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posted by on Friday March 17 2017, @08:04PM   Printer-friendly
from the a-lens-that-blinds-you dept.

The Sun could be used as a gravitational lens to magnify normally hard-to-image targets such as exoplanets. The catch? The equipment needs to be 550 AU away from the Sun:

Now Leon Alkalai from the Jet Propulsion Lab and his co-authors have picked up an earlier suggestion from Italian physicist Claudio Maccone to use our Sun, rather than a distant star, to create what might be the ultimate telescope based on the microlensing principle. Alkalai's team has investigated the viability of the method in detail as a breakthrough mission concept. They also presented their findings at NASA's recent Planetary Science Vision 2050 workshop in Washington, D.C.

To build such a "telescope," detecting instruments would be placed at a point in space where the Sun's gravity focuses lensed light from distant stars. Not only is the idea viable, according to the Alkalai team, it would produce images that separate the distant star from its exoplanet, a critical observation that is the goal of future space telescopes equipped with Starshades. And using the Sun as a lens would result in much greater magnification. Instead of a single pixel or two, astronomers would get images of 1,000 x 1,000 pixels from exoplanets 30 parsecs, or about 100 light years, away. That translates to a resolution of about 10 kilometers on the planet's surface, better than what the Hubble Space Telescope can see on Mars, which would allow us to make out continents and other surface features.

[...] There is a downside, however. The telescope's focal plane instruments would have to be at least 550 AU from the Sun (1 AU, or astronomical unit, is the distance from the Sun to Earth), which is well into interstellar space. The only spacecraft that has reached interstellar space so far is Voyager 1, which covered approximately 137 AUs in 39 years. So we would need a spacecraft that is at least 10 times faster, but Alkalai and his colleagues say this is within the reach of current technology.

Also at Engadget, MIT, and The New Yorker.

Mission to the Gravitational Focus of the Sun: A Critical Analysis


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  • (Score: 0) by Anonymous Coward on Friday March 17 2017, @10:36PM (4 children)

    by Anonymous Coward on Friday March 17 2017, @10:36PM (#480667)

    At that distance, orbiting starts to get a lot less relevant for anything not completely inert.

    Orbiting is always relevant. There is no way you can change your velocity without changing your distance to the primary that's just how the universe works. Kepler's 2nd law will not be violated. There's only one speed that will keep you 550AU from the sun. Of course you can duck into a smaller orbit and push out again, but at a stupefyingly horrendous fuel cost. I don't think an ion/photon/VASIMR engine will even be able to give you the delta v you need and hauling any other sort of engine out that far would be amazingly expensive and of very limited use. It would be cheaper, as others suggested, to have several identical satellites in the same orbit imaging from different angles. Then it's just a case of waiting the necessary years for one of the satellites to be at the angle you wanted.

    (PS you might want to wait for night time too j/k)

  • (Score: 0) by Anonymous Coward on Saturday March 18 2017, @12:11AM (2 children)

    by Anonymous Coward on Saturday March 18 2017, @12:11AM (#480707)

    Thing is, this will be on an escape trajectory -- you have to exceed escape velocity to get there in any reasonable time, and there's no point slowing down to orbital velocity once you reach 550 AU. (Especially since 550 AU is the minimum distance, but at that point you're peering through the corona which limits observable frequencies; as you go farther out, the einstein ring gets bigger. 600-700 is a more practical figure.)

    So rather than thinking about orbits, think about dozens of km/s of radial velocity, and then superimposing whatever tangential velocity you like (subject to hauling the fuel with you) to navigate between targets. Unfortunately, fuel constraints basically mean we can pan between targets within a system well enough, but panning from one stellar system to another will either require impractical fuel supply or be impractically slow. (Roughly: 1 km/s starts panning at a rate of 1 arcminute per year, then another km/s to stop. That's around 700 AU, but of course the actual figures are a function of distance -- the further you go (and you're going further all the time) the harder it gets.)

    • (Score: 2) by Dunbal on Saturday March 18 2017, @12:30AM (1 child)

      by Dunbal (3515) on Saturday March 18 2017, @12:30AM (#480714)

      and there's no point slowing down to orbital velocity

      Except for the minor point of being able to keep and re-use your equipment for a bit longer than the time it takes to get from 550-700 AU or whatever the max feasible upper limit is...

      • (Score: 1, Interesting) by Anonymous Coward on Saturday March 18 2017, @07:28AM

        by Anonymous Coward on Saturday March 18 2017, @07:28AM (#480797)

        But there is no upper limit for the lensing mechanism, or at least none that matters in a practical sense. Just keep improving our end of the communications link to keep up with the increasing distance. In fact, not only does the Sun work better as you go out (because the Einstein ring gets out of the corona), but when you get far enough away (something like 6000 AU, IIRC), Jupiter becomes usable as a magnetic lens, too; while it has less power than the Sun, and the smaller Einstein ring will be harder to resolve, it has the helpful ability to sweep a relatively wide region of space for you every 12 years. And the optics keep working far beyond that; the only thing stopping us from using nearby stars as gravitational telescopes is the fact that none of them are pointed directly at anything interesting. (Moving 600 AU while using our sun as a gravity lens lets us look in any direction we choose; moving 600 AU while using, say, Barnard's Star as a lens only lets us realign by 10 arcminutes or so.)

        The real limit to the useful distance is the fuel requirements to realign the telescope, which get bigger with increasing distance; decelerating to orbit hardly solves that problem.

        (Another way of thinking about it is to note that, thanks to the rocket equation, it's cheaper to send one out on an escape trajectory, then send another when the first is about to run out of maneuvering fuel / get too far away, than to send the first one with enough fuel to slow down when it gets there.)

  • (Score: 2) by Immerman on Saturday March 18 2017, @01:31AM

    by Immerman (3985) on Saturday March 18 2017, @01:31AM (#480730)

    For long term stability, obviously. For more human timescales...not so much Consider:

    At 1AU, the acceleration by the sun is a paltry ~0.006 m/s/s
    At 550AU that falls to 0.006m/s/s *(1/550)^2 = 2x10^-8m/s/s

    So, if we brought it to a total, complete stop, and waited for it to fall back just 1AU (1.5x10^11m), to 549 AU from the sun, it would take about...
    Distance traveled = 1/2 a * t^2, or alternately t = sqrt(2d/a) = ~4x10^9 seconds, or 123 years.

    Plenty of time. And if instead of bringing it to a stop you left it coasting outward so that it would reach 700 AU or so before falling back - well the hardware would likely have stopped working long before it even reached the fallback point.