The two probes have outlived their utility and will be crashed
The twin Grail spacecraft, Ebb and Flow, that have spent the past year studying the moon's gravity are set for a dignified but explosive end by crashing into our natural satellite. The two probes have outlived their utility and with fuel running out they would crash by themselves eventually.Which is why NASA is using this opportunity to learn and carry out one last experiment, by firing up the engines and running them for as long as they work, to figure out how much fuel the probes had left.
This will provide valuable data on fuel consumption, great in simulations and estimates for future crafts.
The two probes will be crashing in a shadowed area of the moon and there's little fuel to cause an explosion, so amateur astronomers won't be able to see the event. At 3,760 mph, 6,051 km/h or 1.7 km per second, the impact will leave little of the craft behind.
Fortunately, NASA is live streaming the entire thing. The stream starts at 5:00 pm EST (22:00 GMT) and the crash itself will occur at 5:28 pm EST (22:58 GMT) later today.