Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 28/07/2011 15:55:30
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How will LISA's (http://lisa.nasa.gov/) three craft be able to precisely stay in the same position five million km apart when they will be detecting minuscule gravity warps via their laser joined triangle ?
I gather their measurements will be extremely accurate so how will it really be possible to keep them aligned so precisely?...how ?
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Each satellite will be in a 12-month elliptical orbit around the sun, about 20° away from Earth. Each orbit will have the same period and eccentricity, but the aphelions will be 120° apart. Consequently, they will follow circular orbits relative to each other. If you do the math, you'll find that identical elliptical orbits 120° apart around a massive body, like the sun, are
circular elliptical relative to one another. (EDIT: The orbit of the three craft relative to each other will have the same shape as their orbit relative to the sun, only smaller.)
It is not necessary to keep them precisely the same distance apart because they are looking for tiny differences between their actual distance and their predicted distance. The predicted distance has slight differences due to the gravity of Earth and other planets, but gravity waves are expected to cause fluctuations in the distance which are not due to known factors.
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When you add 8 other planets, that should also affect their orbits. All 3 will be on the same side of the sun, but the difference in distance to Jupiter, or Earth should be enough for the closest to receive more gravitational pull than the farthest one.
I would assume the press release is very much oversimplified.
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Lisa uses monochromatic lasers that gives a very tight light. Those spacecrafts lasers will form a triangle, traveling 5 000000 km from each other, without losing much of their collimation (narrowness of beam and energy). When the light from 'A' hits 'B' it will be reflected back directly, as long as their relative distance doesn't change, they should get 'coherent' light back, if the distance in between change the slightest fraction the reflected laser light will become out of phase with the sent beam, which Lisa's detectors will discover. As I understands it (well more of a slightly educated guess actually:) they will use the lasers to find/keep a constant range between them. But, would a gravitational wave come it will disturb that balance.
But first they will send up Lisa Pathfinder. "LISA Pathfinder will be launched into a temporary parking orbit before shifting into successively larger orbits using its powerful propulsion module until reaching its destination 1.5 million km from Earth. This is the L1 Lagrange point where the gravitational effects of the Sun and the Earth balance. The propulsion module will then be released to ensure that the remaining fuel on board does not disturb the sensitive scientific instruments.
That one will test "the ultra-high precision technology needed to make a test mass float freely in space so that any effects on its trajectory can only be the result of external gravitational forces. These test masses are two metal cubes which will be placed into gravitational freefall." Lisa Pathfinder - incredible technology. (http://sci.esa.int/science-e/www/area/index.cfm?fareaid=40)
What makes me really flabbergasted is how it even is possible to send such delicate instruments up in the sky, resting on a rocket, without damaging them :)
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"Each of the three satellites will contain two telescopes accompanied by lasers and optical systems. Pointing in directions 60 degrees apart, the telescopes in each satellite will communicate with those in the other two satellites by laser beam. Inside each telescope is a four-centimeter-wide, free-floating cube of gold-platinum alloy, which is used as a reflector for the incoming laser beams. This provides a reference for measuring the distance between spacecraft. When a gravity wave moves through the observation field, it literally changes the shape of space so that there is a slight change in the distance between the satellites. By measuring this change, the strength, direction and polarization of the wave can be derived.
This method is so precise, even the pressure of sunlight on the satellites can alter their position in relation to each other, spoiling the measurement. Because of this, the LISA satellites will have to constantly monitor such extraneous forces and counteract them with their electric thrusters."
It has to be a statistical approach I think, in where you expect certain deviations but also where a gravitational wave from some binary system will make a 'peak' that will stand out. They should be able to make educated guesses on which systems they should/could expect such waves from, and so confirm their results.
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The simple answer to neilep's question is that they will not be fixed relative to each other but that their motions will be clearly measurable, precisely predictable and relatively slow. in space the noise and disturbance will be extremely small unlike on the earth which is very noisy. Gravitational waves produce more rapid and unpredictable variations. This will allow gravitational waves at much lower frequencies to be detected than on the earth.
I will also lay some bets that the instrument will also detect some other slow gravitational variations that were not predicted in the orbital calculations caused by unseen bodies in the vicinity of the solar system.
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How fast do these predicted waves travel? Is it possible that it would take days or months to travel the 5 million km?
Also, the magnetic fields being detected by the Voyager Probes are being described as magnetic bubbles or froth. Likewise, would this gravity experiment be better tested outside of the heliosphere or heliosheath? Not that it is easy to get that far away from Earth, but perhaps one would see gravity differently when very far away from the Sun.
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Gravity waves travel at the speed of light that is 300,000 km/sec so they will take about 15 seconds to do the 5 million km.