Defining the Missions for the Ocean Worlds

This article originally appeared on Van Kane's blog and is reposted here with permission.

One of the major revolutions in planetary science that I’ve seen in my lifetime is the discovery that the solar system contains not just one ocean world—our Earth—but several ocean worlds. Unlike our planet, which has its oceans on the surface, these other worlds trap their oceans beneath a surface layer of ice (or in the possible case of the asteroid Ceres, beneath a rocky shell). For several of these worlds, such as Jupiter’s moons Ganymede and Callisto, the oceans appear to be locked between layers of ice and therefore would be unlikely candidates for abodes of life. For two of the moons, Jupiter’s Europa and Saturn’s Enceladus, the oceans appear to lie directly on top of a rocky core that would provide key elements needed to support life as well as energy from possible hydrothermal vents. Saturn’s moon Titan is a unique case, with seas of liquid ethane, methane, and propane on the surface and a water ocean in the interior that may or may not be in contact with the rocky core and occasionally interact with the surface. (This article and poster give more background on these worlds and their oceans.)

NASA’s managers, at the direction of Congress, have begun to put together an Ocean Worlds program to explore Europa, Titan, and Enceladus. At a recent meeting of an advisory group for NASA, the Committee on Astrobiology and Planetary Science (CAPS), Jim Green, the head of NASA’s Planetary Science Division, and Barry Goldstein from the Jet Propulsion Laboratory, provided updates on plans to explore these worlds. In this post, I’ll report on the highlights of their talks (the presentations will be posted to this site (scroll down to the March 29–31 meeting) sometime in the future).

Europa Multi-flyby Mission

The only currently approved mission in the Ocean Worlds program is the Europa multi-flyby spacecraft. This mission, estimated to cost ~$2 billion, will orbit Jupiter and take approximately 45 toe dips into the high radiation belts surrounding this moon to make close flybys. In between flybys, the spacecraft will have time to transmit the volumes of data it collected up close back to Earth. (This presentation gives a good overview of the mission design and science goals while this presentation summarizes the instrument payload.)

The mission is well into its design phase. At the CAPS meeting, the project manager, Barry Goldstein with the Jet Propulsion Laboratory, updated the committee members on refinements to the design.

Until recently, NASA’s managers had hoped that the main spacecraft could carry a 250 kilogram free flying daughter spacecraft to conduct complimentary studies. Ideas ranged from a simple Europa lander, to a spacecraft that would divert to flyby the volcanic moon Io, to a spacecraft dedicated to flying through any plumes ejecting material from the surface of Europa. NASA had invited the European Space Agency to propose (and pay for) a daughter spacecraft. In addition, a group at NASA’s Goddard Space Flight Center had developed a proposal for a free flyer that would swoop even lower to the surface than the main spacecraft to fly through any plumes while carrying a mass spectrometer more tuned to identifying bio signatures than the main spacecraft’s instruments.

Unfortunately, it appears that NASA has decided to drop the idea of a daughter spacecraft. I’m told that ESA’s managers determined that they had no way to fund such a spacecraft on the timeline for the Europa mission. NASA’s managers may have also decided they lacked the funding to build their own daughter spacecraft.

Dropping the daughter spacecraft opens up new possibilities for launching the multi-flyby spacecraft to Jupiter. NASA’s primary plan for sending this spacecraft on its way is the Space Launch System (SLS) that would enable a direct launch. This extremely large booster could launch the spacecraft directly to Jupiter with a flight time of 2.1 to 2.5 years. It will have sufficient heft to give the project a 33–35% mass margin, providing a cushion should the actual spacecraft as finally implemented weigh more than its designers currently think it will (which usually happens). 

The SLS, however, is still in development and its reliability will only be proven through one or more future flights. In addition, this program is something of a political football, and so assuming it will be funded through development and into the time period of the Europa launch is a risk. It’s also unclear what an SLS launch would cost and whether or not the planetary science program could afford it. The project’s managers, therefore, are designing the spacecraft to also be capable of being launched on a less powerful commercial launch vehicle. 

Currently that backup would be either an Atlas V 551 or Delta IV Heavy booster followed by three Earth and one Venus flybys to receive gravity assist boost that would enable the final flight to Jupiter (known as the EVEEGA trajectory). This extended looping flight would take 7.4 years to reach the Jovian system.

