Next month's launch of NASA's tandem lunar exploration probes will be delayed until at least June 17 to finish analysis between the Atlas 5 rocket's upper stage and the impactor spacecraft payload, officials said Friday.

The LRO spacecraft sits atop LCROSS at the Astrotech facility. Credit: Stephen Clark/Spaceflight Now

The Lunar Reconnaissance Orbiter and Lunar Crater Observation and Sensing Satellite were scheduled to launch June 2, but NASA announced Friday they are delaying the launch about two weeks.

Engineers will use the extra time to "mitigate a potential thrust disturbance" for the LCROSS impactor associated with fill and drain valves on the rocket's Centaur upper stage, according to a NASA statement.

Launch opportunties to reach the moon are available for about four days every two weeks.

The next window opens June 17 for four days. Launch would occur in the afternoon and evening hours each day, officials said.

The Centaur is being used as an impactor by LCROSS, a kamikaze mission that will excavate potential water ice from a shaded impact crater near the lunar south pole.

United Launch Alliance, the launch services provider for the mission, has conducted extensive modeling of the Centaur's predicted performance during the unusual mission. The stage must vent residual hydrogen and oxygen propellant overboard to prevent the fuel from skewing science results.

The mission is looking for water ice, which is also composed of oxygen and hydrogen.

The delay also broadens the scientific return from LCROSS, officials said.

"That launch block looks far superior to the June 2 block for science return," said Dan Andrews, LCROSS project manager at NASA's Ames Research Center.

The spacecraft were scheduled to be encapsulated inside the Atlas 5 rocket's four-meter payload fairing Friday. The payloads are slated to move to Complex 41 no earlier than May 21 to be bolted to the launcher's Centaur upper stage.

"We are essentially ready to launch from a spacecraft standpoint. They are stacked, they're cleaned and they're fueled," said Craig Tooley, LRO project manager at NASA's Goddard Space Flight Center.

Engineers assembed the Atlas 5 inside the pad's vertical hangar late last month. The rocket, minus its payloads, rolled to the pad this week for a countdown dress rehearsal.

LCROSS will stay attached to the Centaur for a four-month cruise through space. The mission's shepherding spacecraft will separate from the Centaur seven hours before hitting the moon, observing the spent stage as it hurtles into a dark crater at the lunr south pole.

The pair will swing past the moon a few days after launch, using lunar gravity to slingshot into a high-altitude Earth orbit, where the spacecraft will loiter until impacting around Oct. 8, according to Andrews.

"What's nice about the mid-June block is it allows us to have a south pole swing-by, which allows us to save a lot of fuel, enabling us to use that for contingency cases," Andrews said.

The trajectory from a mid-June launch would also mean the lunar smash will be at a much higher angle.

"The higher the impact angle, the more ejecta that gets kicked up, and therefore a much better likelihood of good science to come out of it," Andrews said.

The Atlas nose cone stands poised to encapsulate LRO and LCROSS. Credit: Ben Cooper/Spaceflight Now

A slew of sensors and telescopes will be turned toward the moon to observe the LCROSS impact, including LRO, the Hubble Space Telescope and an array of ground-based observatories. The instruments will look for evidence of water ice molecules in the plume of material thrown into space by the impact.

Scientists must reserve time on those instruments well ahead of time.

Officials have to balance all these requirements in selecting a launch date because the mission trajectory is different every day.

"It's a fairly complicated problem to solve, and it looks like the mid-June opportunities are the optimal ones," Tooley said.

Scientists do not have to choose a target crater until a month before the impact. They will use early images and data from LRO to help make a decision. Researchers are particularly interested in signatures of hydrogen embedded deep within permanently dark craters that could harbor water ice.

LRO will activate and test its instruments between launch in June and an impact in October, Andrews said.

"It's nice that we can actually decide which crater we're going to hit after we've launched," Andrews said. "The cost in propellant to go from one crater to another, as long as we stay on the same pole, is very small, almost negligible. It allows us and the science community to really optimize what is the sweet spot and go for that."

LCROSS officials plan to hold a crater selection conference late this summer to finalize their decision. Mission managers are consulting with a broad group of lunar scientists to help pick a target, officials said.

Andrews said there are about seven candidate craters scientists are considering at the south pole. They are ranked on scientific value and the spacecraft's targeting ability.

Shackleton Crater, a favored location for early moon colonization, has fallen down the list of ranked craters. Other smaller craters now top the list, according to mission officials.

"We can actually hit smaller targets, which turn out to be the ones that the science community really wants to go for anyhow," Andrews said.