New studies suggest that the comet's nucleus has a good chance of surviving its close passage of the Sun, but there are a lot of unknowns that could swing the result either way.
Comet ISON refuses to show its hand. Scientific pundits’ predictions have ranged from two pair to a straight flush, but for now the comet is playing it close to the vest.
Work presented at this week’s American Astronomical Society’s Division for Planetary Sciences meeting in Denver adds to speculations about the comet’s upcoming performance. These new observational and theoretical studies suggest that the comet’s nucleus has a good chance of surviving its sungrazing pass but hint that, like a sneaky card player, it might have a trick up its sleeve.
Matthew Knight (Lowell Observatory) and Kevin Walsh (Southwest Research Institute) took two approaches in their attempt to determine whether ISON will survive perihelion. First, they looked at the nucleus’s size. Both theoretical calculations and observations of sungrazing comets by the SOHO and STEREO spacecraft show that comets smaller than about 400 meters (>1,000 feet) across should disintegrate. The best estimate for ISON’s size is between 1,000 and 4,000 meters, so it shouldn’t completely evaporate, even in the 2,800° C (5,000° F) it will feel at its closest approach on November 28th, Knight said in a press conference on October 9th.
But the comet also has to contend with tidal forces. Close to the Sun, the comet will experience a stronger gravitational pull on the Sun-facing side than on its far side. These tidal forces could stretch the nucleus out like a cigar and, if the pull is strong enough, break the nucleus apart. Solar radiation will erode these smaller pieces more effectively, lowering the comet’s chances for survival.
The nucleus’s density and rotation have a big say in what the tidal forces will do to ISON, Knight explained. He and Walsh ran simulations using likely densities and spin speeds for comets and found that, in most scenarios, ISON should survive. But survival is less likely if the nucleus is spinning very fast or prograde (i.e. forward toward the Sun). He puts the odds of survival above 50%, but jokes that the error bars on that estimate are “plus or minus 100%.”
The prediction is so uncertain in part because astronomers don’t know ISON’s spin. Hubble observations by Jian-Yang Li (Planetary Science Institute) and his colleagues indicate that the comet’s rotational axis is pointing at the Sun. The team detected a slight uptick in brightness that they interpret as a jet from the nucleus, and because this jet appears in all the images they took over 19 hours (and in ground-based observations over a two-month period), its stationary-ness suggests it’s quite near the nucleus’s pole. But no one’s seen changes associated with which way the nucleus is spinning.
The sunward-pointing pole is where ISON’s mystery deepens. The comet is following a parabolic orbit, meaning that, thus far, it’s basically been on a straight-shot trajectory toward the Sun. If it’s pointing only one hemisphere at the Sun, the other hasn’t been heated yet — and won’t be until maybe a week before perihelion, when the comet passes within Mercury’s orbit. When that happens, pristine surface material such as carbon dioxide ice could “sublimate like crazy,” releasing a lot of dust and causing an outburst, Li said. But he cautions that his team determined the pole’s location from only one set of observations in April, and the result could change with follow-up. Hubble observed ISON this week, so that follow-up is imminent.
Any outburst could wreak havoc with the nucleus's spin, making its survival even harder to predict.
Of course there’s always the wild card. Knight noted that comets in the solar system spontaneously disrupt for unknown reasons maybe 1% of the time. That’s beyond researchers’ ability to simulate.
In other words, we’ll have to wait and see what we’re dealt.