IBEX -- the Interstellar Boundary Explorer -- is the first U.S. spacecraft designed to map the dynamic interactions taking place "where the hot solar wind slams into the cold expanse of space," NASA said. The spacecraft is to be launched at 1:48 p.m. EDT Sunday, which is the midpoint of an 8-minute launch window.

IBEX will initially be deployed from an L-1011 aircraft over the Pacific Ocean about 125 miles north of Kwajalein. The spacecraft will then be carried into orbit by a Pegasus XL rocket built by Orbital Sciences, which also built IBEX.

Live coverage of the IBEX launch will be provided on the Web but no NASA television coverage is planned. The live streaming video of the countdown and launch will be available on the NASA home page at http://www.nasa.gov.