Thursday, November 28, 2013

SO What Happened to Comet ISON? ANS. It's Still Too Close to the Sun

The Hubble Space Telescope captured this view of Comet ISON, C/2012 S1 (ISON), on May 8, 2013.

 In my Nov 1st blog post I noted that Comet ISON (an acronym for the International Scientific Optical Network in Russia, which discovered it in September 2012) was not yet visible. Earlier, I invoked temporary forecasts to say it ought to be brilliant by the dawn of Nov. 28, today. Well that didn't happen either. Why? The reason is that the comet is actually grazing the Sun's corona today - at a distance of about 730,000 miles (1.2 million kilometers). 

Nor is there any guarantee it will survive this close passage because at such proximity it will be subject to the Sun's heat and gravitational forces, the combination of which could see it disintegrate.  This is the fate forecast by Carey M. Lisse, a senior research scientist at the Johns Hopkins Applied Physics Laboratory during a press conference on Tuesday.

 Readers may recall in my October 19 blog post I cited an ASTRONOMY magazine article ('Comet Ison Blazes Into Glory', November, p. 54):

"Optimists tout numbers that have  many amateur astronomers giddy with excitement. If they're right the comet could rival the full Moon when it passes closest to the Sun at perihelion November 28 and remain as bright as the planet Jupiter for several days around then."

In retrospect and as I will show below, "bright as the full Moon" at perihelion-  was not likely to be a reality after later observations. However, the comet did undergo some brightening as noted at the website: 

"From Nov. 13 to 21, Comet ISON brightened by 3.5 magnitudes on the scale used to determine the brightness of objects in space. That's a 25-fold increase in brightness to the observer! Between Nov. 19 and 21 alone, ISON more than doubled in brightness"

For those who want to observe the comet, the new advice from many websites is to be watching just before sunrise or just after sunset for the 12-14 days after its solar perihelion passage today. I likely won't bother to dig out my binoculars until at least Tuesday or Wednesday. I suspect earlier than that will not likely allow a decent sighting since ISON is still too close to the Sun.

Why the scientific interest in ISON? Mainly because it's an Oort Cloud comet, formed a few million years ago, probably by the gravitational nudge of a passing star. That 'nudge' sent it hurtling toward the inner solar system for the first time and quite possibly the last.   Scientists have never before been able to watch an Oort Cloud comet dive past the Sun.

In all probability - from the data we now have- the early ASTRONOMY estimates of a 'full Moon' brightness at perihelion was overblown. 
As one report noted yesterday, scientists at first thought ISON was several miles in diameter and would become the “Comet of the Century,” "rivaling the brightness of a full moon."   But as the comet passed Mars, the NASA orbiter took pictures showing that it was three-quarters of a mile wide at most, smaller than most comets.   In this case, the comet wouldn't reach the brightness forecast, i.e. in ASTRONOMY.

Michelle Thaller, a NASA astrophysicist appearing this morning on MSNBC, gave an exuberant depiction of the comet from an "optimist's perspective". So she is hopeful it will survive it's close passage.  She compared it to Comet Lovejoy, which also made a close solar pass, and noted that it survived its passage. She said ISON is "quite a bit larger than Lovejoy" was - thereby implying a much better chance at survival. However, with a puckish look on her face he wryly noted "that if it breaks up it's in some ways better for NASA because we'll get to see what's inside".  She added we will have clues to what happened in our early solar system from 4 1/2 billion years ago.  Heady stuff indeed, but I suspect most ordinary folk will be hoping that break up doesn't happen!

As to the observing basics, and assuming it survives, Thaller noted it will be visible just before dawn and very near the Sun. As December progresses, she noted, it will be displaced farther and farther away from the Sun- until by Dec. 17 it will be in the East-southeast sky and also be visible at night. She said that "by the time you get to late December it should be roughly where the Big Dipper is in the sky. So assuming it survives you might be in for a very nice show in the sky".

One other remark of Thaller's is somewhat suspect, when she claimed a comet like ISON could be a "very nice probe of the Sun" adding that when such a comet gets very close it "interacts with the Sun's magnetic field, with the solar in some ways we're doing an experiment".

However, as a recent article in Physics Today ('Comets as Solar Probes', October, p. 27) has pointed out, ISON's perihelion distance of 1.2 million km puts it much farther out in the Sun's corona than Lovejoy's (135,000 km) passage. Since plasma density drops rapidly with altitude in the corona  ISON's solar interaction will be more dominated by waves and turbulence than by collisions and Lorentz forces, i.e. F = q(E + v X B). It is the latter which is truly useful in assessing key parameters for solar MHD or magneto-hydrodynamics.  However, as the the article notes, ISON will allow us to probe a region in the "nascent solar wind". Not as terrific as the denser corona, but not to be sneezed at.

We will wait to see what happens after today!


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