Even at that great distance, Comet ISON is already active as sunlight warms the surface and causes frozen volatiles to sublimate. A detailed analysis of the dust coma surrounding the solid, icy nucleus reveals a strong 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 Comet ISON is no larger than 3–4 miles (5–6 kilometers) across. This is remarkably small considering the high level of activity observed in the comet so far, said researchers. Astronomers are using these images to measure the activity level of this comet and constrain the size of the nucleus in order to predict the comet’s activity when it skims 700,000 miles (1.1 million km) above the Sun’s roiling surface on November 28.
Comet ISON’s dusty coma, or head of the comet, is approximately 3,100 miles (5,000km) across, or 1.2 times the width of Australia. A dust tail extends more than 57,000 miles (92,000km), far beyond Hubble’s field of view.
More careful analysis is currently underway to improve these measurements and to predict the possible outcome of the sungrazing perihelion passage of this comet.
This image was taken in visible light with Hubble’s Wide Field Camera 3. The blue false color was added to bring out details in the comet’s structure.
ISON stands for International Scientific Optical Network, a group of observatories in 10 countries who have organized to detect, monitor, and track objects in space.