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Hacking Cassini To Detect Gravity Waves
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."
There was already a story on this earlier this summer.
and a great page on
space clocks and frequency control technology
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.
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.
(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.
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.
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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