## Solar system science

April 4, 2011

A couple of interesting stories:

Aerial photos of Mars: The Mars Reconnaissance Orbiter has just celebrated five (earth) years of low-Martian orbit and aerial photography. Its main objective is to study the aqueous history of the planet, and for that purpose it is equipped with an array of high resolution cameras. It has already sent back thousands of images of the Martian surface, and to mark its birthday Wired magazine published a sample. As is so often the case with aerial photographs, they are strikingly beautiful. Even if part of the effect can be attributed to the use of false colour, I have never seen Mars look this good before.

Pioneer anomaly: The Pioneer 10 and Pioneer 11 spacecraft were launched in the 1970s on trajectories that would take them out toward the asteroid belt, past Jupiter and Saturn, and then onward and outward into deep space. They are now well beyond Pluto’s orbit and communication with both has been lost for several years. Meanwhile, however, there has been a puzzle about an anomaly in the trajectory of each: the velocity of each spacecraft was decreasing because of the gravitational pull of (principally) the sun, but the data show that each was decelerating more than expected. The difference between the expected and observed decelerations was small — about $10^{-10} m/s^2$ — but still enough to be bothersome. We’re supposed to understand gravity, so what was going on? Explaining the Pioneer anomaly has become one of those tantalizing problems in physics that not many people work on, but that everybody would like to see resolved.

A group of Portuguese physicists have published a recent paper claiming that the anomaly can be explained by a better model of the way the Pioneer spacecraft radiate heat. They argue that the heat (which is just a form of electromagnetic radiation) reflects off the various surfaces of the spacecraft in such a way that its overall effect is to apply a gentle push toward the sun, thereby slowing it down. Granted, this is only one explanation among many that have been proposed, but no previous explanation has satisfied everyone, so a new idea is always welcome. Here is a popular article about the recent paper.

### 3 Responses to “Solar system science”

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2. Mac Says:

Well, that’s a somewhat disappointing explanation for the anomaly. I was hoping it would be the gravitational effects of an alien spacecraft.

Beautiful Mars picture. Did you see the new Mercury ones last week? Not as pretty, but interesting.

3. cburrell Says:

I had sort of been secretly hoping that the Pioneer anomaly would be resolved by an interesting modification of Einsteinian gravity. It would be awfully dramatic. But, then again, I like Einsteinian gravity just the way it is, so I would also be happy if another explanation were found.