Dropping the 250 kilogram free flyer (plus supporting equipment on the main spacecraft) opens up an alternative launch plan. By enlarging the spacecraft’s propellant tanks to allow a large deep space maneuver to set up a single Earth gravity assist (known as the ∆v/EGA trajectory), a Delta IV Heavy vehicle could deliver the Europa mission in just 4.7 years while still providing a healthy mass margin of 34%. (For Falcon Heavy fans, NASA’s managers will consider this booster, too, once they have its final specifications, but they believe it would have similar performance to the Delta IV Heavy.)

This new launch backup is not yet an official plan as engineers and NASA’s managers examine it in more detail. If they decide they can adopt it, the net savings in flight time if the SLS launch is unavailable is 2.7 years.

Titan and Enceladus

The Ocean Worlds program now includes Saturn’s moons Titan and Enceladus as target worlds. Previous mission proposals were for either for expensive Flagship missions (with estimated costs of ~$1.5 billion to ~$6 billion) or the inexpensive Discovery missions (~$450 million). The former doesn’t fit within NASA’s budget and the latter appears to be too little to reach and explore these distant moons. In the past few months, NASA’s managers have opened up the intermediate cost (~$850 million) New Frontiers mission class to explore these worlds.

At the CAPS meeting, Green presented draft science objectives for a possible New Frontiers mission to Enceladus and/or Titan along with example goals for measurements that would meet those objectives. For Enceladus, the goals relate to understanding the composition of the material within the plumes erupting from the moon’s southern pole. What are the organic molecules in the plume detected by the Cassini spacecraft, but which its instrument lacked the sensitivity to analyze in detail? Do these compounds suggest possible present life or a geological origin from hydrological activity? Does the chemistry suggest that the ocean below the icy crust has the necessary chemicals to support life?

The goals for Titan mission broke into two sets. The first related, as with Enceladus, to questions of chemistry. How are complex organic molecules created, modified, and stored in the upper and lower atmosphere and in the surface lakes and seas? Do any of these compounds suggest possible pre-biotic or even biotic origin? The second set of goals focus on the structure of the interior ocean (for example, is it in contact with the silicate core that would provide many of the elements needed for life?) and whether material from that ocean may have reached the surface (as evidenced by past resurfacing).

Previous studies have looked at a number of mission concepts to continue the exploration of these two moons following the Cassini mission. At the high end—and almost certainly outside the cost cap of a New Frontiers mission—were Titan and Enceladus orbiters and Titan balloons. 

In the past two Discovery competitions, three missions to Enceladus and Titan were proposed (but not selected to fly). By law, NASA’s managers can’t reveal the results of their evaluations of these proposals—that’s proprietary information for the proposing teams who may well propose future missions in these competitive selections. However, comments by managers in public meetings have said their science was compelling and that missions to the Saturn system don’t fit within the cost cap of the Discovery program. The implication is that the higher mission costs allowed by the New Frontiers program could enable a mission to the Saturn system. These past proposals for Discovery-class missions suggest possible New Frontiers-class missions to these worlds.

Two of the Discovery proposals replaced orbiters with spacecraft that would—like the Europa multiple flyby mission—study these Saturnian moons with multiple flybys. The Enceladus Life Mission (ELF) would have flown through Enceladus’ plumes with two cutting edge mass spectrometers that would have studied the chemistry of the ocean’s volatiles and silicates. The Journey to Enceladus and Titan (JET) would have carried a mass spectrometer to study the volatiles in the plumes and Titan’s upper atmosphere. It would also have carried a thermal imaging camera that would have imaged Titan’s surface at up to an order of magnitude higher resolution (as fine as 25 m) than the Cassini spacecraft has done. The imager would also have imaged the sources of the plumes on Enceladus’ surface in much higher resolution than Cassini has.

A possible New Frontiers proposal might combine the ELF and JET proposals by carrying ELF’s mass spectrometers and JET’s thermal imager and conduct multiple flybys of Enceladus and Titan. Such a mission could address the composition questions posed by Green for Enceladus and Titan’s upper atmosphere. The thermal imager could address the questions of whether Titan’s surface morphology indicates that the subsurface ocean has interacted with Titan’s surface. 

Possible enhancements to this type of mission might include an ice penetrating radar to study the subsurface structures of their icy shells. Or the thermal imager could be enhanced by adding an imaging spectrometer that would search for variations in the surface composition of Titan. Both of these latter ideas have been included in previous, Flagship-class mission proposals and would address the goal to better understand the structure of the interior oceans and their interaction with the surface.

