The Lunar Atmosphere and Dust Environment Explorer (LADEE) was intentionally crashed into the moon by NASA but why did they crash the craft instead of leaving it in orbit, or was it a part of some mission to study how the impact takes place?
Once in orbit, always in orbit doesn't apply to the moon--the gravity is too uneven. Stuff left in lunar orbit generally crashes into the moon after a while. (All the Apollo landers that were left in lunar orbit are gone by now.)
In addition to observing the crash they also wanted to control the crash--make sure it didn't happen on top of anything sitting on the moon.
LADEE was going to run out of fuel, and they wanted - and succeeded - to collect data of Moon's atmosphere and levitated dust. (LADEE means 'Lunar Atmosphere and Dust Environment Explorer'.)
Fuel is needed to compensate orbital degradation due to Moon's non-symmetric gravitational field.
Online communication was possible due to superfast laser data transmission (Lunar Laser Communication Demonstration, or LLCD).
- The space probes are orbiting around the moon, below a certain altitude, and definitely at altitudes as low as 1 - 2 miles, fuel must be used to keep it in orbit. After it has completed its mission, there will be no/little more fuel left for it to escape the moon's gravity pull again. (Quote from previous reference
At the time of impact, the spacecraft will have used all of its fuel.) So the probes will crash anyways.
- From this New York Times article about a previous planned (of Grail), the purpose of a planned crash is so that the crash site does not destroy a historic landmark, such as Neil Armstrong's footprints. A less sentimental reason is that the crash site can also be strategically planned to help further missions locate a specific place on the moon, using the indentation the impact will create.