Comet ISON is potentially the "comet of the century" because around the time the comet makes its closest approach to the Sun, on November 28, it may briefly become brighter than the full Moon. Right now the comet is far below naked-eye visibility, and so Hubble was used to snap the view of the approaching comet, which is presently hurtling toward the Sun at approximately 47,000 miles per hour.
When the Hubble picture was taken on April 10, the comet was slightly closer than Jupiter's orbit at a distance of 386 million miles from the Sun.
Even at that great distance the Sun is warming the comet enough to trigger outgassing from its frozen gases locked up in the solid nucleus. Hubble photographed a jet blasting dust particles off the sunward-facing side of the comet's nucleus.
Preliminary measurements from the Hubble images suggest that the nucleus of ISON is no larger than three or four miles across. The comet was discovered in September 2012 by the Russian-led International Scientific Optical Network (ISON) using a 16-inch telescope.