Remote sensing comes in handy

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!