(NBC News) -- One year after their arrival at the moon, NASA's twin Grail spacecraft are getting a grand sendoff to oblivion, including live video commentary that will climax with today's well-orchestrated crash into a lunar mountain. But if you're watching the webcast, which is due to begin at 5 p.m. ET on NASA TV and Ustream, don't expect to see any video of the crash itself.
"There is no flash expected," NASA spokesman D.C. Agle said.
Each of the probes is the size of a washing machine, so they won't make a fiery impact — even though they're due to hit the surface around Goldschmidt Crater's rim at a smashing velocity of 3,760 mph (1.7 kilometers per second). What's more, at the time designated for the crash, 5:28 p.m. ET, the impact site will be in shadow.
Eventually, NASA's Lunar Reconnaissance Orbiter will pass over the site and take stock of the crash site, Agle told me. Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said they expected the Grail probe to be completely blown apart by the impact — but there's a chance that the blast might kick up some water ice or something similarly interesting that the orbiter could detect.
The Grail mission's spacecraft entered lunar orbit on Jan. 1 and have completed their $496 million mission to map the moon's gravitational field in unprecedented detail. ("Grail" is an acronym standing for Gravity Recovery and Interior Laboratory.)
Earlier this month, the scientists behind the mission reported evidence that the moon was significantly more battered than they thought, early in its history. The fracturing goes deep into the crust and perhaps down further, into the mantle. Researchers also determined that the moon's crust goes 25 miles (40 kilometers) deep, which is not nearly as deep as previously thought.
Such findings are expected to shed light on the process that influenced the formation of the moon as well as Earth, billions of years ago. Before the mission, some scientists suspected that two moons smashed together to form the modern-day moon, in an ancient event nicknamed the "Big Splat." Grail's findings provided no evidence to back up that hypothesis, however.
The Grail mission made a huge contribution to education and public outreach: The two probes were named Ebb and Flow by elementary-school students from Bozeman, Mont., who participated in a nationwide contest. Both Ebb and Flow were equipped with cameras that could be pointed at targets selected by students from around the world. The MoonKam project was the brainchild of Sally Ride, the first American woman in space, who passed away in July.
The mission's principal investigator, MIT planetary scientist Maria Zuber, said "it is going to be difficult to say goodbye" to the probes. "Our little robotic twins have been exemplary members of the Grail family, and planetary science has advanced in a major way because of their contributions," she said in a NASA news release.
Avoiding Apollo sites
NASA opted for a controlled crash primarily to make sure that the Grail didn't end up hitting a historic site on the moon, such as the landing zones for the Apollo lunar modules or unmanned U.S. or Soviet probes.
The rocket burns that set the stage for the crash were successfully executed on Friday. Ebb is due to be the first of the two spacecraft to go down, at 5:28:40 p.m. ET. Flow's crash should occur 20 seconds later. For what it's worth, Ebb and Flow will pass closest to the landing sites for Apollo 12 and Surveyor 3 during their final orbit.
Grail project manager David Lehman said that before the burns, mission navigators calculated a seven-out-of-a-million chance that one of the probes would hit a historic site. "Now, after these two successful rocket firings, there is zero chance," he reported Friday.
During their final hours of existence, the two probes are being used for one last experiment. Mission engineers are having the spacecraft fire their engines until all their remaining fuel is gone, to compare the computer models for fuel consumption against the actual figures.
"Fuel gauges in space are rather challenging, because fuel doesn't sit on the bottom like it does in car tanks on Earth," Agle explained. Grail's last experiment should help engineers get a better handle on the fuel requirements for future missions to the moon, Mars and other cosmic destinations.
"Our lunar twins may be in the twilight of their operational lives," Lehman said, "but one thing is for sure, they are going down swinging."