The unmanned Falcon 9 rocket is set to blast off at 1905 GMT from Cape Canaveral, Florida. It's the company's second attempt to launch the Deep Space Climate Observatory for NASA, the National Oceanic and Atmospheric Administration and Air Force.
Last-minute radar trouble halted Sunday's countdown. SpaceX skipped Monday because of rain.
NASA says former Vice President Al Gore is returning for the launch. It was his idea in 1998 that led to this observatory.
The spacecraft will fly to a point 1.6 million kilometers from Earth to monitor solar outbursts.