sun looked just like another star, albeit an unusually bright one. Nothing changed for Comet ISON until something big happened. The shove that dislodged ISON might have been the movement of a nearby star, whose gravitational pull dislodged the comet from its slumber. Or it might have taken the combined pull of the entire Milky Way galaxy to send Comet ISON back home. "The gravitational potential of the whole galaxy can destabilize comets," explains Alessandro Morbidelli of the University of Nice. The Oort Cloud is so weakly attached to the sun that the gravitational pull of the galaxy essentially creates tides in the Oort Cloud, squeezing it and causing some vulnerable comets to fall inward. Comet ISON may have been one of those unlucky victims. 6. Comet ISON was a bomb, literally. Nevski and Novichonok were able to discover the comet while it was far from the sun, out past Jupiter, because it was already kicking up a big cloud of gas and dust. That ruckus is part of what made astronomers so optimistic that Comet ISON would be spectacularly bright; most comets don't really get going until they are closer to the sun's heat. Now we have some good ideas why the comet was such a precocious performer. Karen Meech at the University of Hawaii ran computer models showing that Coet ISON was probably coated with a layer of frozen carbon dioxide (CO2) and carbon monoxide (CO), gases that vaporize at very low temperatures. Any comet that has experienced even a trickle of the sun's warmth will have lost those gases. Comet ISON, which has been in deep freeze since birth, still had them. As a result, it erupted vigorously as soon as it hit the critical temperature for CO and CO2 to vaporize. Mike A'Hearn of the University of Maryland notes that the comet must also have been bombarded by cosmic radiation for billions of years while it idled in the Oort Cloud. The radiation shattered molecules in the comet; when heated, those molecules would then recombine explosively, like TNT. When it was first spotted, Comet ISON might have been in mid-detonation. At any rate, Comet ISON's hyperactive phase didn't last. This is another lesson: First-time visitors from the Oort Cloud tend to peak early and then fizzle. That pattern may explain one of the most notorious cometary disappointments, Comet Kohoutek.7. Little things have big consequences. Comet ISON's tail stretched nearly 10 millions of kilometers across. Other comets have reached lengths up to 500 million kilometers--wider than Earth's orbit around the sun! And yet the solid body of a comet is minuscule, typically no more than a 10 kilometers wide. The lesson here is that you don't need much material to capture and reflect sunlight; think of wisps of cigarette smoke. The tail of a bright comet is so empty that it would be considered an extremely hard vacuum here on Earth. By the latest estimates, Comet ISON was a piker, no more than 500 meters across--and now, of course, it is nothing at all. Yet the similarly puny Comet Lovejoy made a beautiful show in 2011, and collectively comets have had a profound effect moving water and organic compounds around through the solar system. Comets may be small, but there are trillions of them...and they sure know how to put on a show.
This is the look of an old, eroded comet--Comet Hartley 2, seen by NASA's EPOXI mission on November 4, 2010. The nucleus is about 2 kilometers long. (Credit: NASA/JPL-Caltech/UMD) 8. There is nothing to fear from Comet ISON. Every comet brings with it some nuttiness from the UFO/conspiracy theory crowd, and Comet ISON is no exception. Now that the comet has disintegrated, the fears linger on. The main concern is a based on the true detail that Earth may pass through the outer edges of Comet ISON's debris trail later this month. But as the CIOC team explains, this will be no more intense than a typical meteor shower. In fact, it will be quite a bit less. Paul Wiegert of the University of Western Ontario estimates that the particles of Comet ISON reaching Earth will be "as fine as wood smoke," 1,000 times smaller than the typical shooting star. The total mass of the comet debris raining down on us? About one pound (half a kilo). Like I said, no fear. 9. We've entered the age of interplanetary astronomy. On November 28, anyone with an internet connection could watch Comet ISON's self-destructive swoop past the sun, nearly in real time. Even in an age of ubiquitous connectivity, the Comet ISON observing campaign marked a true milestone. The venerable Deep Impact comet probe watched from solar orbit. The Mars Reconnaissance Orbiter (MRO) watched the comet from Mars and MESSENGER watched from Mercury. The Hubble and Spitzer observatories scanned the comet from near-Earth space. The SOHO, SDO, and STEREO sun-monitoring probes captured the comet as it swung past the sun. What we are witnessing is the brain transcending the body: Humans now have eyes all across the solar system. 10. Comet ISON is never coming back, but sequels are on the way. Even if Comet ISON hadn't disintegrated, we never would have seen it again. It came in on a hyperbolic course, meaning it passes one time by the sun and then heads back to the Oort Cloud forever, perhaps breaking free from the sun entirely. But sequels are already on the way. On May 24 of this year, Earth will pass through the trail of Comet 209P/LINEAR, possibly producing a spectacular meteor shower. In August, the Rosetta spacecraft will rendezvous with
Comet Churyumov-Gerasimenko, and
then (all going well) drop a lander onto its surface in November. Just before that, on October 19, Comet Siding Springs will squeak 110,000 kilometers from Mars. The comet will pass so close that its coma (glowing envelope) will enshroud the planet and perhaps noticeably alter its atmosphere. Every spacecraft on and around Mars, including NASA’s upcoming MAVEN probe, will be watching. “It happens to be a particularly good time for interesting comets; not just ISON, all these others as well,” says Mike A’Hearn. But it's not just the comets getting better. We are getting better too, and Comet ISON--rest in peace--has already helped.
