The robotic explorer, named Juno, is atop an unmanned rocket at Cape Canaveral, Florida. Liftoff is scheduled for 11:34 a.m. (1534 GMT) Friday.
It will take Juno five years to reach Jupiter, the largest planet in the solar system. The spacecraft will be powered by three huge solar panels. It will be the farthest any solar-powered craft has ever traveled. Previous Jupiter probes have relied on nuclear energy.
Jupiter is believed to be the oldest planet in the solar system. Astronomers hope to figure out the recipe for making planets, by uncovering the ingredients of this gas giant. Juno will spend at least one year circling Jupiter's poles.