Both Discovery proposals included high-speed flybys of Enceladus. While these flybys are relatively easy to set up, the velocity (typically ~4 kilometers per second) could destroy any highly complex organic molecules as they impact the mass spectrometer instruments. One mission option would instead use a number of flybys of the moons Rhea, Dione, and Tethys to lower the orbit over two years to enable Enceladus flybys at ~1 kilometer per second. Affording the additional costs of two years of mission operations likely is hard in a Discovery mission proposal but might be an option that could fit within a New Frontiers budget.

The third Discovery proposal, the Titan Mare Explorer (TiME), would have landed a probe to float on one of the moon’s large polar seas. These lakes are believed to be stews that absorb and release gases into the atmosphere, receive a rain of complex organic molecules created in the upper atmosphere, and interact with the ices forming the shores and bottoms of the lakes. A future mission could replicate TiME’s goal to study Titan’s chemistry. The TiME proposed mission focused tightly on science conducted on a lake. A plusher New Frontiers mission might add instruments that could enhance atmospheric composition measurements as the probe descends to a lake landing as was proposed for a Flagship version of this mission several years back.

There is no assurance that an Ocean Worlds mission will be selected as the next New Frontiers mission, which will launch in the mid-2020s. These missions are selected through open competitions. The other missions on the candidate list—a Venus lander, lunar sample return, comet sample return, a Saturn atmospheric probe, and Trojan asteroid tour—are scientifically compelling in their own right and several may be less risky and expensive to implement. We should learn which mission is selected in 2019.

On a side note, not discussed at the CAPS meeting (at least while I was listening), is the question of international cooperation in exploring Titan and Enceladus. An obvious idea would be to combine the Titan lake lander with the multi-flyby spacecraft that could act as carrier and data relay in addition to its own scientific duties. Fitting both within a New Frontiers budget seems unlikely to me. However, other space agencies, particularly ESA, are also interested in exploring these worlds. It may be possible that NASA would provide one craft within a New Frontiers budget while another space agency provides the complimentary craft. Timing the funding for cooperative missions can get tricky (as shown by the inability of ESA to pay for a Europa mission free flyer on NASA’s schedule), but it is an obvious idea that I’m sure will be explored.

An Ocean Worlds Lander

In addition to providing an update to the Europa multiple flyby mission, JPL’s Goldstein provided the first public look at the current concept for a Europa lander. In the normal progression of exploring a world, NASA would not look at detailed plans for lander until the results from a mission orbiting that world (replaced with multiple flybys for Europa) were in. However, Congress has directed NASA to add a lander to the currently planned Europa mission.

JPL’s engineers have decided to make the lander an entirely separate spacecraft from the multi-flyby spacecraft. To find the spot on this moon that best combines scientific value and landing safety, the multi-flyby spacecraft must first complete its examination of the surface. As a result, a landing would come at least two to three years after the arrival of the multi-flyby spacecraft. The lander spacecraft could either launch with the multi-flyby spacecraft and park itself in Jovian orbit while waiting for the reconnaissance to be complete or the lander could be launch later. (I’m betting on the latter. NASA’s Green described the current design state of the lander concept as “immature” and it’s not clear that NASA will receive sufficient time or funding to mature the design in time for launch with the multi-flyby spacecraft.)

The lander itself would look much like and be about the size of the Mars Pathfinder that landed on the Red planet in 1997 (but without the Pathfinder’s small rover). The lander would be encased in petals that would deploy, allowing the lander to right itself if necessary after touchdown and that could also act as “snowshoes” in case the landing is on a soft surface. A mass spectrometer and a Raman spectrometer would study the composition of the surface material, panoramic and microscopic cameras would provide context and close up images, and a geophone would provide seismic measurements. The lander would include an arm that could scoop or drill samples from the surface to deliver to the instruments. Batteries would power the lander for up to 21 days.

While the initial target for this lander design is Europa, Goldstein pointed out that the design could be used to land on a number of ocean worlds including Enceladus and Jupiter’s Ganymede. (As discussed above, a Titan lander will need enter and descend through a thick atmosphere and then float on a sea. Its design is likely to be quite different.) Perhaps, if the funding gods are kind, we could see both multiple flyby missions to these moons and landers for these moons launch in the next decade or two.

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