An artist's impression of Ebb and Flow mapping the lunar gravity field.

An artist's impression of Ebb and Flow mapping the lunar gravity field. Photo: NASA

NASA will smash two tiny probes into the moon next week after they spent months gathering data from orbit miles above the lunar surface, the US space agency said.

"We're not expecting a big smash, a big explosion," said project manager David Lehman of NASA's Jet Propulsion Laboratory. "Their fuel tank will be empty and they are the size of a washing machine."

Our lunar twins may be in the twilight of their operational lives, but one thing is for sure. They are going down swinging. 

Maria Zuber, Massachusetts Institute of Technology

The probes dubbed Ebb and Flow are set to end their controlled descent on a mountain near the Moon's north pole about 10.28pm GMT on Monday (9.28am Tuesday AEDT).

This image taken by Ebb and Flow shows the variations in the lunar gravity field on the moon.

This image taken by Ebb and Flow shows the variations in the lunar gravity field on the moon. Photo: AFP/NASA

They will hit the moon's surface at a whopping 6050 km/h, or 1.7 kilometres per second.

Unfortunately, NASA will not be able to gather pictures of the impact because the region will be in shadow at the time of impact.

The probes are being destroyed after running too low on fuel and sinking too low in orbit to conduct any further missions.

The probes managed to generate the highest resolution gravity map ever gathered from a celestial body. That will help provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved, NASA said.

"I couldn't have imagined even in my dreams that the mission would be so successful," said principal investigator Maria Zuber of the Massachusetts Institute of Technology.

"It is going to be difficult to say goodbye."

The probes have been flying in formation around the moon since January 1 starting with an average altitude of 55 kilometres above the surface, and then sinking to about 23 kilometres for a closer look.

At some points they were flying just a few kilometres above the moon's tallest mountains.

"Our lunar twins may be in the twilight of their operational lives, but one thing is for sure. They are going down swinging," Lehman added.

"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."

Ebb and Flow will fire their main engines until the tanks are empty, which will allow NASA to determine precisely how much fuel is left. That will help them improve predictions of fuel needs for future missions.

AFP