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