June 27, 2005: In a dress rehearsal for the rendezvous between NASA's Deep Impact spacecraft and comet 9P/Tempel 1, the Hubble Space Telescope captured dramatic images of a new jet of dust and gas streaming from the icy comet. The images are a reminder that Tempel 1's icy nucleus, roughly half the size of Manhattan, is dynamic and volatile. Astronomers hope the eruption of dust seen in these observations is a preview of the fireworks that may come July 4, when a probe from the Deep Impact spacecraft slams into the comet, possibly blasting off material and giving rise to a similar plume of dust and gas.See the rest:
Comets are thought to be "dirty snowballs," porous collections of ice and rock that dwell in the frigid outer boundaries of our solar system. Periodically, they make their journey into the inner solar system as they loop around the Sun. As a comet approaches the Sun, sunlight heats up its icy nucleus. Sometimes the extreme heat opens cracks in the comet's dark, crusty surface. Dust and gas trapped beneath the surface could then spew out of the crack, forming a jet. Sometimes a portion of the crust itself is lifted off the nucleus by the pressure of heated gases beneath the surface. This porous crust might then crumble into small dust particles shortly after leaving the nucleus, producing a fan-shaped coma on the sunward side. Ultimately, for comets bigger, brighter, and closer to the Sun than Tempel 1, the gas and dust that is ejected in jets or sublimated from the comet's surface stretches out in orbit behind the comet, forming its familiar, beautiful tail.
Due to its rotation, irregular shape, and uneven surface composition, the comet's solid nucleus is ejecting gas and dust at a variable rate. So the distribution of material around the comet is also irregular – denser in some places and less dense in others. The darkest wedge on the side opposite the jet is the shadow cast by the inner coma and nucleus.
Astronomers believe that this jet, if it lasts that long, will be on the side of the comet away from the Deep Impact spacecraft at the time of the encounter. So, no changes are planned in the flight path toward the comet.
The observations by the Deep Impact camera and the Hubble telescope complement each other. The camera aboard the Deep Impact spacecraft will provide a brief, close-up view of the comet, from more than 300 miles away. The Hubble telescope, from its distance 80 million miles away, will capture a broader view of the encounter. Hubble also will continue to monitor the comet's activities for several days after the event.
Hubble will study the expected plume of dust and gas that may be ejected from the surface of the comet and from below its surface as a result of the impact. By analyzing the Hubble observations, astronomers will determine the speed of the ejected material, how far the plume extends into space, and something about its chemical makeup. Hubble's observations will include images taken in visible light, as well as images and spectra taken in ultraviolet light. Some of the gases expected to be released, such as carbon monoxide, carbon, and sulfur, can best be seen in ultraviolet light. These gases come from ices that condensed out of the cloud of material from which the solar system formed.