GRAIL / The Moon gravity lab

Two spacecraft. One remarkable Moon.

Follow Ebb and Flow. Explore how the changing space between two orbiters can help us learn about a world beneath them.

NASA GRAIL: Ebb (2011-046A) & Flow (2011-046B)Degree 660 Field

0.02 µm/s Microwave Ranging & The Sally K. Ride Site

Tandem Lunar Orbiters · Slashed Apollo crust estimate in half · 100,000+ MoonKAM Students

Impact Site

Sally K. Ride Impact Site

75.62° N, 333.37° E

Ranging Precision

0.02 µm / s

1/400th of a red blood cell/sec

Mean Crust Thickness

38 km

Down from Apollo 60 km guess

MoonKAM Student Photos

119,830

Led by Dr. Sally Ride

Impact Speed

1.68 km/s (6,050 km/h)

LROC M1115786064R

The 0.02 Micrometer-per-Second Ranging Instrument

As Ebb and Flow flew tandem over craters and mountain ridges, gravity pulled the lead spacecraft forward, stretching the distance between them. The Lunar Gravity Ranging System measured this stretch with Ka-band microwaves.

Tandem Spacecraft Separation:150 KilometersMicrowave Signal Round-Trip: 1.00 ms

75 km (High-Resolution Near-Pass Mode)150 km (Nominal Mapping Separation)225 km (Primary Science Orbit)

Tandem Skimming Flight at 15–55 km Altitude

Ebb (GRAIL-A)

Lead Craft

Flow (GRAIL-B)

Trailing Craft

Ka-band 32 GHz carrier · Relative Velocity Shift Precision: 0.00002 mm/s

Look deeper: mission background

Authentic NASA Mission Context: GRAIL Ebb & Flow

In 2012, NASA's GRAIL mission mapped the gravitational field of the Moon with unprecedented precision. By measuring small variations in the distance between the twin spacecraft down to 20 nanometers per second using Ka-band microwaves, scientists proved the Moon's crust is only 34 to 43 km thick (slashing previous Apollo seismic estimates almost in half) and discovered ancient magma dikes up to 500 km long. Over 100,000 students participated through Dr. Sally Ride's MoonKAM camera program before the twins ended their mission on a mountain ridge named in her honor.