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The Planetary Society Weblog

By Emily Lakdawalla

Past, Present, and Future

Feb. 11, 2007 | 10:37 PST | 18:37 UTC

Weblog Archive

by John Spencer

I'm writing this on Friday on the Washington (DC) Metro, heading out of town after a day at NASA Headquarters (I seem to be the only person on the train with an open laptop -- is there some rule of commuter etiquette that I'm unaware of?). A week ago I was on the other side of the country at JPL, helping to plot the immediate future of Enceladus exploration, but the purpose of today's meeting was to take a longer view.

For several years NASA has been pondering its next big mission to the outer solar system, after Cassini and smaller missions like New Horizons and the planned Juno mission to Jupiter. Europa has been the favorite target for many years, because of the probable ocean concealed beneath its remarkable surface. In 1998, NASA planned a relatively cheap Europa mission, the Europa Orbiter, which turned out to be too small in scope to meet the huge engineering challenges of getting into orbit around Europa and surviving the intense radiation environment for long enough to gather a useful quantity of data. Then there was the peculiar distraction of JIMO, the Jupiter Icy Moons Orbiter, an enormously ambitious (and just plain enormous) battleship of a spacecraft, designed to orbit all the icy Galilean satellites in turn using nuclear electric propulsion, which sank under its own weight a couple of years ago, once engineering and fiscal reality set in. In the third act of this Goldilocks story, NASA is now studying a medium-sized mission to Europa, which we hope will be just right in size and scope.

JIMO
The nuclear-propelled JIMO concept, briefly considered as the next big outer planets mission in 2003 and 2004. Credit: NASA

But much has happened in the outer solar system since the late 1990s, and Europa is no longer a shoo-in as our next big destination. Titan is emerging as a compelling alternative, and Enceladus also needs to be considered. Enceladus has much in common with Europa -- both are intensely fractured, tidally-heated ice worlds with possible liquid water inside. Both are living worlds, but Europa may be sleeping right now while Enceladus is very much awake. There's also a school of thought that favors surveying the entire Jupiter moon system, including Europa, without actually orbiting Europa and enduring the intense radiation there.

So we have at least four possible places to go (Europa, the broader Jupiter system, Titan, and Enceladus), and probably just one big Cassini-class outer planet mission in the remainder of the professional lifetimes of many of us. Which to choose? It's the Cassini extended mission problem, writ large (I know, we are lucky to have such problems!). NASA is funding four studies to investigate missions to each of these targets, and, with Amy Simon at Goddard, I'm co-leading the science team for the Enceladus study. The purpose of today's Headquarters meeting was to give us our marching orders -- what launch vehicles and power sources we could assume, the format of our reports, our total budgets, and so on. We have until late summer to complete our work -- may the best moon win!

Simultaneously, back on the left coast (and elsewhere, via telecon), planning for the more immediate future of Enceladus exploration entered its next phase at JPL today. Now we've chosen Cassini's orbit for the extended mission, we have to figure out how to best use the time, including getting the best science out of those seven precious Enceladus flybys. It's not simple -- if we aim the cameras at the south pole, where the action is, the dust instrument may be pointing the wrong way to analyze the ice grains in the plume, and if we point the main antenna at Earth to track the spacecraft and measure Enceladus' gravity, both the cameras and dust detector may be out of luck. If we fly too close, we won't have time for a good look at the surface, and if we don't fly close enough, we won't get adequate measurements of the dust and gas plumes. And so on. Even with seven varied flybys to bend to our multifarious purposes, it will be a tricky balancing act. The HQ meeting meant I'd miss the Cassini meeting, so with input from other team members, I did some pre-emptive work this week, making a rough-cut "strawman" plan for divvying up the seven flybys -- how we might tweak the trajectory, which direction to point the spacecraft for the benefit of which instrument on which flyby, what nifty special opportunities existed on each flyby, and so on. I passed on my plan to the rest of the Cassini team to discuss without me at today's telecon.

Click to enlarge >

Examining Enceladus' South Pole
Cassini's view of the south pole of Enceladus nine seconds after closest approach to Enceladus on October 9, 2008. The little white square is the field of view of Cassini's wide angle camera. We'd like to move the spacecraft trajectory a bit to the right, so we can use the intense ultraviolet light from the star Bellatrix, in Orion, to probe the gases spraying out of Enceladus' south pole. Credit: David Seal, JPL.

I called into the telecon briefly during breaks in the HQ meeting, and learned that my beautiful plan had not met with universal acclaim. I had made a small mistake in describing the suggested orientation of the spacecraft, because I hadn't had one of my most valuable planning tools (below) with me when I put the plan together. But more seriously, on several of the flybys I hadn't anticipated everyone's needs as well as I had thought. There was a little tension in the air. Solutions that worked for remote sensing didn't work for the plasma instruments, and vice versa. The tour may be decided, but the negotiations are far from over.

Click to enlarge >

My Most Valuable Planning Tool
One of the best ways to visualize the orientation of Cassini is to use a physical model. And it doesn't need to be pretty... Credit: John Spencer

After the Headquarters meeting was over, I walked the few blocks to the National Air and Space Museum. Hallowed ground -- that place always blows me away. There was the actual Apollo 11 command module, as always, and I could come within a plexiglass inch of touching it! My spiritual needs satisfied, I found a quiet corner of the cafeteria and called Amanda Hendrix, one of the leaders of the Cassini telecon, for a debrief on the rest of that meeting. Inspired by the history of space exploration, we work on some of the next steps, and ponder the decades to come.