The Baltimore-based Space Telescope Science Institute says the Comet Ison is being called the "comet of the century" because it could be brighter than the full moon when it makes its closest pass by the sun in late November.
The institute says the pictures taken earlier this month show the comet slightly inside Jupiter's orbit, about 386 million miles from the sun. The institute serves as the science operations center for the Hubble.
Hubble scientists say preliminary measurements show the nucleus of the comet is about three to four miles across. The comet was discovered in September by a Russian team.