For the last two years, NASA has been the shy partner refusing to get
on the dance floor, and Congress has been the aggressive partner insisting on a
dance now. Recently, NASA has said maybe
on another night but only if it’s a cheap date.
While NASA says no for now, Congress looks to be willing to slip the
band a cool $100M – on top of $150M already paid – to keep the music playing,
but (to keep the metaphor going) has not been willing to fully commit itself to
paying the bigger bill to rent the dance hall.
The dance, of course, is the continuing attempt by Congress to have
NASA commit to a mission to explore Europa, and NASA’s attempts to delay a
mission well into the 2020s. NASA is
also seeking ideas for alternatives to the current $2B Europa Clipper concept
that would cost no more than $1B but that also would presumably be less
capable.
Compared to the budget waltz, the scientific case for a mission to
Europa is compellingly simple. After the
Earth, Europa is considered by many scientists to be the most likely location
in the solar system as a home to present life.
It has the key ingredients: an outer layer with lots of water (more than
in the oceans of Earth) in contact with the rocky core (source of key elements
needed to build the molecules essential for life) and energy (from the tidal
heating supplied by Jupiter). And Europa
has had a lot of time for life to evolve.
Its oceans should have been present for most of the life of the solar
system. (This distinguishes it from
Enceladus where the weaker tidal flexing of Saturn may allow its internal ocean
to freeze for long periods of time.) The
recent observation of possible plumes spewing water into space where Europa’s
ocean could be easily sampled has just raised the desire for a dedicated Europa
mission.
 |
| The Europa Clipper would replace a short-lived Europa orbiter concept with a spacecraft that would fly past the moon several dozen times with a highly capable instrument payload. At each encounter, the instruments would do low- to medium-resolution studies before and after closest encounter, but would do high resolution studies during the brief period when the spacecraft is less than 1000 km from the surface . Credit: NASA/JPL. |
The Science Goals
In the 1990s and early 2000s, the Galileo orbiter made eleven of flybys
past Europa. That mission all but proved
the existence of a liquid ocean beneath the moon’s icy shell and globally
mapped the surface features and composition.
Galileo, however, had a crippled main antenna that reduced the returned
data to a tiny trickle of what had been planned, so medium and high resolution
mapping of the moon covers only small areas.
The spacecraft’s vintage 1970’s technology instruments also lacked the
sophistication to identify important substances in the icy surface. It also did not carry instruments that could
probe the structure of the icy shell to look for lakes within the shell or
study the shell’s interface with the ocean.
The standard progression for exploring a world is first flyby it (which
Galileo did), then orbit it for globally studies, and then land on it for
intensive studies in a single location.
Unfortunately, Europa sits well within Jupiter’s harsh radiation belts,
and any affordable orbiter would have weeks to a handful of months to complete
its studies and would carry a minimal instrument compliment.
JPL’s engineers and scientists have developed an alternative strategy
for the proposed Europa Clipper mission: Fly a highly capable spacecraft that
orbits Jupiter, but that toe dips into the radiation belt and conducts its
science during several dozen flybys. The
radiation challenges are still significant, but the science that would have
carried a cost of >$4B as an orbiter can now be done for ~$2B.
The several dozen flybys is key to the Europa Clipper’s ability to
replace an orbiter mission with a multiple-flyby mission.
 | ||
During the few minutes around closest encounter, high resolution studies are done only for a narrow swath along the ground track immediately below the spacecraft's flight. Credit: NASA/JPL. Over several dozen flybys, the high resolution measurements build up into regional studies. Credit: NASA/JPL.
|
I’ll focus on just one set of requirements and how they link to several
key investigations. To ensure that
proposed mission achieves global coverage, the science team has divided Europa’s
map into 14 panels. The science goals
require that the spacecraft fly over at least 8 of these panels at altitudes of
less than 400 kilometers with a desired goal of 11 panels. Within each panel, the requirements specify that
at least two close flybys occur in each on the Jupiter-facing hemisphere of
Jupiter and three on the anti-Jupiter face.
If the minimum 8 panels are evenly distributed between the pro- and
anti-Jupiter hemispheres, then 20 flybys are needed to meet the minimum science
goals.
This one set of requirements for regional measurements within a panel
and a distribution of regional studies across the Europan globe enables several
key studies:
“Characterize the ice shell and any subsurface water, including their
heterogeneity, ocean properties, and the nature of surface-ice-ocean exchange.
·
“Characterize the distribution of any shallow
subsurface water and the structure of the icy shell.
·
“Search for an ice-ocean interface.
·
“Correlate surface features and subsurface
structure to investigate processes governing material exchange among the
surface, ice shell, and ocean.
·
“Characterize regional and global heat flow
variations.
“Understand the habitability of Europa's ocean through composition and
chemistry.
