NASA's moon twins going their own way

NASA's Moon Twins Going Their Own Way
Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's gravity field, as depicted in this artist's rendering. Image credit: NASA/JPL-Caltech

NASA's Gravity Recovery And Interior Laboratory (GRAIL)-B spacecraft successfully executed its first flight path correction maneuver Wednesday, Oct. 5. The rocket burn helped refine the spacecraft's trajectory as it travels from Earth to the moon and provides separation between itself and its mirror twin, GRAIL-A. The first burn for GRAIL-A occurred on Sept. 30.

"Both spacecraft are alive and with these burns, prove that they're kicking too, as expected," said David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "There is a lot of time and space between now and lunar orbit insertion, but everything is looking good."

GRAIL-B's rocket burn took place on Oct. 5 at 11 a.m. PDT (2 p.m. EDT). The spacecraft's main engine burned for 234 seconds and imparted a velocity change of 56.1 mph (25.1 meters per second) while expending 8.2 pounds (3.7 kilograms) of propellant. GRAIL-A's burn on Sept. 30 also took place at 11 a.m. PDT. It lasted 127 seconds and imparted a 31.3 mph (14 meters per second) velocity change on the spacecraft while expending 4 pounds (1.87 kilograms) of propellant.

These burns are designed to begin distancing GRAIL-A and GRAIL-B's arrival times at the by approximately one day and to insert them onto the desired lunar approach paths.

The straight-line distance from Earth to the moon is about 250,000 miles (402,336 kilometers). It took NASA's Apollo moon crews about three days to cover that distance. Each of the GRAIL twins is taking about 30 times that long and covering more than 2.5 million miles (4 million kilometers) to get there. This low-energy, high-cruise time trajectory is beneficial for mission planners and controllers, as it allows more time for spacecraft checkout. The path also provides a vital component of the spacecraft's single , the Ultra Stable Oscillator, to be continuously powered for several months, allowing it to reach a stable operating temperature long before beginning the collection of science measurements in lunar orbit.

GRAIL-A will enter on New Year's Eve, and GRAIL-B will follow the next day. When science collection begins, the spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon. Regional gravitational differences on the moon are expected to expand and contract that distance. GRAIL scientists will use these accurate measurements to define the moon's gravity field. The data will allow mission scientists to understand what goes on below the surface of our natural satellite.

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Oct 07, 2011
Wondering what they use as a reference for making the measurements, since orienting a camera (i.e. to look at reference stars, etc,) or other similar device would change the orbiter's center of mass.

There's no GPS locators on the Moon (That I'm aware of anyway).

They can use radar or Lidar for range finding, but radar is obviously distorted by the topography and even composition of the soil.

In order to detect changes in gravity (presumably to find changes in internal density of the Moon) you need to know both your altitude and coordinates EXACTLY to within a centimeter, maybe even a millimeter.

Maybe I should read up some more on this experiment.

Oct 07, 2011
who references miles per hour to meters per second ?? wouldn't km per hour be the correct equivalent?

Oct 07, 2011
who references miles per hour to meters per second ?? wouldn't km per hour be the correct equivalent?

Actually, meters per second, (m/s,) is the basic SI velocity measurement in physics.

Oct 09, 2011
yes its a basic measurement in physics - but when comparing miles to the SI equivalent km is the most appropriate comparison .. especially velocity when all you are trying to get across is a general since of speed in an article.

I think 90 kph is a better reference for the lay person.

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