Spectacular Comet ISON Pictured and Analyzed by Hubble Space Telescope

Eagerly awaited and spectacular comet C/ISON was now imaged with the Hubble Space Telescope on April 10 using the Wide Field Camera 3 on its approach towards the sun, when the comet was at about the distance of Jupiter: 621 million kilometers from the Sun and 634 million kilometers from the Earth. Scientists look forward to the arrival of the recently discovered and highly active comet that will skim 1.1 million kilometers above the Sun's surface on Nov. 28 this year and has the potential to be very brightly visible from Earth.

The comet, C/2012 S1 (ISON), is highly unusual in that it comes to the inner solar system for the first time and will skirt around the Sun within less than two solar radii from the Sun's surface in November.

Comet C/ISON was discovered in September 2012 when it was farther away from the Sun than Jupiter, and was already active at such a great distance. This is distinct from most other sungrazers – comets that pass extremely close to the sun – that are only discovered and remain visible for several hours nearest the Sun. At such a close perihelion distance from the Sun, sungrazers are expected to be intensely heated by the Sun, and sublimate not only ice but also silicates and even metals, releasing a tremendous amount of dust. The expectation is high that Comet C/ISON will be much brighter and more spectacular than most other sungrazers when it puts on a show late this year.

"As a first-time visitor to the inner solar system, Comet C/ISON provides astronomers a rare opportunity to study a fresh comet preserved since the formation of the solar system," said Planetary Science Institute Research Scientist Jian-Yang Li, who led a team that imaged the comet. "The expected high brightness of the comet as it nears the Sun allows for many important measurements that are impossible for most other fresh comets."

The team is using these images to measure the activity level of this comet and determine the size of the nucleus, in order to predict the comet's activity when it passes perihelion, or closest to the Sun, later this year. Preliminary measurements from the Hubble images suggest that the nucleus, the solid, icy body at the center of the comet, is no larger than three or four miles across. This is remarkably small considering the high level of activity observed in the comet so far. This small size also means that the outcome from its close perihelion passage near the Sun is extremely hard to foresee.

The comet is active as sunlight warms the surface and causes frozen volatiles to sublimate. The comet's dusty coma, or head of the comet, is currently approximately 5,000 kilometers across, or 1.2 times the width of Australia. A dust tail extends more than 92,000 kilometers, far beyond Hubble's field of view.

Whether Comet C/ISON will become a "Comet of the Century" and outshine all other bright comets in the past still remains to be seen.