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Hacking Cassini To Detect Gravity Waves

Posted by timothy on from the ether-net dept.

lennon writes: "With some upgrades to the tracking equipment, NASA is going to try to detect gravitational waves by tracking the speed of the Cassini probe. They've tried this with other spacecraft, but the sensors have evolved since then. Complete press release is here. Looks like a neat hack."

19 of 91 comments (clear)

  1. links by javaaddikt · · Score: 3, Informative

    There was already a story on this earlier this summer.

    and a great page on

    space clocks and frequency control technology

  2. related to string theory? by 2Bits · · Score: 2, Interesting
    Gravitational waves are ripples in the fabric of space and time that are set off by accelerations of massive bodies, such as black holes or supernovas.

    Sounds like string theory in applied science. Could anyone explain/comment how much of string theory affect research in space travel?

  3. Uncertainty? by Zen+Mastuh · · Score: 4, Insightful

    I understand the methodology (well, as much as can be deduced from a press release...) of the measurement, but could other factors cause apparent--or real--shifts in relative velocity? For example: mini planets, large asteroids, or lopsided planets could cause variation in the gravitational force exerted on Cassini and said object, altering the relative velocity of the probe. Someone enlighten me!

    --
    "What is the sound of one belly slapping?"
    1. Re:Uncertainty? by markmoss · · Score: 5, Informative

      could other factors cause apparent--or real--shifts in relative velocity? For example: mini planets, large asteroids, or lopsided planets...

      If they see a doppler shift, it's a real velocity change. Electronics designed to transmit and measure frequency are remarkably accurate and stable, so unless NASA didn't bother to put a good oscillator into the transmitter, any measured shift will be real. The only other thing that could cause an apparent shift would be a warped gravity field between the probe and Earth; if there's anything undetected out there capable of that, it would be much bigger news than detecting gravity waves...

      A large asteroid near the flight path could change the velocity, but I would expect the experiment design to distinguish that effect from the gravity waves they are looking for. The larger asteroids, and anything else big enough to be gravitationally significant inside the orbit of Neptune, are easily visible in moderate-sized telescopes on Earth, so they are pretty sure they have all been identified and their gravitational contribution already calculated. (These long missions would always miss the target if NASA wasn't pretty good at those calculations.) But if there is something they missed, the effect on the probe speed would be a single cycle, like speeding up as the probe approached and slowing down as it went past. If there's a velocity change that lasts more than one cycle, a gravity wave is about the only explanation. Also, an asteroid would change the direction of the probe's orbit as well as the speed. This can't be measured to the same accuracy as a doppler shift, so it might take quite a while to detect the change, but eventually they would see that the probe is slightly off course.

      Finally, "lopsided planets": Earth is slightly irregular in shape and density, causing a measurable effect on satellites in low orbit. Presumably other planets are similar, and the irregularities have not been well mapped. But once you are out a bit from the planet, this effect is no longer measurable. All the nit-picking measurements astronomers took on the Moon over several centuries never showed that Earth was anything but spherical, nor did close observation of other planets' moons ever show irregularities, so it isn't going to affect something much farther away from any planet than the Moon is from Earth.

    2. Re:Uncertainty? by Zen+Mastuh · · Score: 3, Interesting

      Now it makes more sense--thanks for the enlightenment. Also I finally grokked that the procedure will be performed a few more times over the next year, when the positions of planets and asteroids are significantly different from today's positions.

      I learned in physics that waves don't have mass, then learned later (on /. maybe???) that they can be affected by gravity (and other forces, ostensibly). I'll play devil's advocate here and suggest that the transmitters' waves can themselves be affected by other forces besides the elusive gravity waves. Agree/disagree.

      --
      "What is the sound of one belly slapping?"
    3. Re:Uncertainty? by Anonymous Coward · · Score: 3, Informative

      Pretty good analysis. One thing that people unfamiliar with the search for gravity waves tend to assume is that it'll be like a seismograph and you can watch the data scroll by and say "ooh, there's a gravity wave!". In reality (at least with current instruments), you have to do some simple or not so simple data analysis to see what really makes up your signal. The simplest form of this would be to perform a fourier transform on the data and look at what frequencies make up the signal. I work at LIGO (annother project searching for gravity waves, its mentioned in the article) and hardly anyone looks at signals without running an FFT on it. You look for spikes at certain frequencies to figure out what exactly is on the signal (i.e. "there's a spike at 60hz again, #*$! the power cabling" or "there's a broad hump around 450hz, we must have the gain up to high"). Then you can decide whether the signal has nothing of interest on it, some known noise source, or an unknown noise source that could be from gravity waves. The real-time values are really only used for certain tasks (i.e. aligning the mirrors, when you want to maximize the signal on that readout, minimize it on that one, etc.)

