A Europa-Io Sample Return Mission

A couple of weeks ago the Obama Administration released its proposed budget for FY2015.  NASA’s budget (which is almost certainly subject to change by Congress as has been the case for decades) would stay essentially flat at $17.5 billion and Planetary Science would get nearly $1.3 billion or just $65 million less than what Congress approved for the current fiscal year.  Included in the budget is $15 million for continued studies of a mission to Jupiter’s moon, Europa, in the 2020s.  But instead of a full blown flagship-class mission, the Administration is proposing that the Europa mission have a cost target of under a billion dollars.  This is just a fraction of the cost of missions like the proposed $2.1 billion Europa Clipper currently under study which itself is a fraction of the cost of the earlier proposed $4.7 billion Europa Orbiter.  As the planetary science community scrambles to figure out how to meet such a tight budget cap to such a difficult-to-reach (but scientifically fascinating) target like Europa, I would like to make a suggestion: A sample return mission to Europa.

A little over a year ago, the popular space press was filled with a flurry of stories about a proposed, low-cost sample return mission to Saturn’s moon, Enceladus.  Proposed by a team led by Peter Tsou of Sample Exploration Systems in La Canada, California, the LIFE (Life Investigation For Enceladus) mission would have a spacecraft fly through the geyser plumes in Enceladus’ southern polar region and use an aerogel collector (of the same sort successfully employed by NASA in the Stardust mission to return samples of cometary dust in 2006) to secure samples of the plumes’ icy particles for return to the Earth where they could be studied in detail.  Earlier concepts of the LIFE mission also included sample collection of Saturn’s E-ring (believed to be generated by particles that escaped Enceladus’ geysers into orbit around Saturn) and even the atmosphere of Titan.  The proposed mission offers an affordable means of securing samples from the watery (potentially life-supporting) environment beneath this moon’s icy crust using readily available technology.

In a presentation made last June at the LCPM-10 conference at Caltech, Tsou and his team outlined their vision for their proposed Discovery-class LIFE mission [1]:  The 15-year LIFE mission would be launched in the early 2020s and employ a spacecraft with a ~800 kg dry mass possessing a ~3000 m/s Δv capability.  Assuming a storable, bipropellant propulsion system with a specific impulse of ~300 seconds like those typically used by other NASA planetary spacecraft, the launch mass would be something on the order of ~2,200 kg.  There are a number of possible mission profiles depending on the launch date and other factors, but in one proposed mission scenario, LIFE would be launched in November 2021 and use a VEEGA (Venus-Earth-Earth Gravity Assist) trajectory to gain the speed needed to reach Saturn using a smaller (and more affordable) launch vehicle.  The Venus flyby would take place in April 2022, the first Earth flyby would occur in March 2023 and the final Earth flyby in June 2026.  The spacecraft would then hibernate (to save on mission operation costs) until just before reaching Saturn in May 2030.

After entering orbit around Saturn, LIFE would use five close passes by Saturn’s largest moon, Titan, over the course of 128 days to gradually alter the probe’s orbit so that it could make multiple low-speed (3.7 to 4 km/s) passes through the geysers in the southern polar regions of Enceladus where an aerogel collector would secure samples of the plumes’ ice particles.  This is lower than Stardust’s 6.1 km/s encounter velocity with Comet Wild 2 in January 2006 and would result in better preservation of fragile ice particles.

After the encounters with Enceladus are completed, seven additional Titan flybys over 152 days would gradually pump up LIFE’s orbit in preparation for its departure.  After spending about two years in orbit around Saturn, LIFE would use its propulsion system to escape Saturn and begin the ~4.5 year long voyage back to Earth.  Sometime in late 2036, the return capsule would detach from the main spacecraft and reenter Earth’s atmosphere at a speed of 16 to 18 km/s.  The total Discovery-class mission cost would be about $425 million, excluding launch, and is advertised to generate flagship-class quality science on a Discovery-class budget.

