Science and Plutonium
A political thing that is relatively uncontroversial for your (American) election day. But, first! A story!
So, yesterday folks in my research group were talking about various things, as we do, and somehow the Cassini end of mission plans came up. If you're not familiar with what I actually do, Cassini is the mission I work on, that currently orbits Saturn and has for seven years. We plan on running it until 2017, since outer Solar System missions are pretty hardy, and it'll always be cheaper to use what you got than to build a new mission and wait til it gets out there. Plus, there are other giant planets to visit.
Now, NASA has a thing about space missions near potentially habitable places cleaning up after themselves. Anything going to Mars gets sterilized like it carrying the zombie plague. NASA dropped the Galileo probe into Jupiter at the end of mission, because it could hit Europa, which has an ocean under tens of kilometers of ice. Since Saturn's moon, Enceladus, also has something going on -- it has geysers, which could mean liquid water near the surface -- NASA insists that before Cassini runs out of thruster fuel or power, we drop it into Saturn so it will never hit Encealdus.
Consequently, Cassini scientists figured out the coolest way to do so, considering it's a bit of a tricky problem, since... well, Saturn has big old rings, not like Jupiter's puny ones. And we're apparently not allowed to crash Cassini into the rings instead*. So, the plan is, first to bring Cassini so that its closest approach to Saturn crosses just outside the Main Rings, but inside the braided F ring. It orbits like this for a while, then we use a close encounter with Titan* to change the orbit again so that closest approach passes between the rings and the planet. Stepping down like this lets Cassini make the transition from staying away from the rings full of spacecraft-killing ice boulders to 'lets drop into the planet'.
So, basically we want to spend a year orbiting in 'Proximal Orbits'. We can use these orbits to get the mass of the rings and information about the inside of Saturn. What we do invokes a lot of math, but basically involves Cassini sending radio waves back at us and telescopes at Earth measuring them. Then scientists turn that radio signal back into a position and velocity of Cassini, and use it to look for the nudges of gravity: of the rings, say, or of a slight difference in gravity based on the size of Saturn's core.
Now, if Cassini loses funding -- which, well, economy in the toilet -- we'll have to do this early. And in addition to the loss of Important Cassini Science that is not this, it turns out that the proximal orbits we set up won't be as good if we have to throw it together at a moment's notice. The plan works best when Cassini does its closest approach when it is directly between Saturn and us. If it's off to the side, we get less accurate measurements. And the alignment takes planning and precision and can't be done just by saying 'time to drop the spacecraft into Saturn' -- end of mission scenarios took months being argued over.
I mention this because the current mission to Jupiter, Juno, is planning on doing the same kind of interior measurement that Cassini will do at Saturn. Juno doesn't have this optimal orbit -- quite the opposite in fact, which makes it less useful. The reason is, that Juno can't spend time in Jupiter's shadow, because, unlike Cassini, Juno is solar powered. Cassini is powered by radioactive plutonium-238, and doesn't care if it goes into Saturn's shadow -- in fact, some of us rings people like it, since we're not allowed to point Cassini's main cameras near the Sun unless a planet or moon is blocking it***.
Besides this being a nerdy story in of itself, I mention this because of a story on NPR this morning about a problem astronomers have known about for a while, one that could hinder our ability to launch new missions outside of the asteroid belt. Basically, we need to make plutonium-238. And, well, we're not. And we're running out of the stuff -- in addition to the fact that radioactive materials decay (making delicious heat and energy), we've used a lot of it. Cassini has some. New Horizons, heading to Pluto, has some. Both can't be done without it -- the Sun is too faint out there. In fact, Juno is the first mission past the asteroid belt that took solar panels. We can also use it for a bit more power than solar panels can provide: the old Viking landers and the new Mars Rover Curiosity (launching the day after Thanksgiving, weather permitting!) have plutonium so they could do more science. Curiosity has a fricken laser beam, after all!.
Anyway, so NASA uses plutonium to do Cool Things. We aren't making plutonium, so we're running out. NASA would like to make more. The Department of Energy, which owns the labs that make more, would like to help. Congress even agrees that 'yes, this is a thing you should do', which... well, Congress never likes spending money.
The problem is that right now Congress is arguing over whether NASA or the DoE should pay for it. Which... look, everyone's budget comes out of the same pot of money, and it's $15 million a year for five years, which is not a lot compared to even a small NASA Mission. Let alone the DoE's nuclear research budget. It's the type of thing that is trivial, but it holds up actually doing something about it.
Astronomers have been beating their heads against the wall over this issue, so it's nice to see it get press.
* It's weird how gleeful I can be about crashing a mission that I've spent my dissertation on, and hope to continue working on in the future. Some of that is just... well, I want Cassini to go out with a bang, and doing Important Cassini Science. And, unlike the solar-powered Mars rovers, say, which run until something breaks, we know Cassini will run out of either sort of fuel and possibly when.
** Since we couldn't bring that much thruster fuel to Saturn, Cassini navigates mostly by stealing momentum from Titan, with the thrusters to make sure Titan throws it in the right direction. Since Titan is a giant moon and Cassini is a bitty spacecraft, Titan doesn't notice that Cassini is borrowing a bit of motion from its orbit.
*** Half of Cassini's cameras don't have shutters. Sensitive astronomical cameras designed to look at faint things don't like bright sunlight. As a result, the spacecraft planning software has a Sun-avoidance mode so Cassini won't get its cameras near the Sun on accident even when turning to look at something else.