What I Learned Today

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!

Today was the first day of my Remote Sensing class, which I am attending remotely through USC’s Distance Education Network. In this 2.5-hour class, we covered an introduction to what remote sensing is, what kinds of instruments are used, and some highlights in terms of scientific discoveries that have been obtained through this technology. Here are some tidbits I took away that were surprises:

• Some of the earliest “remote sensing” involved sending cameras up with balloons, then retrieving them and developing their film. This kind of surveillance was used as far back as the Civil War!
• The (spectral) width of atmospheric absorption bands varies with atmospheric pressure, and therefore with altitude; as pressure increases, they spread out to cover adjacent wavelengths.
• Sea-surface height varies with water temperature (this is how they track El Nino) and with water depth (useful for mapping the ocean floor from orbital observations of the sea surface). Separating the two effects (lower sea-surface can mean colder water and/or an oceanic trough) would seem to be a significant challenge.
• Landsat-7, unlike its predecessors, can detect clouds in images on-board and decide to discard cloudy images.