With the announcement in December of the possible detection of plume activity over the southern polar region of the Jovian moon, Europa [2, 3], the possibility exists of using a spacecraft essentially identical to that employed in the proposed LIFE mission to secure samples from the Europan plume(s).  And as an added bonus, it may prove possible during the same mission to sample one or more volcanic plumes of Io (the next moon in from Europa) which have been observed to reach as high as 500 km above this active moon’s surface – easily within the reach of a passing spacecraft.

To investigate the feasibility of this idea of a Europa-Io sample return, I have checked available trajectories for a round trip mission to Jupiter using the Mission Design Center Trajectory Browser available on-line [4].  This database indicates that there are four favorable launch windows in the early 2020s available (the same time frame as the launch of the proposed LIFE mission) that employ a single Earth flyby to provide the extra boost needed to reach Jupiter (the same gravity assist strategy used by Juno currently en route to Jupiter): June 2021, October 2022, March 2023 and October 2023.  The most energetically favorable (from the stand point of combined launch energy and total in-flight Δv requirements) with the shortest total mission time is the first launch window in June 2021.

In my notional Europa-Io sample return mission scenario, launch would occur in June 2021 or just five months before the proposed launch of LIFE (at least in the scenario described at the LCPM-10 conference).  The spacecraft would make a deep space maneuver amounting to ~570 m/s just over a year later near aphelion.  After a flyby of the Earth at an altitude of 1,150 km in July 2023, the probe would be flung towards a rendezvous with Jupiter in October 2025.

Once at Jupiter, the spacecraft would use its propulsion system to enter an elongated Jovian orbit to keep the spacecraft away from the worst of Jupiter’s radiation belt for most of the mission.  Using repeated gravity assists from Europa, Ganymede and Callisto, the spacecraft’s orbit would be gradually decreased in size to allow multiple, relatively low velocity encounters with Europa’s polar plumes so that samples could be gathered using an aerogel collector.  With a spacecraft perijove distance equal to Europa’s mean orbital distance from Jupiter, encounter speeds of 3.7 to 4.0 km/s (the Enceladus encounter speed in the LIFE mission scenarios) are possible if the apojove is ~2.0 to ~2.4 million km – just beyond the orbit of Callisto.  By coincidence, this potential Europa encounter orbit would be in a 3:1 resonance with Europa (i.e. Europa would orbit Jupiter three times for every orbit of the spacecraft allowing repeated encounters with minimal orbit adjustments) with a apojove of ~2.1 million km.  Such a resonance would facilitate repeated encounters with Europa if the orbit is properly phased.

After months of observation from a safe distance to spot the most promising target, one final close pass by Io located deep inside Jupiter’s radiation belt could be made to sample one of its volcanic plumes.  This encounter with Io and (probably) during insertion into Jovian orbit would be the only two times this spacecraft would be required to pass through the most intense portions of Jupiter’s radiation belts thus helping to minimize the spacecraft’s total radiation exposure.  Keeping the encounter velocity with Io down to ~4 km/s will be more difficult but perhaps a greater velocity would be acceptable since the mineral grains from Io’s plumes will be more robust than any icy particles from Europa’s plume.  Depending on any limitations on the spacecraft’s orbit around Jupiter, it might even prove possible to sample Jupiter’s outer Gossamer Rings associated with Jovian moons Amalthea and Thebe thus allowing indirect sampling of these bodies as well (since these rings are believed to originate from material that has escaped these small inner moons).

After the spacecraft’s sampling mission is completed, repeated gravity assists from Jupiter’s Galilean moons would be used to pump up the probe’s orbit.  After almost two years orbiting Jupiter, the spacecraft would use its propulsion system once again in September 2027 to escape Jupiter and start the 2.8-year long trip back to Earth.  Reentry into Earth’s atmosphere at a speed of 18 km/s (a speed similar to that of the proposed LIFE return capsule) would occur in July 2030 ending the nine-year mission.

