Odds looking good Comet ISON will beat the heat

Adam Block took this beautiful closeup of Comet ISON at Arizona’s Mount Lemmon Observatory on Oct. 8. Credit: Adam Block

Looks like Comet ISON may have what it takes to take a licking and keep on ticking. A new study by scientists at the Lowell Observatory and Southwest Research Institute (SwRI) indicates that Comet ISON is big enough to survive both the intense heat and gravitational beating it will receive from the sun when it passes just 730,000 miles above its surface on November 28.

Sizes of five comets we’ve visited and photographed with spacecraft. Click to enlarge. Credit: NASA/JPL-Caltech/UMD

Size matters in the cometary world. Comets smaller than about 650 feet (200 meters) almost always break apart and boil away when passing as close as ISON will to the sun. But preliminary measurements by both the Hubble and Spitzer Space Telescopes coupled with computer simulations show that the comet’s nucleus measures between 0.5 to 3 miles (1-5 km), large enough to remain intact unless it’s unusually fluffy.

Artist view of Comet ISON during its extreme encounter with the sun on Nov. 28, Thanksgiving Day. Its surface will broil in temperatures up to 5,000 degrees F. Credit: NASA

Not only will ISON experience temperatures approaching 5,000 degrees F (2,760 C) when it careens around the sun moving at more than 800,000 mph, the flying ball of dirty ice will be stretched and pulled by the sun’s powerful gravity much like the way the moon’s gravity stretches the Earth’s oceans and crust to create the tides.

View of ISON taken with the SECCHI H-1 camera on NASA’s STEREO-A solar probe on Oct. 10. STEREO is now the 7th spacecraft to observe the comet. Click photo for a short movie and more information. Credit: NASA

While ISON could be stretched to the breaking point, it would have to be unusually porous or fluffy for that to happen. The best estimates of the similar comets’ densities point to ISON remaining intact despite the sun’s best efforts to take it apart. This according to a study by Matthew Knight and Kevin Walsh that appeared in the Astrophysical Journal on Sept. 24.

Every planet, asteroid and comet in the solar system including ISON spins or rotates on its axis. For sungrazing comets, the direction of that spin can affect its fate. A “backward spinning” comet – one that rotates from east to west instead of the typical west to east of the sun and most of the planets – helps to cancel out the some of the powerful tidal forces felt by the comet around the time of perihelion.

It ain’t much, but it’s there. Comet ISON in the “SWAN” instrument on the Solar and Heliospheric Observatory on Sept. 28. The SWAN instrument detects the glow hydrogen in the coma of comet ISON – information scientists can use to estimate the rate at which ISON is producing water. Credit: NASA/ESA

But if ISON rotates in the forward or “prograde” direction, it will be spun up by the sun around the time of closest approach, which would put more stress on the nucleus. Not to worry yet. The study indicates that ISON’s packed together tightly enough to resist a rotational breakup.

One of the most tantalizing predictions for the comet comes from research scientist Jian-Yang Li at the Planetary Science Institute. Li and his team imaged the comet in detail and discovered it rotates with its north pole presently facing the sun. In other words, ISON’s icy nucleus rotates on its side compared to Earth.

Said Li: “We measured the rotational pole of the nucleus. The pole indicates that only one side of the comet is being heated by the sun on its way in until approximately one week before it reaches it closest point to the sun.”

Since the “dark side” or opposite pole of ISON wouldn’t become exposed until the comet moved within the orbit of Mercury, fresh ice suddenly exposed to the heat of the sun could elicit a massive outburst of material. Presumably the comet would vault in brightness as a result.

I’ll take that as more good news. Current best guesses put the comet at about the same brightness as the planet Venus (-3 to -5 magnitude) around the time of perihelion.

This image shows the color change of ISON’s dust coma. The white dot at the center of the coma marks the location of the nucleus. ISON’s dust coma appears to be less red near the nucleus than it is further away from the nucleus. Although the color change is actually very small, it could be an indication of relatively more water ice particles near the nucleus. Those icy particles evaporate, as they move outward, makes the coma appear redder. Credit: NASA, ESA, J.-Y. Li (Planetary Science Institute) and Hubble Comet ISON Imaging Science Team

Li and team also measured the color of the comet’s coma or tenuous atmosphere of gases, dust and boiled-away ice grains around the nucleus, and discovered that the outer coma is slightly redder than the inner, implying that ice grains released by the nucleus are vaporizing into gas as they’re pushed tailward by sunlight.

Typical sungrazing comets are discovered only days or hours before their close encounter with the sun. Most get zapped in the solar heat like the little sungrazer found in SOHO images earlier this week. ISON was nabbed more than a year before perihelion and may have “the right stuff” to complete its fiery flyby AND put on a great show later this fall.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

8 thoughts on “Odds looking good Comet ISON will beat the heat

  1. All 3 of the brightening trio of comets should be between magnitude 9 and 10 now. But, I have heard no reports of 1 of them brighter than 10 yet.

  2. The Sun and ISON are coming closer to each other at over 20 miles a second. That is over 30 miles a second, closer to Earth. The comet is continually gaining speed.

    • Edward,
      The comet’s velocity increases continuously as it approaches the sun. Careful though of wording – while the two are getting closer at over 20 mps, ISON’s the one doing the moving in the direction of the sun.

  3. Bob,the PSI study is especially interesting – and potentially important. Almost like Ison is ‘saving its ammo’ for perihelion and beyond. It may be that those ‘breaking-up’ reports were somewhat premature.Ison may be something special.

  4. Hi…I think we just saw Ison fly over the Iotla Valley in Franklin NC tonight. It was a huge golden orb and it passed the entire length of the sky from Wayah Bald to Cowee Mountain, and then went out of sight. it was pretty fast and had no sound. We cannot find any other sightings in NC, so if anyone can clarify if the comet would have been over Western NC, that would be great :) we live right at the airport, and the orb did not have a flashing aviation light or anything. We saw it about 10PM on October 18, 2013. Thanks so much!

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