NASA may have to wait a few days before launching a set of twins to the moon.
The two spacecraft _ Grail-A and Grail-B _ are due to blast off aboard an unmanned rocket Thursday morning. Forecasters say there's a 60 percent chance that storms will delay the flight. The weather should improve this weekend.
NASA is sending the probes on a long, roundabout trip to the moon. The spacecraft will orbit the moon, chasing one another in circles so researchers can measure the gap and the gravity below.
It will be the first lunar mission devoted to studying the insides of the moon. By measuring the entire gravity field of the moon, scientists hope to learn what the moon is made of all the way to its core.