What I Learned Today

The EPOXI mission was born out of the desire to make use of the Deep Impact spacecraft, after it successfully hit Comet Tempel 1 with a separate smaller spacecraft. Two missions were selected to make use of Deep Impact: EPOCh (Extrasolar Planet Observations and Characterization) and DIXI (Deep Impact eXtended Investigation). If you stick DIXI and EPOCh together, you get… EPOXI. The proposers got kudos from NASA HQ for this acronym.

EPOCh has just finished its main investigation, which involved observing seven stellar targets that were believed to have planetary companions. I recently attended an excellent talk summarizing the results by Dr. Drake Deming, the deputy PI for EPOCh. They used Deep Impact’s camera to watch for the characteristic dip in stellar brightness when a planet transits across it. Since the camera was not designed for observing distant stars, it had no automatic stabilization, and the star would appear to wander all over the CCD. Tracking the star in the data once it was downlinked to Earth, and applying a different correction for each pixel in the CCD, makes ground processing challenging. However, they’ve been able to analyze this data and extract some interesting findings.

• They studied a Neptune-sized planet (radius about 4 times that of Earth) orbiting the red dwarf star GJ 436. It has an eccentric orbit that is likely to be influenced by a second, smaller planet. EPOCh has searched industriously for a signal from this smaller planet, so far not yet finding it (down to 1 Earth radius, the limit of what they can see with this instrument).
• A secondary transit happens when a planet goes behind its host star, from our perspective. This also causes a (smaller) dip in total brightness because the planet no longer reflects light from the star. This dip can help provide an upper bound on the albedo (brightness) of the planet. (Neat!)
• They also observed the Earth from Deep Impact, treating it as if it were an exoplanet and trying to see if they could accurately infer its properties. These observations serve as the perfect validation set to help us do a good job of interpreting similar observations of other planets, when we get to the point of having them. More details will appear in a paper on this subject of how an alien observer would view planet Earth.

Even better, the data collected by EPOCh will be released to the public in the spring. So you can try your hand at analyzing it, too!

And of course, stay tuned for news from the Kepler mission, set to launch on March 6. It will stare at (relatively) nearby stars specifically seeking Earth-sized planets in the “habitable zone” (where liquid water is stable). It will survey so many stars that even a null result (if they don’t find any Earth-sized planets) would make an interesting statement about the distribution of planets in the galaxy. It’s far more likely, though, that they will find such planets. We live in such exciting times!

Many of us have a mental image of Mercury that’s black-and-white, like 1950’s TV. We liken it to the Moon: a pale, devastated landscape pock-marked with staccato craters and blanketed with the finest grey dust imaginable. But this view, it turns out, is somewhat mistaken–probably influenced by the classic view at right, captured by Mariner 10 in 1975. Mariner 10 did take some color images as well, but they are lower resolution, and only about half of the planet was imaged in color, so they get less press.

Enter MESSENGER, the NASA mission that just executed its second close flyby of Mercury, on its way to a 2011 orbital insertion. With its more capable imaging system, MESSENGER has captured close approximations of “true color” for Mercury, although as always this isn’t as simple as snapping a photograph (CCDs just don’t respond to color the way the human eye does!).

Further, false color is often more scientifically useful than the our mineralogically impoverished RGB views. MESSENGER recently released a color movie that pans across the surface of the planet in fascinating detail. Color differences here yield clues to compositional differences. Note also that you can see a definite change in resolution; later in the movie, the crater edges are less crisp and somewhat fuzzy. I assume that this is because it was imaged during a flyby, not in regular orbit; the spacecraft was only 200 km from the surface at closest approach, but then would have been moving rapidly away. The movie has been adjusted to correct for the geometric distortion, but the reduced resolution remains. Check it out!

I saw the International Space Station with my own eyes for the first time today. It crossed over Los Angeles from about 8:52 p.m. to 8:57 p.m. (You can get a list of upcoming local flybys based on your zip code.) It was bright: magnitude -3.0, according to that website (“magnitude” of sky objects is inversely proportional to brightness). I watched it sail overhead, from the northwest towards the southeast. It was a solid light, moving so quickly that you might mistake it for an airplane, but then it became obvious that this was no low-altitude craft.

It was nearing Jupiter (within about 8 degrees by my estimate) and I was comparing their brightness. At first, I was sure it was brighter than Jupiter (which, my StarPilot software claims, is -2.6 magnitude, so that checks out). But then it started to get dimmer. I blinked, but I wasn’t imagining it. It got dimmer and then began to turn red, and as I watched, it entirely vanished!

Some quick geometric reasoning suggested that I did not just witness the fiery demise of the ISS. Instead, it must have crossed into the Earth’s shadow while I was watching. Given the time of day and the relative position of Sun, Earth, and ISS, that makes sense. The red color I would not have known whether to expect to see visually, but presumably it’s the same effect we see at sunsets: low-angle sunlight must cross through more atmosphere, absorbing more of the shorter-wavelength colors, and at the limit, red will preferentially shine through due to refraction even as the Sun is partly occluded. So the light I was seeing definitely wasn’t coming from some airplane’s headlight; it was reflected sunshine!

And not only did I see the ISS, but I witnessed an ISS eclipse.

I came across an article today describing the latest extraterrestrial ocean hypothesis — that one exists beneath the crust on Titan. That’s right, a subsurface ocean, in this case probably composed of water and ammonia. This is in contrast to the so-last-year news about methane lakes on Titan, which are widely accepted. Usually Enceladus or Europa get all of the press with regards to potential oceans, so this is pretty unexpected. From the article:

“Using data from the radar’s early observations, the scientists and radar engineers established the locations of 50 unique landmarks on Titan’s surface. They then searched for these same lakes, canyons and mountains in the reams of data returned by Cassini in its later flybys of Titan. They found prominent surface features had shifted from their expected positions by up to 19 miles. A systematic displacement of surface features would be difficult to explain unless the moon’s icy crust was decoupled from its core by an internal ocean, making it easier for the crust to move.”

I’m curious about these observations. I’m sure that the scientists involved have already applied an appropriately sized dose of skepticism to this subsurface ocean theory, but my gut reaction would be that an error in measurement is far more likely than a decoupled crust and core separated by a liquid ocean (where are you, Mr. Occam?). (In fact, if the displacements really are “systematic” then a measurement bias/error is an even more likely candidate explanation.) I’ll have to look for the upcoming article in Science!

On Friday, I attended a talk at work on the Europa Explorer study, a flagship NASA mission concept that is currently being considered, in competition with three other candidates, for a 2015-2025 launch window. This is a big mission (a budget of about $3 billion) and would orbit Europa for a full year. The orbiter includes a host of remote sensing instruments to tackle the big science questions, such as “Is there really a liquid ocean beneath the icy surface?” and “What processes are currently active on Europa?”

At one point, the presenter noted that they’d like to have lower-orbit “dips” late in the mission, to improve the quality of subsurface sounding (searching for that ice/ocean boundary, which is expected to be tens or hundreds of kilometers deep). One person in the audience asked, “Why do you have to dip down? There’s no atmosphere to scatter the signal,” and without thinking I said, “Because signal power falls off as R^4,” which was straight from the material we’ve been covering recently in my Remote Sensing class. (R is the range from the instrument to whatever it is imaging, and active instruments pay an R^2 penalty in both directions.) Technically, it’s the signal-to-noise ratio that falls, but it’s such a big hit with the two-way signal path that it matters a lot. Even though there’s no atmosphere, the signal is attenuated by the distance.

Having a bit of new knowledge pop up when it’s relevant is a nice payoff for the time invested in this class. Two more lectures, one homework, and one final to go!