Update 6:18 p.m. EDT: the orbiters have launched! Follow their mission progress.

At approximately 5:12 P.M. EDT today, a pair of robotic lunar “scouts” will hopefully be launched aboard an Atlas V rocket. Their mission is to map the moon’s surface, search for ice, and assess levels of radiation in the environment. The flight is the first U.S. mission to the moon in more than a decade, and the first step toward returning humans by 2020.

The first of the two lunar probes is a spacecraft called the Lunar Reconnaissance Orbiter (LRO), and it is part of NASA’s Vision for Space Exploration. Richard Vondrak, a project scientist for LRO, told me during an interview last year that “LRO is the most advanced lunar satellite NASA has built. It will provide information that would have been impossible to collect a few decades ago.”

LRO will carry seven instruments, including a cosmic-ray telescope, to measure the effects that lunar radiation could have on humans, and a laser altimeter, to map the surface of the moon. It will also take high-resolution images and temperature measurements.

The second lunar probe is a smaller spacecraft called the Lunar CRater Observation and Sensing Satellite (LCROSS), and it has a simpler mission: to crash into the surface of the moon where experts believe that ice may be present. Unlike LRO’s four-day cruise to the moon, it will take LCROSS months to get there. It will ride on the Atlas V upper stage and will first observe the upper stage’s smackdown before flying through the plume and crashing into the moon’s surface minutes later. LRO and the Hubble Space Telescope will observe the collisions, hopefully viewing and sensing any water ice that gets thrown up.

Engineers are anxious to get the probes into space–the $583 million mission has been delayed since October 2008, and has endured more setbacks this week due to Space Shuttle launch delays. In addition, China, Japan, and India have launched lunar probes within the past three years. (China’s Chang’e 1 and Japan’s Kaguya both launched in 2007 and crashed into the moon this year.) Russia and Europe have more recently joined in, announcing their own lunar ambitions but not fully disclosing their plans. The race to return humans to the moon is well under way.

The probes have three chances to launch this evening, and a few opportunities on Friday. But the flight window closes on Saturday, and then it will be another two weeks before the spacecrafts get another shot at liftoff.