      Currently, if we see an unknown noise source, we start looking for what part of the electronics is screwing up our data. Even after we finnaly do see a gravity wave in our results, expect lots of discussion for a year or so until the scientific community will accept that it isn't just some unknown source of noise in our equipment. (And with just cause, some of the sources of noise in this thing can be very strange, and some of the current noise sources still aren't fully understood.) Of course, there are some better and more complex analysis methods in development for when we get the noise down to a state where we have a chance at seeing gravity waves, but for now a simple FFT meets most needs.

      -Too Lazy to Create Account

    4. Re:Uncertainty? by markmoss · · Score: 2

      Thanks. Sounds like you know this stuff better than I do. (I bailed out of physics and into engineering 25 years ago...) I would guess that a major reason for doing this experiment in interplanetary space is to avoid much of the interference that ground-based equipment is subject to. But now that I started to think about just how tiny an effect they are looking for, I wonder -- wouldn't a truck driving by the receiving antenna cause it to bounce up and down a little, giving a periodic doppler effect? So you've got to record on several widely separated antennas and compare the results...

      Could you comment on one bit of arithmetic in the article? "Cassini's speed relative to Earth ... will typically be about what it would take to zip from New York to Chicago in five minutes. In contrast, this experiment could detect any change in speed so small it would lengthen or shorten that trip by a mere fraction of a second." One second in 5 * 60 = 300 is 0.33%, so I suspect this is off by several orders of magnitude (unless you think a fraction of a microsecond and a fraction of a second are equivalent).

  4. Through the sun??! by Zapman · · Score: 2

    How on earth are they communicating with the probe if it is on the other side of the Sun?

    Though that might be the source of the gravitational waves they are measuring... hrm...

    --
    Zapman
    1. Re:Through the sun??! by Xzzy · · Score: 2

      You misread.

      If you drew a line from the satellite to the sun, earth would be somewhere on that line between the two.

  5. The really interesting thing here... by nikoftime · · Score: 5, Insightful

    What I find truly interesting about all this is not just that they are measuring the velocity changes (the acceleration) of the Cassini probe, but that after getting back the information of the forces at work, they will have to somehow determine exactly where the gravitational waves are coming from.

    Think of it this way: If two planets are on opposite sides of the probe, and both are emitting gravitational forces, then the probe will be subjected to the net forces of the two planets. The equation for relative force of gravity comes to mind here, and I assume they will be using it when calculating multiple sources of gravity.

    (GmM)/(R^2) gives the acceleration of the system for two masses in space, so any resultant force must take into account that it could come from several different masses.

    JPL engineers have carefully instrumented a large dish antenna at the Deep Space Network's Goldstone complex near Barstow, Calif., to send and receive the higher frequencies with unprecedented Doppler sensitivity. The upgrade includes refined pointing capability needed to exploit the higher frequencies, said Sami Asmar, supervisor of JPL's Radio Science Group. Other new equipment at Goldstone will allow researchers to correct for the atmosphere's distortion of radio transmissions and improve performance of the search.

    As I see it, the trickiest thing here will be taking the "exquisitely accurate measurements" and turning them into real, useable models of gravity given off by our neighboring planets.

    1. Re:The really interesting thing here... by who+what+why · · Score: 3, Informative

      (GmM)/(R^2) gives the acceleration of the system for two masses in space

      That is newtonian gravity. By definition, gravitational radiation is a general relativistic effect. The source of gravitational radiation is likely to be a fairly close supernova, or perhaps a binary black-hole system etc.

      The weak-field effect or nearby planets will be taken into account, I presume, but will not contribute to gravitational radiation.

