Space

This year we give thanks for an historically influential set of celestial bodies, the moons of Jupiter. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, and the Jarzynski equality.)

For a change of pace this year, I went to Twitter and asked for suggestions for what to give thanks for in this annual post. There were a number of good suggestions, but two stood out above the rest: @etandel suggested Noether’s Theorem, and @OscarDelDiablo suggested the moons of Jupiter. Noether’s Theorem, according to which symmetries imply conserved quantities, would be a great choice, but in order to actually explain it I should probably first explain the principle of least action. Maybe some other year.

And to be precise, I’m not going to bother to give thanks for all of Jupiter’s moons. 78 Jovian satellites have been discovered thus far, and most of them are just lucky pieces of space debris that wandered into Jupiter’s gravity well and never escaped. It’s the heavy hitters — the four Galilean satellites — that we’ll be concerned with here. They deserve our thanks, for at least three different reasons!

Reason One: Displacing Earth from the center of the Solar System

Galileo discovered the four largest moons of Jupiter — Io, Europa, Ganymede, and Callisto — back in 1610, and wrote about his findings in Sidereus Nuncius (The Starry Messenger). They were the first celestial bodies to be discovered using that new technological advance, the telescope. But more importantly for our present purposes, it was immediately obvious that these new objects were orbiting around Jupiter, not around the Earth.

All this was happening not long after Copernicus had published his heliocentric model of the Solar System in 1543, offering an alternative to the prevailing Ptolemaic geocentric model. Both models were pretty good at fitting the known observations of planetary motions, and both required an elaborate system of circular orbits and epicycles — the realization that planetary orbits should be thought of as ellipses didn’t come along until Kepler published Astronomia Nova in 1609. As everyone knows, the debate over whether the Earth or the Sun should be thought of as the center of the universe was a heated one, with the Roman Catholic Church prohibiting Copernicus’s book in 1616, and the Inquisition putting Galileo on trial in 1633. …

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Here’s a pretty picture from JPL, based on data from NASA’s Mars Reconnaissance Orbiter. Click to see a larger version. (Note that the image is highly doctored, in the best NASA tradition; not just false-color, but they’ve “reprojected” so that a satellite image now looks like it was taken by a flying helicopter!)

“Water on Mars” is one of those things (like “black holes” or “the missing link”) that seems to be discovered over and over again. That’s because we’re not really finally discovering it once and for all; we’re slowly gathering new evidence, and also evidence for different manifestations. It seems clear that frozen water exists in the polar regions of Mars; also, there’s good reason to think that there used to be running water at some point. This new finding would be evidence for running water right now.

In this case, NASA scientists have noticed seasonal changes in hillside patterns such as this one. The dark streaks seen in the image appear in the spring and summer, then fade again in winter. (Kind of like the Los Angels River, but backwards.) The best idea we have for an explanation is running water. Not that the darkness is water itself, but some change in the underlying substance as a result of water. It’s a very good idea — likely true — but still not quite like we’ve filled up a cup and done a chemical analysis.

Anything with any tenuous connection to “life on other planets” runs the risk that everyone wants it to exist and is looking very hard; consequently, skepticism is always warranted. Still: awesome pictures!

Okay, that’s a bit alarmist. But Stephen Hawking has generated a bit of buzz by pointing out that contact with an advanced alien civilization might not turn out well for us backward humans. In fact, we should just try to keep quiet and avoid being noticed.

“If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans,” he said.

Prof Hawking thinks that, rather than actively trying to communicate with extra-terrestrials, humans should do everything possible to avoid contact.

He explained: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet.”

To which I can only say: yeah. Sounds about right. If aliens were sufficiently enlightened to be utterly peace-loving and generous, it would be great to have back-and-forth contact with them. But it’s also possible that they would simply wipe us out — not necessarily in a Mars Attacks! kind of invasion, but almost without noticing (as we have done to countless species here on Earth already). So how do you judge the risk? (Dan Drezner gives the interplanetary-security perspective.)

It’s like the LHC doomsday scenarios, but for real — the sensible prior on “murderous aliens” is much higher than on “microscopic black hole eats the Earth.” Happily, a face-to-face chat seems unlikely anyway. Nothing wrong with listening in, on the unlikely chance that the aliens are broadcasting their communications randomly throughout the galaxy. Besides, a little advance warning wouldn’t hurt.

Update: I had forgotten that we had already discussed this a couple of years ago. Old bloggers tend to repeat themselves.

If you haven’t heard that Planck has seen first light, you haven’t been reading the right cosmology blogs: see Andrew Jaffe, Peter Coles, and Planck’s own Twitter feed. Planck is of course the European Space Agency’s microwave background satellite experiment, which was launched back in May. Since then it’s been tumbling in space about once every minute, doing a leisurely scan of the sky. The survey is not nearly completed, but all systems seem to be running smoothly.

Here’s the region it’s looked at so far, superimposed over a visual-light map of the Milky Way:

And here’s a zoom in on one region, as seen in two different wavelengths:

So far the scientists are playing with the data to learn about the instrument, not so much about the microwave background. Andrew predicts a big splash of papers from Planck in August 2012. We’ll be looking for a bunch of things: Are the overall features of the CMB consistent with predictions from inflation? Are there “non-Gaussian” features indicating extra power in some regions? Is the strength of the perturbations equal on all scales, or does it gradually diminish at smaller distances? Did we learn anything surprising from the polarization, such as tensor modes that could come from inflation or an overall rotation that could come from quintessence? Does the universe have a preferred direction?

I’m sure it will be front-page news, whatever that news turns out to be. Stay tuned.