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 or 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 will 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.
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| The current baseline plan for launching the Europa multiple flyby mission is the Space Launch System. A new backup plan under consideration could reduce the alternative flight from over seven years to less than five years. Credit: NASA/JPL |
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.
 |
| Science objectives for Enceladus and Titan presented by Dr. Green. Credit: NASA |
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 –that would 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.
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| Examples measurements a New Frontiers mission to Enceladus or Titan might make to meet the science goals. Credit: NASA |
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.)
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| A Europa lander, whose design could be used for landing on other ocean worlds, would consist of four major elements, a carrier craft that would also relay the lander's data, a solid rocket motor to slow the lander, a sky crane descent stage, and the lander itself. |
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.
 |
| The proposed Europa lander would look much like and be roughly the same size as the Mars Pathfinder lander (bottom). Credit: NASA/JPL |
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.
Additional Material
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| Current launch plans for the Europa multiple flyby mission. Credit: NASA/JPL. |
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| The proposed multiple flyby tour of Titan and Enceladus for the Journey to Enceladus and Titan (JET) mission (top) and the ground tracks below each flyby (top panel of bottom slide) and the imaging resolution for Titan and the height of the flyby through the plumes for Enceladus. Credit: JPL |
