Most of the comets that astronomers study have made previous journeys into the torrid inner reaches of the solar system, often multiple times, as they orbit the sun. As a result, they have been cooked, evaporated and eroded in ways that erase much of their original nature. It is more like the dinosaur itself. Most sungrazing comets are scrappy objects spotted just hours before they race past the sun.
The first phase of that plan kicked off around the beginning of , as observers around the world began monitoring Comet ISON and sending reports to the Minor Planet Center in Cambridge, Mass.
From those observations, she documented a vigorous outpouring of gas and dust from the comet. Around the same time, Li led a team working with the famed Hubble Space Telescope, which clarified the origin of the outpouring.
The Hubble images captured an energetic geyser, almost 2, miles high 4, kilometers , shooting out from the sunward side of the comet. That geyser partly explains why Nevski and Novichonok spotted the comet so far ahead of perihelion, but it opened up a deeper mystery. The most common ingredient in a comet is ordinary ice, so comets typically remain fairly sedate until they receive enough sunlight to vaporize water. Comets do not generally get that hot until they approach Mars.
Meech thinks such flamboyant behavior reflects the unusual makeup of comets that have never experienced any warmth from the sun. She notes that as a new arrival from the Oort Cloud, Comet ISON might be covered with a layer of frozen carbon monoxide and carbon dioxide, gases that vaporize rapidly when exposed to even a slight trickle of heat. Solar wind creates a huge magnetic bubble, known as the heliosphere, that protects Earth and the other planets from energetic subatomic particles that constantly zip around in deep space.
You get lots of unstable molecular fragments. When they warm up, even a little, they recombine and tend to do so explosively. The super-reactive molecules had mostly been consumed, and ISON was now behaving more like an ordinary comet. At that point, astronomers realized they might have misjudged Comet ISON based on its front-loaded performance. Drawing on his data from the Hubble observations, Li estimates that the nucleus — the solid body of the comet itself — is no more than about 2.
On the other hand, the dramatically changing nature of Comet ISON as it boils away is exactly what scientists were hoping for.
With each new part of the comet exposed, a little more of its past — and our past — is revealed. The comet passed behind the sun as seen from Earth, and for two months, comet researchers were effectively blinded. That gave them some much-needed time to plan their next round of studies and to anticipate what to look for when the comet rounds the sun on Nov. The kinds of questions that observers like Li investigate are informed by an origin story that is substantially different from the one that was written into textbooks just a few years ago.
In the old view, the planets formed in an orderly manner, born from a swirling disk of gas and dust, known as the solar nebula, into stable orbits at their present locations from the sun. That interpretation left many details unclear, though, including how comets like ISON got tossed all the way out to the Oort Cloud while others were sent crashing into Earth. Then the doubts multiplied. The discovery of strangely ordered planetary systems around other stars showed that the formation process cannot be so tidy after all.
Some planets hug their stars, move in wildly stretched orbits or even circle backward. Such arrangements are possible only if newborn worlds interact and migrate in complicated ways. Another surprising clue came from comets that apparently derived some of their material from relatively warm parts of the young solar system. According to this theory, the other giant planets also shifted locations as they interacted with each other and with the thick disk of material swirling around the still-forming sun.
Comet ISON got swept up in the drama. As Jupiter migrated, its intense gravity stirred up everything around. It siphoned off raw material that would otherwise have reached Mars. The size has been estimated using images taken from the Hubble Space Telescope , which in reality only give us an upper limit.
It might even be smaller. How can that be? When the comet gets near the Sun this ice warms and turns directly into a gas. It escapes the weak gravity of the nucleus, forming the fuzzy coma around it. But on Nov. Since ISON was about million km 90 million miles from Earth at the time, that would put the coma at a size of about , km 80, miles.
Once the gas and ejected dust in the coma is out in space, it can be affected by both the solar wind and the pressure of sunlight. It streams away, forming one or more long tails.
The density of atoms in a typical tail can run up to about 50, atoms per cubic centimeter. Sound like a lot? If ISON is a typical mix of ice and rock, it has a density of about kg per cubic meters.
That may sounds like a lot, but remember, ice is far less dense than rock. A small rocky mountain would be far more massive. Doing the math, I get that this is about three tons per second — enough to fill an Olympic pool in about ten minutes.
In some cases the comet can disintegrate spectacularly. As of right now it seems to be OK, but who knows what the next few days will bring.
The closest it will get is on Dec. Right now, ISON is bright enough to see naked eye, and easily with binoculars. However, sometimes comets like this get incredibly bright when they are close to the Sun. But it failed to brighten at the rate expected as it hurtled towards the sun. If it is, the best time to look will be around 6. Looking east, the comet will rise before the sun, with its tail pointing straight up into the sky.
O'Brien is optimistic. We'll know in the next few days. The dramatic dimming is similar behaviour to a previous sun-grazing comet. In the comet Lovejoy skimmed the surface of the sun, passing eight times closer than Ison.
It too faded dramatically but then survived to develop a new tail as it sped away from the sun. For now, scientists are keeping a close eye on the situation as it unfolds. This article is more than 7 years old.
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