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Tue, 13 Dec 2005
A number of scientists, including one of my colleagues, have been working to resolve the mystery of the Pioneer Anomaly. This anomaly is an unexplained slowing down of the Pioneer 10 and Pioneer 11 spacecrafts in the outer solar system.
Many learned theories have been proposed, including observational errors, gas leaks, thermal radiation, electric charges, new physics that changes gravity, etc.
My own theory is based mostly on Star Trek. In Star Trek, the Motion Picture, we find out that V'ger is none other than a Voyager spacecraft, with dust obscuring the letters "oya". That is the evidence on which I base my theory - space is dusty and the crafts are gathering dust as they travel, thus increasing their mass. They don't have deflector shields like warp-powered spacecrafts. The shower of charged particles they have received probably makes them attract dust like a television screen. Gathering dust means more mass and more mass means greater gravitational attraction toward the centre of the solar system.
I admit I haven't done the calculation of how much dust would have to accumulate how quickly for this to work. If a craft is gathering a large amount of dust, there should also be a loss of momentum as it accelerates the dust particles to its own speed. There is still the problem that the deceleration seems linear, while gravitational forces decrease with the square of the distance. If the spacecraft keeps gathering dust, then maybe its mass is increasing at just the right rate to compensate for the decrease in gravitational pull of the solar system.
Alternatively, maybe it's the solar system that's gathering dust. We all know about the accretion of new matter on Earth - our planet gets heavier by the day with all the meteorites that enter its atmosphere. This is why you always have to dig when doing archeology: anything that isn't swept regularly eventually gets buried in space dust. Planets with an atmosphere are particularly good at catching stuff.
In the calculations of the predicted speed of Pioneer, it is assumed that the mass of the solar system is constant. That may not be the case. Besides the solar system's own dust accumulating on the planets, the solar system is itself sweeping a path (sweeping is the operative term) through the galaxy, gathering the dust left by the poor housekeepers next door.
We assume that we know the mass of the solar system: we can measure the mass of the planets from their positions, (and vice-versa, partly a circular argument). Any extra mass would show up in the orbits of the planets, it's said. But there are all sorts of possible distributions of mass that would have no measurable effect on orbits, especially since the mass of the planets is only known indirectly.
As anyone who has tunnelled through the centre of the earth can tell you, you experience weighlessless at the centre, and as you go up your apparent weight gets greater the further from the centre you are, until you reach a point where all of the mass of the planet is below your feet. A different relative distribution of the total mass can have a significant effect on the gravitational forces exerted on a small object moving near it. There is likely to be a lot of matter in the Kuyper Belt and the Oort Cloud beyond the solar system. Explaining the anomaly as a measurement of this mass distribution seems more likely than fancy unknown linear forces.