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.
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.
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.
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.
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.
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.