Jupiter’s moon Europa has been a priority destination for NASA’s
planetary program since the mid-1990s.
With a deep ocean trapped beneath an icy shell on top and the rocky
surface below, Europa is believed to have the chemicals and energy needed to
host life. Over the course of almost two
decades, I’ve seen plans for a better, really cheaper, faster mission that just
needed a lot of new technology to be developed.
As if to balance that plan out, there was a plan for the planetary
equivalent of a Battlestar Gallatica mission that was both unaffordable and
also required technology that still doesn’t exist. I thought we were close with the Jupiter
Europa Orbiter (JEO, circa 2010) until new cost estimates showed that it, too,
was unaffordable.
Now we have a proposed mission, the Europa Clipper, that doesn’t require
substantial technology development and that has a cost estimate (~$2B) that
puts it well within the cost range of NASA’s larger science missions. However, in today’s era of declining US
federal budgets, the Clipper’s price tag is deemed unaffordable.
In a conversation with scientists on a NASA advisory panel, the head of
the space agency’s Science program, John Grunsfeld discussed whether NASA
should look at a Europa mission for half that of the Clipper mission. If it could be done, then a Europa mission
could fit in the established New Frontiers program of planetary missions. (I want to emphasize that Grunsfeld’s
conversation was informal and wasn’t announcing a policy decision.)
Grunsfeld’s comments made me curious.
Estimates for the last two serious Europa proposals have come in at $4.7B
(JEO) and ~$2B (Clipper). Was a mission
for ~$1B (the recommended cost cap for future New Frontiers missions) credible? In my post today, I report on the results of
my thought experiment .
To give you my conclusion first, yes, a Europa New Frontiers mission
seems to be a credible idea to examine.
However, I come away even more impressed with the Europa Clipper
proposal and that’s the mission I want to see fly.
Depending on the instruments carried, a mission to Europa could study the surface morphology and composition, structure of the icy shell, or size and composition of the ocean and its interface to the rocky sphere below. Credit: JPL/NASA.
The analysis below is somewhat wonkish as I document the assumptions
and rational behind my thought experiment.
Based on emails I receive from readers, getting any mission to Europa is
a desire of many. I want to be clear on
the analogies I’m drawing and assumptions I’m making. And remember that this is a thought
experiment by an interested layman. It’s
all just fun speculation until a team of planetary scientists and engineers
does the real work to evaluate the feasibility and science return.
A mission that orbits Jupiter faces a number of technical
challenges. The spacecraft must be
powered (and sunlight is dim at Jupiter).
Jupiter possesses electronics-frying radiation (and Europa sits within
the high radiation belt). That is in
addition to the normal challenges of any planetary mission to operate a suite
of instruments, store their data, and return the results to Earth.
We have proof that a capable Jupiter spacecraft can be built within a
New Frontiers budget because NASA did so with the Juno spacecraft that is en
route to Jupiter now. NASA qualified solar
panels to work at Jupiter and developed shielding strategies to protect
electronics from the worst of the radiation.
The Juno spacecraft also carries a highly sophisticated instrument
package to study the giant planet. (I
was surprised to find a technical report on the mission that lists the total
mass of the instruments at a large 155 kilograms. This, however, might have been a preliminary
figure.)
In some ways, Juno is a simpler spacecraft than one that would study
Europa. Juno spins like a top to provide
stability rather than having to provide the more expensive rigid and precise
3-D pointing that would be needed for the cameras and other instruments to
study a moon’s surface. Juno’s
instruments also produce relatively small amounts of data compared to the
instruments required to study Europa.
That additional data for a Europa spacecraft requires expensive data
storage and a more capable communications system and more power for the
spacecraft.
In 2010, however, NASA completed studies of two New Frontiers-class
missions to study Jovian moons as part of a planning process known as the
Decadal Survey. I looked at those
reports for clues about the capabilities a Europa New Frontiers mission might have
in a credible design that meets the technical and environmental challenges of operating
in the Jovian system. One of the reports
described a multi-flyby spacecraft to study the volcanic moon Io (the Io
Observer) and the other described a small orbiter for the icy moon Ganymede.
|
Io observer
|
Clipper
|
Instrument mass
|
42 kg
|
108 kg
|
Encounter number
|
6 to 10
|
45
|
Time between flybys
|
60 days
|
7 to 21 days
|
Telemetry data rates
|
50 kbps
|
134 kbps*
|
Total data
|
240-400 Gbytes
|
~1440 Gbytes
|
Comparison of the overall capabilities of
the proposed New Frontiers Io Observer mission and the proposed Flagship-class
Europa Clipper mission. *Kilobits of
data per second.
