Sunday, December 14, 2008

Io Missions: Part One

In case you haven't noticed, there's a poll just to the right of this blog entry with readers' votes on their preferred destination for the next New Frontiers mission. The current voting has Venus as the favorite destination, with Io as a distance second but well ahead of the third favorite, a lunar sample return. A previous (and future) blog entries discussed options for a Venus mission. This and the next two entries looks at options for a return to Io.

Topics will be (1) Background on options for a return to Io, (2) science goals for an Io mission, (3) the proposed Io Volcano Observer, and (4) ideas for extended missions. This entry addresses the first topic, and entries later this week will address the other topics.

On a side note, I'm writing this while traveling to the American Geophysics Union (AGU) fall conference. While this conference is usually jam packed with results from missions in progress (and it will be again this time), this meeting will also have two sessions devoted to future Venus missions as well as poster presentations on several other proposed missions. It will take me awhile to work through the riches, but I will report on all the sessions over the next few weeks.

Background on options for a return to Io

A return to study Io has been a goal of a subset of the planetary science community (and based on our poll, the public that follows planetary exploration) ever since the end of the Galileo mission. While the Galileo spacecraft made several close flybys of Io, relatively little scientific data was returned in part because of the malfunctioning main antenna and in part because the intense radiation near Io caused the spacecraft to repeatedly go into safe mode just prior to closest approach. I don't want to downplay the value of the data returned. It was scientifically invaluable. However, only small portions of the moon were imaged and temporal coverage of this very dynamic moon was limited. One presentation I saw stated that just 0.2Gbytes of data were returned. That represented a very tiny window of data for what is a very dynamic moon.

The planetary science community has made a return to Io a priority. It is one of a handful of missions listed a priority targets for the next New Frontiers mission ($650M). (For a list of all the targets, see the previously mentioned poll.) Why is Io a priority?

o Because of extreme tidal heating, Io is the most active volcanic body in the solar system. The style of volcanism is believed to resemble that of the terrestrial planets in their extreme youth.

o Io is the best place to study tidal heating, a phenomenon that maintains a liquid ocean beneath the crust of Europa and possibly Enceladus.

o Io is a geologist's dream with interesting surface chemistry, grand mountains....

o Io is intimately connected to the Jovian magnetosphere and has even been called the heartbeat of the magnetosphere.

Currently, there are three possible routes for a return to Io. The most likely (in that there's a 50-50 chance the mission will be selected to fly) is the next Flagship mission (~$3B). (Titan is the other candidate target in this selection, which will occur in the next few months.) If Jupiter is the target of the next outer planets Flagship mission, it will include three close flybys of Io for science, including one pass that would go through (presumably the outer fringe) of a volcanic plume. (On Jupiter orbiter insertion, a flyby would also be done to use an Io gravity assist to reduce speed, but scientific observations apparently would not be done, at least with the remote sensing instruments.) Following the Io flybys, continued long range observations would be made for the remainder of the mission. If this mission flies, I doubt that any of the other possible missions that would target Io would be selected.

Let’s assume here, however, that Titan is chosen for the flagship mission. In that case, a New Frontiers mission to Jupiter would still be in the running. (The decision on the New Frontier's target will come in a couple of years.) Return missions have been studied for at least the past decade. I have a paper copy of a proposed Jupiter orbiter/multiple Io flyby Discovery program mission from about ten years ago. I believe that Io missions have been proposed for the Discovery program at least once and if not multiple times. None have ever become finalists.

If the mission(s) were proposed, two factors probably kept them out of the running. First, a $300-450M (budget caps have increased over time) budget is very tight for an outer planets mission. Both the New Horizons Pluto flyby craft and the Juno Jovian orbiter required the approximately doubled budget cap of the New Frontiers mission. In addition, radioactive power sources have not been allowed for Discovery missions. Solar cells can be used at Jupiter (the Juno Jupiter mission will use them, for example.) However, the intense radiation fields in the inner Jovian system where Io lies will gradually degrade performance of the solar cells, limiting the number of close passes. I've not seen a definitive answer as to how much, although at least a couple of studies have suggested that even the Europa orbiter mission (which would face much higher high radiation levels) could be done with solar cells.

Whatever its power source, an Io mission would orbit Jupiter and conduct multiple flybys of Io. The radiation field at Io is so strong that an Io orbiter's lifetime would be measured in hours to days – assuming the craft survived the radiation to even enter orbit. The total number of flybys would ultimately be limited by the cumulative radiation damage to the craft's electronics or (if used) solar cells. The craft can be designed to tolerate high levels of radiation exposure. An Io mission with the same radiation hardening as the proposed Europa flagship mission could do 50 – 100 flybys of Io within its radiation tolerance. (Note that this number is an extrapolation from a couple of presentations with different assumptions about radiation hardening; if any readers have better information, please pass it along.)

An Io mission study group that provided input to the 2003 Decadal Study proposed an Io mission that would encounter Io multiple times (up to 50) at the same place in its orbit. This way, lighting conditions would remain identical with each encounter. Different lighting conditions can make it difficult to determine whether apparent surface changes at Io are the result of true changes in the surface material or changes in lighting.

The Decadal Survey mission concept assumed a very capable craft with a radiation tolerance half that of the then proposed Europa mission. I don't know what degree of radiation hardening can be purchased within the $650M New Frontiers budget, but it is likely less. Radiation is a major design driver for the Juno Jupiter orbiter, which uses a “hole” in the radiation belt near Jupiter to minimize radiation exposure. I suspect that providing radiation hardening to the level of a Flagship mission is probably outside the budget of a New Frontiers mission.

On the other hand, we know that the New Frontiers budget cap is large enough to implement a mission to the outer solar system. Both the New Horizon Pluto and the Juno Jupiter missions are New Frontier missions. It is a reasonable guess that a robust Io mission could also be implemented within the budget cap.

In in the third part of this series, I'll report on a proposal to implement an Io mission within the much tighter Discovery mission budget.

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