Later this year, Comet ISON is expected to become a naked-eye object when it skims through the atmosphere of the sun. The Hubble Space Telescope has just obtained a sneak preview.
Hubble photographed ISON on the 10th April. At the time, the comet was 386 million miles from the sun (394 million miles from Earth), just inside the orbit of Jupiter. Even at that great distance the comet is already active as sunlight warms the surface and causes frozen gases to vaporize. A detailed analysis of the image reveals a strong jet blasting dust particles off the sunward-facing side of the comet’s nucleus.
Preliminary measurements suggest that the ISON’s nucleus is no larger than three or four miles (~5 to 6 km) across and the comet’s dusty atmosphere, or “coma”, is approximately 3,100 miles across, or 1.2 times the width of Australia. A dust tail extends more than 57,000 miles, far beyond Hubble’s field of view.