Like the Europa Clipper mission, the Io Observer would perform multiple
flybys of its target moon to study its features. The cheaper Io Observer mission, however,
would carry far few instruments, return much less data, and encounter its moon
many fewer times than the Europa Clipper mission.
The two reports also showed that the New Frontiers missions would carry
just four instruments each.
(Technically, the magnetometer and plasma instrument for each mission
are distinct, but they support each other’s measurements and because they are
low mass, I’ve combined them in the following table.)
|
Io Observer
|
Ganymede minimal orbiter
|
Laser Altimeter
|
|
23
|
IR spectrometer
|
|
20
|
High resolution imager
|
19.5
|
|
Thermal imager
|
13.8
|
|
Mass spectrometer
|
10.5
|
|
Magnetometer/Plasma
instruments
|
3.2
|
9.73
|
Moderate resolution
imager
|
|
4.6
|
Total mass (kg)
|
47
|
57.33
|
List of instruments and their masses for
two New Frontiers missions proposed to study Jovian moons.
The science team for the Europa Clipper study has grouped the science
goals into four categories and identified nine instruments to meet them. I looked at subsets of the proposed Clipper
instruments that would have a similar mass to the instruments proposed for the
Io Observer and fulfill one or more science goals. (A real mission’s instruments also need to
fit within the spacecraft’s electrical power and data limits. However, I couldn’t find a combination of
instruments that fit within the mass limits but exceeded the power and data
limits, so I’m showing only instrument masses.)
Science goals and instruments for the
proposed Europa Clipper mission. Masses
for the instruments are shown at the end of this post.
One option for a Europa New Frontiers mission would study the icy shell
that covers the ocean and that has been shaped by the movement of ice blocks
and plumes of ice and water within the shell.
Understanding the icy shell would allow scientists to understand the
forces that shaped the shell and that likely brought water from the oceans
below to the surface. The topographic
imager would be a camera that would image much of the surface in multiple
colors at 25 to 200 m resolution.
The ice penetrating radar would study the structure of the ice and any
“bubbles” of water within the ice. The
radar proposed for the clipper mission would be particularly capable. Operating in its shallow mode, it would
penetrate just 3 kilometers into the shell but would have a vertical resolution
of structures of 10 m. Operating in its
deep mode, it would penetrate 30 km but with a vertical resolution of 100
m. Less capable, and lighter, radar instruments
are possible. The European JUICE mission
to the Jovian system will carry a radar instrument that is just 12 kg but that
can penetrate just 9 km with a vertical resolution of 30 to 90 m.
Icy shell studies
|
|
Ice penetrating radar
|
42
|
Topographic imager
|
4
|
Total (kg)
|
46
|
|
|
Potential instruments for a mission that
focuses on the icy shell. All instrument
masses are from the Europa Clipper studies unless otherwise noted
A second instrument option would not include the heavy (and data and
power hungry) radar instrument, but would instead focus on studying the
composition of the surface with an infrared spectrometer and a mass
spectrometer. (The latter would “taste”
molecules blasted from the surface by Jupiter’s radiation or expelled from
beneath the surface by possible plumes of gases.) A topographic imager again would study the
surface geology which would also provide data on the forces that structure the
icy shell. The magnetometer and plasma
instrument would study the interaction of Jupiter’s powerful magnetic field
with the salty water in the ocean to study the extent and salinity (important
to understand the composition) of the ocean beneath the icy shell.
Composition/geology/ocean
|
|
IR spectrometer
|
19
|
Mass spectrometer
|
7
|
Magnetometer/plasma
instruments
|
9
|
Topographic imager
|
4
|
Total (kg)
|
39
|
Potential instruments to study the
geology and composition of the surface ice as well as the hidden ocean.
Recently, there’s been one reported observation of a plume of water
being expelled by Europa. If this is
confirmed, and the plumes are shown to be persistent and reliable, then a
Europa mission might focus on studying those plumes. By doing so, it would study the composition
of water either from reservoirs trapped within the ice or from the ocean
below. Either way, this would be a
unique opportunity to perform the kind of exciting science that the Cassini
spacecraft has been doing with the plumes of Saturn’s moon Enceladus.
For this instrument list, I have duplicated the minimum list of
instruments recommended by a Decadal Survey report on potential missions to Enceladus. For this list, I show the mass of a more
capable mass spectrometer than is currently planned for the Clipper
mission. (For the technically inclined,
the current Clipper mass spectrometer would measure only neutral particles and
not ions. The alternative, and several
times more massive mass spectrometer, would measure both and have capabilities
similar to the Rosetta mission’s ROSINA mass spectrometer.)
