Buzzing the Moon
NASA’s twin GRAIL spacecraft, Ebb and Flow, crashed into the Moon recently. Their fuel was exhausted, their mission to map lunar gravity complete. Fare thee well, fine ships. The video above is a view of their final days, skimming a mere 6 miles above the gorgeous lunar surface. I’m jealous.
"You are go for fly-by, GRAIL. The pattern is not full."
The two spacecraft orbited our rocky satellite, one lagging behind the other, sensing slight fluctuations in each other’s orbits caused by slight differences in the Moon’s gravity. For instance one passed over a spot with slightly stronger pull, it would dip ever so slightly. Communicating via microwaves, the other spacecraft would sense that dip. And so they flew, bobbing and weaving and mapping.
Technically, the Earth and the Moon aren’t perfect spheres. However, for all intents and purposes we can pretend they are, as they are certainly more perfectly round than a billiard ball. The Earth actually bulges slightly in the middle from the tug of the Moon’s gravity, like a tectonic high tide.
We know that everything with mass exerts gravity. Even the coffee cup currently next to me is pulling me toward it, and I’m pulling it toward me, however infinitesimally imperceptible that pull may be. Actually, that tug might be because I need coffee, but you get the idea. What most people don’t realize is that objects like the Earth and Moon don’t have evenly distributed mass, and likewise don’t have completely even gravity.
Everywhere on the Moon that there’s slightly denser, heavier rock, there’s slightly more gravity exerted above that spot. The GRAIL mission mapped the Moon’s blips and bulges in the greatest detail ever, giving us this abstract-art-like map:
If you want to read more about Earth’s lumpy gravity, check out this post by Phil Plait.
NASA’s Grail Paints the Moon Abstract
It looks like some sort of modern art piece, but this actually represents the most high-resolution gravity map of the Moon ever created. Wait, scratch that. The highest resolution gravity map of anything in space, including Earth. The Earth, Moon and other celestial bodies look round, but they (and their gravity) aren’t perfectly even and spherical.
The GRAIL mission uses a pair of satellites orbiting the Moon who sense the slight bumps and blips in gravity by watching how their position relative to one another changes. We’ve looked at how that works before. The colors in the photo represent tiny fluctuations in the Moon’s gravity, just like we see on Earth.
It allows what would normally be obscured away from Earth’s view to peek around, just for a short time, thanks to the oscillation of the Moon’s Earth-side face. This image was captured by NASA’s GRAIL spacecraft, and chosen by MoonKAM students.
That’s us there, the little dot peeking around the edge. Hey, Earth!
GRAIL Mission Returns First Video of Moon’s Far Side
The twin GRAIL satellites have sent back this first video of the far side of the moon, taken with the awesomely named “MoonKAM”. Quite a view of something we don’t ever get to see (unless you’re an Apollo astronaut or a space probe).
In less than three days, NASA will deliver a double barreled New Year’s package to our Moon when an unprecedented pair of science satellites fire up their critical braking thrusters for insertion into lunar orbit on New Year’s Eve and New Year’s Day.
NASA’s dynamic duo of GRAIL probes are “GO” for Lunar Orbit Insertion said the mission team at a briefing for reporters today, Dec. 28. GRAIL’s goal is to exquisitely map the moons interior from the gritty outer crust to the depths of the core with unparalled precision.
“GRAIL is a Journey to the Center of the Moon”, said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology (MIT) in Cambridge at the press briefing.
After a three month voyage of more than 2.5 million miles (4 million kilometers) since launching from Florida on Sept. 10, 2011, NASA’s twin GRAIL spacecraft, dubbed Grail-A and GRAIL-B, are now on final approach and are rapidly closing in on the Moon following a trajectory that will hurl them low over the south pole and into an initially near polar elliptical lunar orbit lasting 11.5 hours.
GRAIL is a really exciting project to map the interior of the moon by detecting tiny gravitational differences and relating those to interior structure. They call it a “CT scan for the Moon”.
Here’s a superb video from NASA detailing the mission, why there’s a pair of satellites instead of one, and what they hope to learn:
These twin GRAIL spacecraft are setting out on a mission to map the Moon’s gravity. Here’s how it will work, from EarthSky:
Here’s how GRAIL’s gravity-mapping ability works. As one body in space orbits another, changes in the larger body’s topography – its hills and valleys, for example – minutely affect the orbital path of the smaller body by slightly increasing or decreasing the amount of gravity exerted upon it. As GRAIL records these changes, it will reveal details about mountains, craters, and other features on the moon, including those below the moon’s surface.
Once arriving at the moon, the twins will spend about two months sliding into orbit with GRAIL B following GRAIL A. After the proper orbit is established, an instrument on board each craft will measure relative changes in velocity, which can then be translated to map lunar gravity. The instruments are so precise they can detect a change in the distance between the two GRAIL orbiters the diameter of a red blood cell.
That’s some sensitive machinery, eh?