  6. Nice experiment to prepare the way for LISA... by Tsar · · Score: 3, Interesting

    It will be interesting to see whether this experiment gets the results everyone seems to be anticipating, or mirrors the 'success' of the Michelson-Morley experiment.

    The Laser Interferometer Space Antenna (LISA) (launching in 2009) should return significantly better data, but it'll be nice to get a sneak preview from Cassini.

    "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'" — Isaac Asimov

  7. How long before the FCC ... by OmegaDan · · Score: 5, Funny

    how long before the FCC declares itself the guardian of the gravtational frequency band and starts selling portions to the highest bidder? :)

    --
    Free Techno/Jazz/DNB/MI Music by guys obsessed with monkeys!
  8. Spce empty enough? by TACD · · Score: 3, Insightful
    As I understand it, the differences in speed will be "barely perceptible"... doesn't this potentially mean that the calculations could be wrecked by the tiny impacts of hydrogen atoms on the probe? It souds like something this fragile could be offset by anything. But tell me if I'm wrong. :-)

    Side info: If you held open a matchbox in space, it would contain about 6 hydrogen atoms.

    --
    Security through promiscuity is no better than security through obscurity.
  9. Re:Sounds familiar by pclminion · · Score: 3, Informative

    That isn't exactly what they are doing here. The gravitic maps of Earth show how the static G-field varies as a function of latitude/longitude. What they are attempting to measure here are dynamic variations in the background G-field due to the propogation of gravitational waves. These waves are generated by accelerating masses in the same way that accelerating charges radiate electromagnetic waves. For instance a black hole and a star orbiting each other will emit G-waves, and by doing so lose orbital energy.

  10. NOTHING to do with string theory. by MillionthMonkey · · Score: 4, Insightful

    People always want to talk about string theory at the drop of a hat. But there is so much fascinating stuff in physics that holds a possibility of actually being true. :)

    Maybe string theory enters into the picture on the Planck scale, or when you're going to talk about individual gravitons, but it's completely irrelevant as far as this experiment goes. Gravitational waves are a classical phenomenon, predicted by GR (which is a classical theory). They have not been detected as of yet because they are so weak. The coupling coefficient is c^4/(8*pi*G), which is really large. So space time is elastic, but it is extremely stiff. It takes a lot of force to warp it even a tiny bit. The earth emits something on the order of 1 watt of gravitational radiation as it orbits the sun. Jupiter emits something like 30 watts. (Don't ask me for a source on those numbers- I think I read them on the Internet somewhere.) But any laboratory source won't emit anything that can be measured. Gravitational waves are even more esoteric than neutrinos, since we know how to detect neutrinos that we have created. The only sources of gravitational waves that are even remotely detectable are binary star systems, where two neutron stars are in a close orbit. The orbital periods of some of these systems have been determined to be decreasing in a manner characteristic of energy loss from gravitational radiation.

    Personally I've always thought it's a bit premature to be speculating on the stringlike nature of gravitons when we can't even detect gross macroscopic things like gravity waves or even gravitomagnetism. It's as if we're blind snails wanting to talk about photons.

    1. Re:NOTHING to do with string theory. by spiro_killglance · · Score: 3, Interesting


      Quite true. At present String theory doesn't
      even seem to be sciences best bet for a
      quantum theory of gravity. A theory called
      Loop Quantum Gravity, that describes space-time
      as network of lines each labelled with a spin,
      is rapidly become a much more promising theory.
      String theory still requires a space-time for
      strings to move in, where as LQG, describes how space time is built. There are already some great results in LQG, including the formula for the Entropy of a black hole, a description of a big bang at zero time, no not a singularity, at that time the universe has a finite but huge curvature equal to 256/(81 G h-bar)

      Have a look at the review paper i mentioned above, its excitted
      work.

    2. Re:NOTHING to do with string theory. by MillionthMonkey · · Score: 3, Funny

      Sorry, but your posting isn't going to help much if you just toss around technical terms without explaining them.

      So... you must be new here? Welcome to Slashdot.

  11. GRAIL by vinylat33 · · Score: 2, Informative

    Professor Frossati working at Kamerlingh Onnes Laboraty at the University of Leiden, leads the project 'Gravitation Radion Antennae In Leiden', alias GRAIL, which tries to measure gravitation waves.

    Website : www.minigrail.nl