Plume studies
|
|
Topographic imager
|
4
|
Mass spectrometer
|
24
|
Thermal imager
|
9
|
Dust analyzer
|
4
|
Total (kg)
|
41
|
Potential instruments to study possible
Europan plumes. The mass for the dust
analyzer is from the Decadal Survey Enceladus mission studies.
Before examining the question of whether a Europa New Frontiers mission
would be a good investment, I want to emphasize that the instrument lists given
above are to illustrate possible capabilities to show that good science likely
could be done within the limits of a New Frontiers mission. A professional science and engineering team
studying such a mission would almost certainly come up with a better
alternative than any of these.
So, would a Europa New Frontiers mission be a good investment at ~$1B
if ~$2B couldn’t be found to do the Europa Clipper mission? The answer would have to come from a study
conducted by planetary scientists and engineers. I’ll suggest some of the questions they may
ask to get to the answer.
One way to ask the question is whether we would end up knowing far more
about Europa than we do today following the Galileo mission of the 1990s. The members of the Decadal Survey answered
‘yes’ to this question for missions of similar capability to study Io and
Ganymede. (The former is on the list of candidate
missions approved by the Survey. The
latter was left off that list based on the hope that a European mission called
JUICE would be approved (which it has been) to orbit Ganymede.) I don’t see how a similar capability mission
to Europa would be less valuable.
However, there is now an approved European mission, JUICE, which will
arrive at Jupiter in the late 2020s.
While its focus will be on Jupiter itself and Ganymede, it will make at
least two close flybys of Europa. Right
now, almost a decade before launch, the JUICE team is committing to just those
two encounters. They know, though, of
the importance of studying Europa, and I wouldn’t be surprised if they don’t
eventually do a handful of flybys. (The
limitation on the number of flybys will be the high radiation exposure each
Europa encounter brings. Designing in
additional radiation hardening is expensive.)
The JUICE spacecraft will carry a more capable instrument suite than a
New Frontiers spacecraft would carry. So
would a New Frontiers spacecraft with six to ten flybys provide enough
additional science over what JUICE would do with two flybys? Or maybe five JUICE Europa flybys?
There are ways that a New Frontiers mission could compliment rather
than compete with JUICE’s measurements.
Because of orbital mechanics, the JUICE encounters will occur at nearly
the same point in Europa’s orbit around Jupiter. This means that only one hemisphere of the
moon will be sunlit for imaging and infrared spectroscopy. A New Frontiers mission could arrange its
orbits to encounter Europa at a location where the other hemisphere is
lit. (The Clipper mission proposes to
vary its orbit to encounter Europa at two locations for near global mapping.)
If the reported Europa plumes are real and persistent, there’s a hint
that they operate only (or most strongly) at the point in Europa’s orbit where
it is furthest from Europa. Unless the
gods smile upon us, this is unlikely to be the same point where JUICE will
encounter Europa. The New Frontiers
craft could tweak its orbit to encounter the plumes where they are most active
for repeated passes through them.
It’s also possible for the two spacecraft to carry complimentary
instruments. JUICE will have good, but
heavy, instruments that would allow it to search for and study any Europan
plumes from a distance and over time. A
New Frontiers spacecraft then could do the up close measurements going to plume
locations spotted at a distance by JUICE.
A New Frontiers craft could also carry a dust counter and thermal imager
that won’t be aboard JUICE to better characterize the plume structures and
their sources.
The last measure of whether a New Frontiers mission would be good
enough would likely be the toughest.
Would the science done be good enough that NASA and other space agencies
could avoid having to refly a similar mission later at another $1B+ to meet the
science goals? This is a different
question than asking whether a second spacecraft to make different measurements
would be needed. If two New Frontiers
missions at ~$1B each did the same science as a $2B Clipper, then doing two
cheaper missions is just buying on the installment plan. But what if those cheaper missions couldn’t
do the right measurements or enough of them to answer the priority science
questions?
Here the likely small number of encounters for a New Frontiers
spacecraft may be the critical issue. The
Clipper has the budget to design in the radiation hardening into the spacecraft
and instruments to last for a planned 45 orbits. To keep costs low, a New Frontiers mission
likely would not have the radiation hardening for that many orbits. The Io Observer study team thought that six
to ten encounters was doable on a New Frontiers budget. (It’s difficult for a layman to compare the
radiation exposure for an Io flyby and a Europa flyby, so I don’t know how to
translate the Io limit for a Europa mission.
