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LISA is officially go!


Streetwind
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It was more or less expected, after last year both confirmed the existence of gravitational waves and saw the tech demonstration precursor LISA Pathfinder blow all testing requirements out of the water within a day of setting up:

The European Space Agency has formally greenlit the implementation of its ambitious Laser Interferometer Space Antenna (LISA) as a flagship "L-Class" mission. It will consist of three spacecraft, flying in a triangle formation several million kilometers apart, performing laser interferometry between each other with the goal of measuring gravitational waves. The incredible precision of this instrument will far outmatch anything that could ever be built on the curved surface of Earth. LIGO, our only currently operational gravitational wave detector, only has two beams with a few kilometers of distance to work with, by comparison.

The mission is currently slated for launch in 2034. "Why so late?", I hear you ask. Well, it's simple, really: it's going to be ESA's third L-Class mission - and the first one hasn't even launched yet! Before LISA gets its turn, we will see the Jupiter Icy Moons Explorer (JUICE) in 2022, and the Advanced Telescope for High Energy Astrophysics (ATHENA) X-ray observatory in 2028. Also, keep in mind that ESA's budget is only roughly 1/4th that of NASA, and thus they can't quite pound out missions like these in rapid succession. Finally, ESA's plans for a gravitational wave observatory were delayed from an originally more aggressive timeline when the US congress made NASA pull out of a cooperation agreement in 2011. Without this delay, it might have gotten the 2028 slot.

Now, while the two other L-Class missions are both impressive in their own right, the sheer ambition behind LISA cannot be overstated. It might be the most revolutionary mission that ESA has ever undertaken, and I am very pleased to see it move forward. :) 

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It's a pretty dang impressive mission, that's for sure. As I understand, the three LISA satellites will be using tiny thrusters to keep station with test masses contained within a vacuum inside them; the outer satellite shields the inner from any non-gravitational forces, and uses minute thrusts to keep station and make sure they don't bang into those test masses.

All so that they can serve as multi-million-kilometer detector arms for a laser interferometry experiment to detect gravitational waves.

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35 minutes ago, Starman4308 said:

It's a pretty dang impressive mission, that's for sure. As I understand, the three LISA satellites will be using tiny thrusters to keep station with test masses contained within a vacuum inside them; the outer satellite shields the inner from any non-gravitational forces, and uses minute thrusts to keep station and make sure they don't bang into those test masses.

All so that they can serve as multi-million-kilometer detector arms for a laser interferometry experiment to detect gravitational waves.

Actually, it was just LISA Pathfinder that used the test mass. The real satellites won't need it. Pathfinder used the test mass to validate the technology for ultraprecise stationkeeping that's required to coordinate a formation of satellites like that. Basically, the satellite tried to maneuver in such a way that the freely drifting mass was always kept dead center. In other words... the world's most expensive game of "I'm not touching you...! I'm not touching you...!" :P 

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13 hours ago, Streetwind said:

Actually, it was just LISA Pathfinder that used the test mass. The real satellites won't need it. Pathfinder used the test mass to validate the technology for ultraprecise stationkeeping that's required to coordinate a formation of satellites like that. Basically, the satellite tried to maneuver in such a way that the freely drifting mass was always kept dead center. In other words... the world's most expensive game of "I'm not touching you...! I'm not touching you...!" :P 

The actual one will do that - the mass will be reflectors test masses (see below). I'm not sure but I think the idea was that these free-flying arrangements will have their position perturbed "solely" by gravitational waves. The satelites just tracks the position of these masses, which then reveals the distance.

I mean, come on, why would you need to keep the distance the same when you do want it to be perturbed ?

Edited by YNM
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The satellites won't maintain distance; it'll actually vary quite a bit over their orbit, and is accounted for in software. It's about individual position and attitude.

If you say that the full-scale system will still use free-falling reflectors, though, then I dunno. You might be right. I certainly don't know enough to refute you. :wink:

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11 hours ago, Streetwind said:

The satellites won't maintain distance; it'll actually vary quite a bit over their orbit, and is accounted for in software. It's about individual position and attitude.

If you say that the full-scale system will still use free-falling reflectors, though, then I dunno. You might be right. I certainly don't know enough to refute you. :wink:

Alright, I'm slightly wrong. It's not reflectors. It really is just test masses.

From wikipedia (yeah, not really the primary source but close enough) :

Quote

To eliminate non-gravitational forces such as light pressure and solar wind on the test masses, each spacecraft is constructed as a zero-drag satellite, and effectively floats around the masses, using capacitive sensing to determine their position relative to the spacecraft, and very precise thrusters to keep itself centered around them

Test mass is just to ensure they follow actual gravity-defined trajectories.

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3 hours ago, Streetwind said:

I see! An odd method for sure, but hey, if it works for them...

Reportedly it has been used in previous other probe for gravity. Including GOCE (the one shaped like airplane with ion thrusters) and Gravity Probe B.

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On 6/23/2017 at 3:21 AM, YNM said:

Reportedly it has been used in previous other probe for gravity. Including GOCE (the one shaped like airplane with ion thrusters) and Gravity Probe B.


Measuring spacecraft position relative to a proof mass was first used by NAVSAT (Transit) to allow accurate orbit determination.

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That info is not listed in most sources I presume ! Quite amazing then for something as old as the earliest space rockets... they used verniers back then or what ?

Though, most sources will instead try to constrain their orbits by looking it up every time from a ground-located (the old-way measured) locations... so that's plenty of infos then ?

Edited by YNM
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