Nasa is preparing to bring its latest lunar mission to an end - by crashing two space probes into a mountain on the Moon.
The pair of spacecraft, Ebb and Flow, have been orbiting the Moon since their launch in September 2011 to create a gravity field map of its surface.
But with not enough fuel to carry out further experiments, the American space agency has opted for a "controlled descent" to avoid the risk of obliterating Neil Armstrong's footsteps on the moon.
The washing machine-sized probes will be propelled into the lunar body at 3,760mph on Monday at around 10.30pm UK time (5.30pm EST).
Maria Zuber, principal investigator on the Gravity Recovery and Interior Laboratory (Grail) mission , said the pair's destruction would be a poignant moment.
"It is going to be difficult to say goodbye," she said.
"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."
Ebb and Flow's final experiment will involve each firing their propellant tanks until they are empty, to establish how much fuel remained and help devise fuel consumption models for future missions.
But no-one on Earth will be able to see the collision as it will happen on the dark side of the Moon.
Grail project manager David Lehman said: "Our lunar twins may be in the twilight of their operational lives, but one thing is for sure, they are going down swinging.
"Even during the last half of their last orbit, we are going to do an engineering experiment that could help future missions operate more efficiently."
Nasa has deliberately crashed spacecraft into the Moon at the end of their useful life in the past, but has attempted to dispose of its rubbish more carefully in recent times.