Puli Space Technologies

Small Step ClubPuli Space Small Step Club


August 2011: The Puli number of the month - The gravity of the situation

1622. No, not the year 1622, 1.622 m/s2, the gravitational acceleration on the surface of the Moon, basically the amount of gravity pulling you, sticking your... boots to the lunar soil (bare feet are not recommended). That's, as you may recall, about one sixth compared to the g here on Earth which is 9.81 m/s2. And it has some consequences.

John Young, commander of Apollo-16, does the Big Navy Salute. He jumped up over knee height in a heavy spacesuit. Twice.

First, you'd – or your rover would – “feel” lighter. Just look at the Apollo astronauts. Their space suits were bulky and very heavy, about 80 kg (180 lbs) so they walked and worked quite slowly here on Earth during training. Yet they moved around quite easily, in a funny, sort of hopping way on the Moon. That's because their body and muscles had to control only one sixth of the weight they were used to. But not only the astronauts moved differently. Physics applies universally, so things they dropped or threw fell slower and flew farther. Or watch the astronauts themselves doing “gymnastics“ in those space suits. On Apollo-14, a piece of tape started to swing as a makeshift pendulum, creating another, unintended but very effective demonstration because time the pendulum swings around depends on the length and the amount of gravity. (Physics teachers, take note: you can replicate the pendulum in classrooms and compare the cycle length and effects of air resistance with the Apollo footage!)

Gravity check: the Surveyor-6 probe jumped up 3 meters (10 ft) by reigniting it's three landing engines. The imprints of the three crushable blocks and two footpads are visible, the third is under the lander. The rockets didn't blasted craters, confirming that the surface was solid enough to support large weights, such as the Apollo Lunar Module.


Another, more practical point is, if you want to send a spacecraft to the surface of the Moon, you've to be prepared. That means a lot of things – surviving the rocket launch, vacuum, radiation, heat from the Sun and cold in the shadows, but also, once there, you'll have to cope with lower gravity too. First, you'll have to decide how you build your rover. Once on the Moon, it only has to support 1/6 of it's own weight. (More precisely, 1/6 of the force it experiences here on Earth.) That means lighter support structures, saving weight that can be filled with more more scientific instruments. Sounds nice, doesn't it? There is a catch though – you can test your systems here on Earth only, with it's 9.81 m/s2 gravitational acceleration. So you either build a different rover for testing that stands our gravity, or build some clever support structures to compensate the forces. Or both.


The gravity simulator concept of Astrobotic Inc. NASA awarded the team and the Carnegie Melon University $599,000 to develop a system in two years.


The latter could also be used to the test the rover's mobility. It'll have to drive on a terrain filled with craters, rocks and boulders. The safe angles of crater walls the rover can climb and biggest rock sizes it can roll over have to be determined – you can't and presumably won't avoid all of them, otherwise what's the point of the mission? More ambitious missions may carry a drill or shovel to probe the surface. The forces those instruments are allowed to apply are vitally important to determine, especially if we want to prevent the spacecraft from accidentally heeling or tumbling during digging attempts. Such parameters can be quite reasonably addressed here on Earth but the closer you are to the actual lunar conditions the better. Simulating reduced gravity is easy enough as long as the rover doesn't move – you just pull it upwards somewhere above it's center-of-gravity with 5/6 g. The fun starts when it starts to roll because the whole support structure has to move with the rover. And when the rover pitches, the attach point has to remain above the center-of-gravity that's usually somewhere inside the spacecraft or otherwise not very accessible. All that fuss just to drive 500 meters on the Moon. Sounds pretty difficult, but hey, it's rocket science!

Image sources:

1.) NASA, Apollo Archive

2.) NASA / image reconstruction by Philip Stooke, University of Western Ontario

3.) Astrobotic Inc.

Last Updated (Wednesday, 24 August 2011 09:16)

XPRIZE_GOOGLE_RM_all grey facebookyoutubetwitterfacebook