A detailed mission study, like those already performed by Tsou et al. for their proposed Enceladus mission, needs to be performed to design the mission around Jupiter properly and determine the total Δv requirements but we can attempt a ROM estimate using NASA’s Galileo mission to Jupiter as a guide.  Galileo’s Δv to enter its initial 215,000 x 19 million km orbit around Jupiter was 643 m/s.  About 97 days later, Galileo performed a maneuver with a Δv of 377 m/s to raise its perijove to a safe altitude and set Galileo on course for multiple encounters with the Galilean moons.  A total Δv of 145 m/s was allocated for Galileo’s primary mission with multiple encounters with Europa, Ganymede and Callisto [5].  For lack of more detailed guidance, for now I assume the same Δv requirement for this sample return mission.

The Δv required to escape Jupiter depends critically on its final orbit around Jupiter and how much it can be pumped up with gravity assists from Jupiter’s moons in the available time (and if more time is needed, trajectory options exist for much longer stays at Jupiter with minimal impact on total Δv requirements) not to mention the requirements for departure back to Earth.  Let’s use the same Δv that Galileo used to assume orbit around Jupiter –  643 m/s.  Add to this the total Δv of 1050 m/s required for this particular choice of mission trajectory and 280 m/s for course corrections and other contingencies (the same amount allocated by Tsou et al. in their LIFE study), and the total Δv for this mission would be on the order of 3,100 m/s or about the same estimated Δv as the proposed LIFE mission.

The total Δv requirements for the other three launch windows in 2022 and 2023 would be only modestly higher with total mission times up to 1 to 5 years longer for a given stay time at Jupiter.  With a C₃ launch energy requirement of 28 km²/s² for this particular choice of mission trajectory and a launch in June 2021, an Atlas V 400-series or Delta IV Medium-series launch vehicle should have the needed performance to send a ~2,200 kg LIFE-like sample return mission to the Jovian system.  The other launch windows have slightly lower C₃ launch energy requirement of ~25 km²/s² (the advantage of which is more than offset by the greater post-launch Δv requirements for these trajectories).

While a lot more work is required to flesh out the details of a Europa-Io sample return mission (especially more information on the nature of Europa’s purported plumes), at first blush it does appear to be feasible using the same hardware proposed for the LIFE mission to Enceladus employing readily available launch vehicles.  This proposed mission also nicely complements the investigations of the LIFE mission by returning samples from yet another set of plumes on a potentially life-bearing moon with the added bonus of sampling volcanic material from a second target of keen interest to planetary scientists –  Io, the solar system’s most volcanically active world.  In addition, it does so with a shorter mission (9 years versus 15 years) and with a mission time line that does not appear to interfere with the currently proposed LIFE mission.  In fact, this proposed Europa-Io sample return mission could be completed just as LIFE would arrive at Saturn.  For minimal additional costs (i.e. a second spacecraft and launch vehicle along with the incremental cost increase of running two missions in parallel), this scientifically interesting mission could be flown in parallel with LIFE and greatly increase its total science return.  And it could probably do so within the Administration’s proposed billion dollar price cap for a Europa mission.

 

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Related Reading

“Sampling the Surface of Europa”, Drew Ex Machina, May 29, 2015 [Post]

“The Next Mission to Pluto”, Drew Ex Machina, July 15, 2014 [Post]

 

References

[1] P. Tsou, D.E. Brownlee, C.P. McKay, A. Anbar, H. Yano, Nathan Strange, Richard Dissly and I Kanik, “Low Cost Enceladus Sample Return Mission Concept”, Low Cost Planetary Mission Conference – 10 (Pasadena, CA; June 18 – 20, 2013), 2013 (Presentation)

[2] Lorenz Roth, Joachim Saur, Kurt D. Retherford, Darrell F. Strobel, Paul D. Feldman, Melissa A. McGrath and Francis Nimmo, “Transient Water Vapor at Europa’s South Pole”, Science, Vol. 343, No. 6167, pp. 171-174, January 10, 2014 (Abstract)

[3] “Hubble Space Telescope Sees Evidence of Water Vapor Venting off Jovian Moon”, News Release No. STScI-2013-55, December 12, 2013 (Press release)

[4] Mission Design Center Trajectory Browser, NASA Ames Research Center (Web site)

[5] “Galileo”, in Janes Space Directory 2001-2002, David Baker (editor), pp. 457-461, Janes Information Group Ltd., 2001