ESA gives gravitational wave space probe LISA the nod for a 2035 launch

The European Space Agency (ESA) has signed off on the Laser Interferometer Space Antenna (LISA) mission to detect gravitational waves from space. The plan is to send three spacecraft, trailing the Earth as it orbits the Sun, forming a highly accurate equilateral triangle in space. Each side will be 2.5 million kilometers long …

[KarMann]

Why three?

Three satellites are great for detecting gravitational waves in a plane, presumably along the ecliptic, but will lose sensitivity towards the poles. Why not four satellites, to completely cover the sphere? I would think that the marginal cost would be reasonably small, given that the design would be the same. But maybe they came up against a hard limit of the payload mass of a particular launcher they're targeting (if they're expecting to launch all three in one go), or it could well be too hard to come up with orbits that keep four satellites at least roughly equidistant–I haven't even modelled that in KSP, much less anything real.

But I would love to see a proper foursome, for full coverage.

[Anonymous Coward]

Re: Why three?

Appreciate the sentiment, but putting 3 satellites into the same plane as the earth is considerably easier than the delta-V needed to put something into an orbit at 90 degrees. I don't think we have anything remotely powerful enough for that - maybe project Orion?

Though I am sure something with a few fractions of a degree could contribute usefully in regards building up a fuller picture.

[jdiebdhidbsusbvwbsidnsoskebid]

Adding sound...no need, already done

"If we imagine that, so far, with our astrophysics missions, we have been watching the cosmos like a silent movie, capturing the ripples of spacetime with LISA will be a real game-changer, like when sound was added to motion pictures."

Interesting quote from the project scientist there. When the first gravitational waves were detected by LIGO, the chirp was already in the right frequency band to be audible without any jiggery-pokery, just feed the signal into an audio output. Google for LIGO chirp and you'll find videos of it (with sound).

Ok, so I admit I am massively underselling the considerable effort that got us to the "just feed the signal into the studio output" stage, but when I saw the quote about adding sound, that's what I thought of.

[Eclectic Man]

Curvature of Space-Time

LISA spacecraft will be at the vertices of an enormous equilateral triangle. When C F Gauss was required to map Germany, he discovered that the angles in a triangle on a sphere added up to more than 180 degrees. I wonder how accurately the LISA craft can measure the angles of their triangle, and of they could detect any curvature of Space-Time. Probably need billions of kilometres an edge for a chance, but with measurements of distance accurate to less than the width of an atom, I'm hoping

[KittenHuffer]

Re: Curvature of Space-Time

How will LISA measure the flatness of the Universe, when it is not measuring the angle between the three satellites but rather the distance between them.

The Universe is flat. Here’s what that teaches us. - Courtesy of Starts With A Bang

How will three satellites a few million km apart measure the curvature when it is already pretty certain that any curvature is on the scale of 250 times the expanse of the visible (92 billion LY) Universe?

I'm afraid it would take a different design of mission, and much larger scales.

[Eclectic Man]

Re: Curvature of Space-Time

If the satellites are at the corners of an equilateral triangle, then they need to know exactly the angles in order to bounce lasers off both the other satellites at the same time.

As for your comments on measuring the curvature of Space-Time. Well, yes, but if LISA did find the angles between the satellites did not add up to 180 degrees, that would be an unexpected event of Nobel-prizewinning proportions (until they discover their protractor is wonky).

[richardcox13]

Re: Curvature of Space-Time

> If the satellites are at the corners of an equilateral triangle, then they need to know exactly the angles in order to bounce lasers off both the other satellites at the same time.

Or make the three edges the same length: which equally defines an equilateral triangle in flat space.

[ravenviz]

Re: Curvature of Space-Time

The curvature of the Earth’s gravity well ought to be much easier to detect than any wholesale curvature over cosmological distances..

[jmch]

detect gravitational waves

By my trivially little knowledge of physics, the effects of gravity travel at light speed (classic example, if the sun disappears it would take earth 8 minutes to 'fly off' into space). So surely all of the gravity waves from the sun, moon etc are already all around us all the time - so is it that the detector is actually detecting a change in gravity* (in effect a ripple that very slightly changes gravity in a detectable way).

Or, strictly speaking, detecting changes in expected motion from which we deduce change in gravity

[Eclectic Man]

Re: detect gravitational waves

Check out the LIGO web site, and the FAQs at https://www.ligo.caltech.edu/page/faq?highlight=How%20does%20LIGO%20work?

"If a gravitational wave stretches the distance between the LIGO mirrors, doesn't it also stretch the wavelength of the laser light?

While it's true that a gravitational wave does stretch and compress the wavelength of the light in the arms ever so slightly, it does NOT affect the fact that the beams will travel different distances as the wave changes each arm's length. And the only thing that matters to LIGO is how far the beams travel in each arm before being merged once again.

LIGO is designed so that as long as the distance the laser beams travel is exactly the same in both arms, they will make their trips in exactly the same time. When recombined, the beams totally destructively interfere with each other. In other words, they cancel each other out and no light emerges from the instrument. When this is occurring, we know the interferometer and its components are stable and the Universe is quiet.

Suddenly, a gravitational wave passes! What happens?

A gravitational wave causes each of LIGO's arms to change length in an opposite fashion, i.e., when one arm gets longer, the other gets shorter. Then they switch--the longer arm becomes the shorter arm and the shorter arm becomes the longer arm. This opposite oscillation in length occurs for as long as the waves pass, one getting longer while the other shorter, then vice versa, and so on, until the waves dissipate.

..."

LISA is basically 'LIGO in Spaaaace'.

Incidentally, if you have any questions about LIGO or black holes, there is an Ask LIGO page where a LIGO boffin will explain things (as far as possible) in lay terms. Very useful for anyone without a Nobel prize in cosmology.

[ldo]

Difference Between LISA And LIGO

As I understand it, LIGO, being limited by the dimensions of the Earth, has to bounce the light between its mirrors lots more times to detect very small phase shifts. As a consequence of this (or perhaps for some entirely different reason altogether), it can only detect higher-frequency gravitational waves.

LISA, however, having a baseline much larger than the Earth, is able to detect lower-frequency waves.

Consider the case of two black holes colliding: initially they will be some distance apart, and the orbital period will be longer, so the waves will be of a frequency more likely to be detected by LISA. As they get closer and speed up, their orbital period drops, and the frequencies shift up into the LIGO range.

In short, the combination of the two will let us observe such events from earlier in their inception.

[Eclectic Man]

Re: Difference Between LISA And LIGO

LISA may also allow us to observe gravitational waves created by objects much less massive than black holes or neutron stars colliding. Not just observing a black hole collision over a longer period of time.