Posted on 03/17/2004 7:44:05 AM PST by PatrickHenry
Gamma rays streaming from the centre of our galaxy could be the signature of elusive dark matter, astrophysicists claim. The rays support an exotic theory about dark matter: that it consists of very light particles.
Physicists know that a large proportion of the universe's mass cannot be accounted for by objects we can see, such as stars and planets. In galaxies such as our own, there could be as much as ten times more dark matter than normal matter.
One popular idea suggests that the 'missing' dark matter consists of as yet unidentified subatomic particles that are much heavier than entire atoms of normal matter but that hardly interact with it, except through gravity. They are called weakly interacting massive particles, or WIMPs.
But Céline Boehm of Oxford University in England and her colleagues think that dark matter particles need not be massive at all. Instead, they think they could be between ten and a thousand times lighter than a hydrogen atom.
Integral research
Their evidence comes from data collected by a satellite called INTEGRAL, operated by the European Space Agency, which searches the skies for gamma rays. The satellite has mapped out the cosmic sources of gamma rays with an energy of 511,000 electronvolts (511 keV). Such rays are about 200,000 times more energetic than visible light rays and are produced when electrons and their antimatter equivalents, positrons, annihilate one another.
Last January, the INTEGRAL team announced that the 511 keV gamma rays come from a source that is evenly distributed throughout the central bulge of our galaxy1.
Intriguingly, dark matter is known to be concentrated in our galaxy's central bulge, thanks to observations of how the missing mass affects the orbit of stars.
Boehm's team says that if dark matter were made up of particles with a low mass, these particles could generate positrons and electrons when colliding with antimatter. When these products collide, they generate gamma rays.
The researchers calculate that the number of such particles needed to produce the intensity of 511 keV gamma rays seen by INTEGRAL fits well with the amount of dark matter that the galactic bulge is estimated to contain. "The numbers are really reasonable," says Boehm. They report their findings in the current issue of Physical Review Letters2.
"It would be very exciting if it turns out to be real", says gamma-ray astronomer Jürgen Knödlseder of the Centre d'Etude Spatiale des Rayonnements in Toulouse, France, who works with INTEGRAL data.
But Knödlseder cautions that it is not yet clear if Boehm's dark-matter theory is really needed. The source of the positrons could be exploding stars called supernovae, rather than exotic particles. "They are still the most plausible source," he says.
He suggests that very accurate measurements of the distribution of the 511 keV gamma-ray emissions might enable researchers to work out whether the source is dark matter or exploding stars.
How many have been tested? Personally, I've been steeped in the math all my life, and I have the damnedest time feeling any intuitive sympathy for the notion that you have to slow down to go up, once you are in orbit. It sure doesn't apply when you're stuck on this mudball, so there's no obvious reason to have developed such an instinct.
Well, sort of, which is why I originally said you had to fire the jets more or less backwards. You should get a slight rise from strict forward firing, since you are firing along a tangent to a circular orbit, not a straight line. However, that is probably too slight a consideration to matter. The question posed, however, regards obtaining higher orbit. If you just slow down, of course you will eventually de-orbit.
Did Bruce Banner find this discovery?
Doesn't everything start out that way?
Here is another non-intuitive repositioning delta-v. For a geostationary satellite, you fire the thrusters in the same direction you want the vehicle to move. What is happening is you are changing the velocity of your vehicle that directly correlates to Kepler's third law. So if you fire the thrusters away from (behind) the direction of flight, causing the satellite to increase its altitude just a tiny bit, its velocity in respect to the velocity of the surface if the Earth will actually be slower. This allows the Earth to turn underneath it faster and the satellite’s subpoint (the point directly below the satellite) will move westward (or backwards in the same the direction you fired the thrusters).
If you fire the thrusters in the direction of flight (eastward), the satellite will drop to a lower orbit causing it to speed up relative to its subpoint and it will move relative to the surface of the Earth in the direction you fired the thrusters once again.
With only two firings (this is a Hohmann transfer orbit BTW) you can reposition a geostationary satellite.
I make a habit of never trusting my instincts over what the math says when I can't see out the airplane window. Your recipe will work, but it isn't the most efficient, or the safest. If you want the spend the least amount of fuel to get to a given higher orbit, you will aim the jets to decelerate your forward motion, and just slightly down toward the earth, hook the jet controls in a negative feedback loop through a gyroscope, and attempt to keep yourself in fixed circular orbit as you rise in one single, continuous blast.
If you simply try to blast straight up, you will then be in an extremely elliptical orbit because you are at the wrong speed for the circular orbit at your new height, which you must then correct. All you gained was a lot of incorrect angular momentum, at the unnecessary cost of a whale of a lot of fuel. If you plan to go to the moon, sure, point the dern thing straight up. But that wasn't the problem that was set.
What is an "orbit plane". Is it the same plane I was discussing in high school geometry? If we are talking about changing the height of a roughly circular orbit, where the old and new orbits are occupying roughly the same imaginary plane, cutting throught the earth--then do we have a case where hohmann transfer is the most efficient transfer strategy? (We are talking fuel-efficient, I presume.)
Indeed. Fuel is precious and heavy.
Where did I say a geostationary orbit could either be higher or lower than a specific altitude? If I did, I sure messed up in a big way.
As to the first part of the question, I am a bit surprised to hear that a slow burn during which you are never lacking for a circular orbit, is not the most fuel-efficient way to raise orbit. By intuition, any deviation from that will have to cost additional fuel to correct, in the end.
You mean the "combined plane change" transfer?
Here is a comparison: (note, even though this is an exercise, it will still show what is going on)
http://www.stk.com/resources/help/help/stk44/gator/tx-planes.htm
You can get the STK basic for free. I use it every day. :-)
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