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Comment by robocat | Hacker News Reader

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

Does the design only capture ripples travelling in the ecliptic?

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.)

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.

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