Astronomers are expecting a lot out of Comet ISON. Due to pass incredibly close to the sun on November 28 of this year (700,000 miles from the star’s surface), the frozen rock will make for a stunning sight, potentially outshining even the full moon. A Hubble Space Telescope photo released yesterday provides the clearest view of the comet so far and suggests a few surprises.
This image of ISON was snapped while the comet was about 286 million miles from the sun (slightly closer than Jupiter’s orbit). It shows the distant object should be as fantastic a sight as astronomers expect, while also clueing them in to some of its surprising properties.
Like all comets, ISON’s heart is a “dirty snowball” of a nucleus—a solid mix of rock, dust, and frozen gases. The nucleus is smaller than expected, measuring a mere three or four miles across, according to early measurements from the Hubble data, which were gathered April 10. This is surrounded by the coma, also known as a comet’s head, a much larger mix of gas and dust. In ISON’s case this is roughly 3,100 miles across, or 1.2 times the width of Australia.
As ISON continues to travel at about 47,000 miles per hour toward the sun, the heat will begin to evaporate the gases in the comet’s nucleus. In fact, Hubble’s close look at the coma showed that the sun-facing side of the nucleus sports an unexpectedly strong, 2,300-mile jet of warming gases and dust. And bringing up the rear, the comet’s tail extends more than 57,000 miles, past the edges of this photo.
The more astronomers know about Comet ISON, the better they’ll be able to predict its behavior as it approaches us and the sun. Not only is this useful for skygazers hoping to get the best views of what could be the “comet of the century,” but it will also teach astronomers more about conditions in the frigid outer reaches of the solar system, where ISON originated. Unfortunately, its unusually small nucleus makes a precise forecast of its trajectory and activity extremely difficult. But despite this minor uncertainty, the comet poses no threat to us (on December 26 it’ll pass about 40 million miles from Earth) and should prove to be one of the most spectacular sights of the sky in recent memory.
Technically known as Comet C/2012 S1 (ISON), the comet is named for the International Scientific Optical Network, an international collaboration of observatories that discovered the comet in September 2012. To find out more about the comet, check out the in-depth coverage in Astronomy Magazine.