[HN Gopher] Capturing the ripples of spacetime: LISA gets go-ahead
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Capturing the ripples of spacetime: LISA gets go-ahead
Author : ssl232
Score : 146 points
Date : 2024-01-25 15:40 UTC (1 days ago)
(HTM) web link (www.esa.int)
(TXT) w3m dump (www.esa.int)
| bonyt wrote:
| "Eddies," said Ford, "in the space-time continuum."
|
| "Ah," nodded Arthur, "is he. Is he."
| roblh wrote:
| "You are tearing me apart, LISA!"
| rqtwteye wrote:
| Such a cool project. The precisions involved in gravitational
| astronomy are simply mind blowing.
| robocat wrote:
| I presume it doesn't capture ripple direction very well.
|
| Does the design only capture ripples travelling in the ecliptic?
| wanderingstan wrote:
| 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.
| dylan604 wrote:
| 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?
| genman wrote:
| Yes, such a relatively low additional cost missed
| opportunity, if you think about it.
| Tepix wrote:
| 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.
| dylan604 wrote:
| even keeping 3 satellites in formation in a single plane,
| you have to be able to keep station by adjusting in three
| dimensions.
| genman wrote:
| 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.
| dylan604 wrote:
| 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
| ars wrote:
| 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.
| dylan604 wrote:
| okay, but now you're adding 3 new satellites instead of
| just one
| ragebol wrote:
| 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.
| ssl232 wrote:
| 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...
| doormatt wrote:
| https://en.wikipedia.org/wiki/Big_Bang_Observer
|
| They're already planning for something like that!
| scheme271 wrote:
| 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.
| BrainBacon wrote:
| 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.
| sbierwagen wrote:
| 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.)
| hossbeast wrote:
| "The launch of the three spacecraft is planned for 2035, on an
| Ariane 6 rocket."
| JumpCrisscross wrote:
| Wait, is this Eurospeak for never? (We schedule Moon landings
| the same way across the pond.)
|
| Less flippantly, is LISA funded?
| timeagain wrote:
| 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...
| SiempreViernes wrote:
| 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"
| eigenket wrote:
| 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.
| cypherpunks01 wrote:
| 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.
| sbierwagen wrote:
| 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
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