An unmanned rocket was set to blast off from Cape Canaveral, carrying twin probes for moon exploration. But the launch had to be delayed due to gusty winds.
NASA said it will try again Friday, even though the weather doesn't look too promising.
The so-called Grail mission is the first in 50 years dedicated to measuring the moon's gravity.
The twin spacecraft named Grail-A and Grail-B will go into orbit around the moon and chase one another. By measuring the varying gap between the two satellites, scientists will be able to create a precise map of the moon's gravitational field. That will shed light on the composition of the moon's interior, as well as the moon's evolution over the past 4 billion years.
The Grail mission - short for Gravity Recovery and Interior Laboratory - costs $496 million.
Launch time is 8:33 a.m. EDT. The space agency has just two single-second launch windows every day.
The Associated Press contributed to this report.