My gut says it’s probably many, many fewer encounters than is planned
for the Clipper mission.)
I don’t know the answer to this question, but suspect that the science
definition team would wrestle with it.
So what’s my personal takeaway from my thought experiment? A Europa New Frontiers mission seems like a
credible idea to explore. However, it
would take three New Frontiers-class spacecraft to fly all the instruments
planned for the Europa Clipper (assuming ~40kg of instruments per
spacecraft). The sum cost would be
considerably higher than the cost for the Europa Clipper. In addition, the Clipper would have 45 flybys
of Europa compared with just 6 to 10 flybys planned for the Io Observer (and by
analogy for a Euorpa New Frontiers spacecraft).
I am deeply impressed with the capabilities planned for the Clipper
mission, and if it flies it will be awesome, and the right science will be done
to sufficient depth that we won’t have to do it again. I want to see the Clipper mission fly.
I’m also not sure that it will be any easier getting $1B than $2B for a
mission. NASA’s current budget can’t
afford either one until early in the next decade. So doing either class of mission requires
additional funds beyond what is currently planned. I view Uncle Sam like an ornery, miserly
uncle who gives you hell whether you ask for $100 or $1000, so you might as
well ask for the top figure. (Actually
three ornery uncles: the President’s budget office, and each house of Congress
since they are controlled by different political parties.)
John Grunsfeld gets paid the big bucks to make the decision on whether
to study and then propose a New Frontiers mission instead of the more expensive
Europa Clipper.
I just want to get to Europa for an in-depth study of that world, and I
hope Grunsfeld and his managers can pull at one of these magic rabbits out
of their hats.
Appendix: Approved JUICE instruments and proposed Europa Clipper
instruments. Several of JUICE’s
instruments will be more focused on studying Jupiter and its magnetosphere than
on the moons.
|
JUICE
|
Clipper
|
Ice penetrating radar
|
11.7
|
42
|
Visible-IR imaging spectrometer
|
26.1
|
19
|
Particle environment package/ mass spectrometer*
|
19.5
|
11
|
Hi-resolution Camera
|
16.9
|
18
|
Ganymede laser altimeter
|
15.2
|
|
Sub-mm Wave instrument*
|
12
|
|
Radio & plasma wave investigation
|
11.8
|
|
Thermal imager
|
|
9
|
UV spectrograph
|
7.4
|
|
Radio science experiment
|
6.8
|
|
Magnetometer
|
2.9
|
5
|
Topographic imager
|
|
4
|
Total (kg)
|
130.3
|
108
|
|
|
|
*Package of 6 instruments on JUICE 2
instruments on Clipper
Alternative more capable Clipper mass spectrometer adds 17 kg
|
Some notes on New Frontiers mission costs: In the past, NASA
selected New Frontiers missions that had a cost target of ~$750M for costs that
would be managed by the missions’ Principal Investigators (PIs): the spacecraft, instruments, mission
operations, and data analysis. Additional
costs borne by NASA included the launch, cost overruns (if any), increased
costs due to factors such as delayed funding and schedule slips. (For example, NASA delayed the launch of Juno
to pay the costs of other missions, which increased the time the Juno
development team had to be paid.) The
final quoted cost for the Juno mission is $1.1B with all these costs included.
A number of New Frontiers missions were studied for the Decadal Survey,
and all had engineering team cost estimates, which included launch costs. Four New Frontiers missions had full cost
reviews that included launch costs and reserves for were termed as “threats”: overruns,
delayed funding, and the like. Including
all these costs, these four mission concepts were estimated to have total costs to
NASA of $1.3B to $1.4B. Based on
similarities in mission goals and broad design requirements, a simple
multi-flyby Europa mission might have similar total mission costs.
Based on the results of the cost estimates, the members of the Decadal
Survey recommended that future New Frontiers missions have Principal
Investigator cost caps of ~$1B. NASA
would also need to cover launch costs and any cost overruns from project delays
or PI cost growth.
The commonly quoted cost estimate for the Europa Clipper, ~$2B, is an
approximate mean of costs for several alternative implementations. Launch
costs and additional costs from “threats” would be added costs to NASA. Given these, a New Frontiers mission might
cost somewhat less than half the cost of a Clipper mission but would likely do
much less than half the science (taking into account both a smaller instrument
compliment, lower data rates, and fewer flybys).
My take is that on a dollar per science return basis, the Europa
Clipper is likely a much better investment than a New Frontiers mission. If the latter is formally studied, I’ll be
interested to see if the professionals reach the same conclusion.
