Thunderstorms and the threat of lightning early today forced NASA officials to scrub their launch of the agency's first space probe to Mercury in 30 years.

The poor weather, an expected offshoot of Tropical Storm Alex churning off the eastern U.S. coast, delayed the Mercury-bound MESSENGER spacecraft's launch from Cape Canaveral Air Force Station in Florida for at least 24 hours. Thick cloud cover also factored into the scrub decision.

"Lightning and launches don't mix well," said Omar Baez, MESSENGER's assistant launch director at Kennedy Space Center (KSC) in Florida, during countdown operations.

The launch attempt was officially aborted at 2:11 a.m. EDT (0611 GMT) during a scheduled hold with just four minutes remaining in the countdown. The spacecraft was originally set to ride a Delta 2 rocket into space at 2:16:11 a.m. EDT (0616:11 GMT) and had a 12-second launch window to make the space shot.

Pad crews are now working to launch MESSENGER Tuesday at 2:15:56 a.m. EDT (0615:56 GMT) - also with a 12-second window - when weather conditions are expected to be more favorable. The mission itself has a 12-day launch window starting Aug. 2 in order to reach Mercury.

MESSENGER, an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, is the first NASA spacecraft to Mercury since Mariner 10, which swung by the planet in 1974. It is expected to pick up where the Mariner left off, the first step of which is filling in the gaps of a global map of Mercury that is currently only half complete.

"Here, we've got Mercury so close to us in the solar system and we've only seen one side of it," explained Ralph McNutt, a MESSENGER science team member with the Johns Hopkins University Applied Physics Laboratory, during the countdown.

Researchers also hope MESSENGER will answer questions about Mercury's above average density for its size, as well as its composition, surface and magnetic field. The spacecraft will take about seven years to reach Mercury, swinging by Earth, Venus and Mercury itself before entering orbit around March 2011.

Weather difficulties expected

Weather officials at Cape Canaveral were aware that poor weather conditions could affect MESSENGER's launch, and estimated a 30 percent chance that the space shot could be delayed.

"While everyone here is looking forward to the next seven years, I'll be very happy to get through the next 36 hours," said Joel Tumbiolo, U.S. Air Force Delta 2 weather officer, during a prelaunch press briefing on July 31.

While the Tuesday launch date also carries a 30 percent risk of weather violations, launch officials said that regional thunderstorms should largely be confined to daylight hours when tower roll back procedures are scheduled. The lightning conditions threatened by clouds from Tropical Storm Alex should not be a factor during the countdown, they added.