LISA Pathfinder free fall test beats expectations European Space Agency (ESA) boffins are tossing hats in the air as data from their LISA Pathfinder mission suggests its gravitational wave detection kit is going to live up to expectations. In the popular mind, LISA – the Laser Interferometer Space Antenna – has been upstaged by the Laser Interferometer Gravitational-wave …
I assume it uses lasers to balance the weights... um "masses" to keep them in freefall and from hitting the edges? Or does it use magnets, and the lasers just for measurements?
The lasers are for measurements, monitoring the position of of the masses 40 centimetres apart to an accuracy of less than 0.01 nanometres. Other systems, including physical pusher rods, are used to position the cubes. After getting oriented, the satellite uses micro-thrusters to stay centered on one test mass and monitors what happens to the other in comparison. It should be in an extremely clean free fall environment.
The Lisa Pathfinder mission isn't going to do much with respect to spotting gravity waves. It's proving out the hardware that will go on the follow-up eLisa mission.
Why did they not have one of these in the spaceships in sci-fi?
Gravitational detectors are abundant in sci-fi. They're a key sensor system in David Weber's "Honorverse," for example.
I read somewhere that they hope now to do the full size version on a shorter timescale.
" I assume it uses lasers to balance the weights... um "masses" to keep them in freefall and from hitting the edges? Or does it use magnets, and the lasers just for measurements?"
The laser is used essentially as ruler, to measure the distance apart of the blocks.
Nothing is holding the blocks in place (relative to the spacecraft), other than inertia. That's why it's at the Lagrange point.
L1 points are unstable, so spacecraft actually orbit them from rather large distances (100,000-200,000 km according to Wikipedia). Considering the small number of objects, I'd say the risk of collision is pretty limited.
From a gravitational point of view, a 1-ton satellite would exert 10^(-15) g at a distance of 10^5 km, which is within range of the experiment's requirements.
L4 and L5 - yes. They are stable and even if you shove something a bit out of it, it tends to try to go back there.
While unstable, L2 is interesting because it is in perpetual shade with the smaller body providing it as it orbits the larger. L3 and L1 - not so much. They are both not stable and solar wind will probably push you out of them at the end of the day. So a long term competition for "real estate" at these locations is highly unlikely.
Isn't a Lagrange point pretty small and valuable?
Nope. Notice the breadth of Jupiter's Trojan asteroid collection: the L4 and L5 groups each cover an arc a couple of AU long.
Orbits around L1 and L2 points can be large, too. The SOHO probe follows an orbit around the Earth-sun L1 point that is roughly 400,000 x 1,300,000km.
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