Capturing the ripples of spacetime: LISA gets go-ahead | Hacker News Reader

Capturing the ripples of spacetime: LISA gets go-ahead

esa.int

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How's this thing powered? Are solar or big RTGs enough? I would think power requirements for these scale lasers, detectors, and transmissions would be pretty large. Isn't it an extraordinary amount of data that needs to be constantly sent back to Earth? I thought LIGO was generating a crazy amount of data per day beyond what I'd think would be possible to stream live back to Earth from so far away.

Also, how can the test masses remain in free fall, while each satellite is undergoing course corrections? Or are corrections infrequent enough for satellites in this earth-following orbit that it wouldn't cause any real amount of downtime? Of course Earth is free-falling.

The 2017 proposal is here: https://web.archive.org/web/20171017043638/https://www.elisa... This information-dense PDF got lost in the shuffle and its contents smeared across a hundred slides on the LISA website, because I guess PR people think only cell phone users exist.

Yes, solar. Laser power is only 5 watts. Total spacecraft power is 760 watts. They're downlinking a mere 334 megabytes a day, sampling at 3.3Hz.

>how can the test masses remain in free fall, while each satellite is undergoing course corrections?

That's the hard part. The test mass floats in the middle of the spacecraft, which maneuvers around it, occasionally firing ultra low-impulse cold gas thrusters to counteract drag. They put up a spacecraft back in 2015 to test that concept: https://en.wikipedia.org/wiki/LISA_Pathfinder

Such a cool project. The precisions involved in gravitational astronomy are simply mind blowing.

"Eddies," said Ford, "in the space-time continuum."

"Ah," nodded Arthur, "is he. Is he."

"You are tearing me apart, LISA!"

I presume it doesn't capture ripple direction very well.

Does the design only capture ripples travelling in the ecliptic?

I wondered about this too. My hunch was that they might be able to use the (comparatively) small detectors on earth to help localize the direction, while using the massive scale of LISA to analyze the details of the waveforms.

What if they added a 4th satellite to form a pyramid to now make it 3D? sure, you're increasing the number of lasers, but so?

Yes, such a relatively low additional cost missed opportunity, if you think about it.

You would have to add a laser to all satellites and add another one. It probably makes the satellite design a lot more complex and i don't know enough about orbital mechanics but wouldn't be surprised if it would get a lot more complicated as well to fly a 3d formation through space.

even keeping 3 satellites in formation in a single plane, you have to be able to keep station by adjusting in three dimensions.

I can imagine how all of it adds up but still, it's way less than doing it again in 3D in who know in how many years. But maybe you are all right, it's too much for start.

to be fair, i can't imagine that an international group of very smart people with all sorts of PhDs sat around for however many years developing a plan after discussion after discussion would have their idea "bettered" by some rando on the internet. it's just fun to think about and wonder

https://news.ycombinator.com/item?id=39140232 gave you the actual answer why it wouldn't work.

Instead they might consider a second formation in orbit 90 degrees away from the first.

okay, but now you're adding 3 new satellites instead of just one

One station would always have to be closer to the sun, which means it should go faster than the other stations. So it gets out of formation.

Indeed, a 4-satellite orbit is probably not a stable one unlike the 3-satellite orbit proposed for LISA. But the 4th satellite already isn't needed due to the conical orbit of the 3-satellite constellation. Fig 5(c) in "On the antenna pattern of an orbiting interferometer" [1] shows the antenna pattern for a LISA-like constellation and it's practically isotropic over a year, and for shorter periods the signals in LISA's sensitive band will be observed for long enough that they will not occupy its less sensitive directions for long and can thus still easily be detected with high SNR.

[1] https://academic.oup.com/mnras/article-pdf/289/1/185/1819985...

https://en.wikipedia.org/wiki/Big_Bang_Observer

They're already planning for something like that!

They won't be able to do that. LISA will be able to pick up signals with frequencies that can't be detected using LIGO/VIRGO/etc. LISA will be the only detector. However with three arms, it might be possible to localize the signal somewhat.

According to the ESA "As the orientation and direction of motion of the spacecraft triangle changes along the orbit, it will be possible to determine the direction of incoming gravitational wave sources." - From https://sci.esa.int/web/lisa/-/lisa-technology-interferometr...

Here's a pretty great infographic on how they plan to collaborate with the Athena x-ray observatory to detect black hole mergers ahead of time - https://www.esa.int/ESA_Multimedia/Images/2019/05/How_can_LI...

It ends up being about 0.4 square degrees in the sky that they're able to narrow in on before the event. That narrowing takes roughly a month.

The triangle spins as each spacecraft in the constellation travel along their orbits: https://en.wikipedia.org/wiki/File:LISA_motion.gif So the dead zone is constantly moving over the course of a year.

(In fact, it has to: in order to keep each leg the same length, each spacecraft must be the same distance from the Sun. There's no way to do that in a free falling orbit and have the formation flat to the ecliptic-- the craft closer to the Sun would orbit faster and drift away from its brothers.)

"The launch of the three spacecraft is planned for 2035, on an Ariane 6 rocket."

Wait, is this Eurospeak for never? (We schedule Moon landings the same way across the pond.)

Less flippantly, is LISA funded?

From the PR, it sounds like contractors will be chosen in the next year, which sounds funded to me. But I don’t know Eurospeak either…

The funding status of big science collaborations is always a bit opaque, as there are a lot of individual actors with independent funding streams, and there's always a lot of R&D, the cost of which you can only guess in advance.

Formally ESA has moved the LISA project into a "implementation phase", meaning they intend to build it, but no money has been "pledged" as far as I know. However, LISA is a flagship Space Science programme, and the SSPs are part of the ESA budget all member states must contribute to.

Basically there's a fixed, overall budget that is then split for each project adopted. So there is definitely money but I'm not sure there is a public statement that "this particular pile of money belongs to LISA"

My understanding is that ESA has agreed to fund their portion, but other partners haven't finished that process yet - e.g. the UK's science agency has to go to the treasury and make a case for it. Similar with NASA & the other partners involved.

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