Gravity Recovery and Interior Laboratory
Mission Logo
Gravity Recovery and Interior Laboratory ( GRAIL ) was a lunar mission of NASA, which was conducted as part of the Discovery Program. The name alludes to English. (Holy ) Grail to = (Holy ) Grail.
GRAIL consisted of two jointly launched in September 2011 spacecraft circling the moon since the beginning of 2012. The aim of the mission was the accurate measurement of the lunar gravity field and gravity anomalies order to gain insights on the internal structure of the moon.
Structure of the probes
The mission was directed by the Jet Propulsion Laboratory. The probes were built by Lockheed Martin Space Systems based on the XSS -11 satellite. The power was two fold, but not pivotable solar modules ( 763 W), the lithium ion batteries boosted (30 Ah at 28 V). Hot gas thrusters and reaction wheels were used for position control. The data is transmitted through an S -band radio communication to the Deep Space Network (DSN).
The probes worked as the GRACE satellites, according to the SST- principle (Satellite -to- Satellite Tracking ): They rounded the moon on the same path and measured with the Lunar Gravity Ranging System ( LGRs ) by means of electromagnetic waves in the Ka-band continuously the mutual distance. This irregularity of the gravity field with high precision, could be analyzed.
As an additional payload, for educational purposes for pupils, was located on each of the two probes, the camera system I / PO MoonKam with five different cameras, the images of the lunar surface, the Earth and the probe transmitted itself. The probe is named after a nation-wide competition for school classes " Ebb " ( low tide) and " Flow" ( flood ).
Mission History
The two GRAIL probes launched together on a two-stage Delta 7920H -10C rocket on September 10, 2011 at 13:08:52.775 UTC from Cape Canaveral Air Force Station.
After the start there was a three and a half month transfer phase on the L1 Lagrange point of the Sun-Earth system to the moon, to keep the required velocity change is small and on the other hand, to achieve the very low lunar orbit of only about 50 km altitude as accurately as possible. The orbital period of the probes to the moon was 113 minutes. At this height, the two satellites following each other in about 175-225 km distance.
The actual science mission began on March 7, 2012, and had a duration of 82 days. This was divided into three cycles of 27.3 days. Twice a day were the probes for eight hours with the Deep Space Network (DSN ) in conjunction and transferred the data.
After completion of the primary science phase, an extension of the mission, it was decided that was to last from August 30 to December 3, 2012. For the probes decreased their amount to an average of 23 km, in order to still perform accurate measurements. After the end of the mission in December 2012 was a five-day decommissioning phase, after which the two satellites should open later on the moon, in order to avoid an uncontrolled crash on any significant sites such as the Apollo landing sites.
Both spacecraft collided with the scheduled mission end on 17 December 2012, an unnamed mountain between Philolaus and Mouchez near the lunar north pole. In order 22:28:51 UTC beat GRAIL A ( " Ebb " ) first on the lunar surface, about 30 seconds later, GRAIL B ( "flow" ). The point of impact of the two satellites was named by NASA after the deceased astronaut Sally Ride.
The GRAIL probes on the launcher
The Delta 7920H -10C before the start
The Delta 7920H -10C shortly before take-off
Thickness of the lunar crust