·
“Characterize the composition and chemistry of
the Europa ocean as expressed on the surface and in the atmosphere
·
“Determine the role of Jupiter's radiation
environment in processing materials on Europa
·
“Characterize the chemical and compositional
pathways in Europa's ocean.
“Understand the formation of surface features, including sites of
recent or current activity, and characterize high science interest localities.”
(Quotes are from the Europa
Clipper Science Traceability Matrix.)
The science goals similarly require a number of well distributed flybys
to study the interaction of Europa’s ocean with Jupiter’s intense magnetosphere
to estimate the depth and salinity of the ocean and to study tides to estimate
the ice shell’s thickness.
The current Europa Clipper mission concept goes well beyond the minimum
science goals to deliver on almost all the extended goals that the science
community has set. However, cutting the
current mission concept from its 45 Europa flybys to a minimum of 20 or fewer
flybys seems unlikely to cut the mission costs in half. Once you build and fly a spacecraft to Europa
that can withstand 20 encounters, operate a suite of instruments, and return a
large volume of data between encounters, I suspect that you've already incurred
most of $2B cost (but remember that I am neither an engineer nor a planetary
scientist).
Is Cheaper Credible?
NASA managers have formally asked if a credible Europa mission could be
done for half the Clipper cost estimate, or around $1B. This is a good news/bad news scenario. NASA took the initiative to propose an
in-depth study by suggesting spending $15M next year ($85M less than the House
of Representatives appears ready to approve for next year, see below). However, next year’s study would be followed
by several years before any mission conceived would actually begin development
and a decade or more before it might launch.
NASA issued a Request For Information on concepts for a $1B mission. It has required that proposers, “meet the
majority of the five science goals set forth in the Decadal Survey [priorities
set by the scientific community], including the goal to characterize
scientifically compelling sites to prepare for a potential future lander
mission to Europa.” Those scientific
goals are, in priority order, to:
” Characterize the extent of
the ocean and its relation to the deeper interior;
“Characterize the ice shell and any subsurface
water, including their heterogeneity, and the nature of surface-ice-ocean
exchange;
” Determine global surface
compositions and chemistry, especially as related to habitability;
“Understand the formation of
surface features, including sites of recent or current activity, and identify
and characterize candidate sites for future in situ exploration;
” Understand Europa’s space
environment and interaction with the magnetosphere.
“While characterizing landing sites for future in situ exploration is
the fourth scientific priority in the Planetary Decadal Survey, NASA places
high programmatic priority on this goal to enable a potential future lander
mission to Europa.”
The request goes on to list the challenges of implementing the
mission. “The primary challenges facing
any mission to Europa involve the harsh radiation environment and planetary
protection requirements… Planetary Protection requirements for Europa are very
strict and involve ensuring that the probability of introducing a viable Earth
organism into Europa is [less than one in 10,000].”
The request's details make it clear that proposers must do a solid amount
of science and engineering analysis to show that they have a credible concept
that could cost less than $1B (not including the launch costs) and make their
case in 15 pages
It is common for government agencies to issue these “Requests for
Information” to learn whether an idea is credible and worth pursuing. This request doesn’t commit NASA to any
follow up studies, but if its managers judge any of the proposals to be
credible, it presumably would follow through with more detailed analyses.
Is a $1B mission idea credible?
I did a
thought experiment in a previous post and concluded that technically it
likely is. The Juno spacecraft that will
orbit and study Jupiter cost ~$700M. A
mission to fly by Jupiter’s moon Io, deeper in Jupiter’s radiation field, six
or so times has been estimated to cost ~$1B.
The European’s JUICE mission will reach Jupiter next decade, flyby
Europa twice, and then orbit the moon Ganymede for ~$1.2B.
If the goal simply is to fly by Europa a few times with a spacecraft
with a small number of instruments, then by analogy with these other Jupiter
missions, it likely can be done for ~$1B.
The bar, though, for a scientifically credible mission is higher. A follow-on mission has to substantially
enhance our scientific understanding of Europa to justify a cost of $1B to
$2B. Most of the key studies identified
by the science team require numerous flybys distributed across the globe.
However, Europe’s JUICE 2020’s JUICE mission to the Europa system is
committed to two flyby of Europa with a highly capable spacecraft and
instrument suite. To be justified, a
NASA mission must produce significantly better science than the already funded JUICE
mission will. (While the JUICE mission,
which is still in design, it has committed to just two flybys of Europa. I suspect
that if the engineers conclude it is safe, the mission’s managers will consider
one or two additional flybys closer to launch.)
So is there hope for a $1B mission that is scientifically
compelling? Color me skeptical (and several
of NASA’s managers are reported to have said they are skeptical, too), but if there is, I
suspect that it will come in one of two forms:
- A proposal suggests a clever way to redefine the science goals in a way that returns the core science with a simpler and cheaper spacecraft. The team that proposed the Juno mission on its way to Jupiter did this for studies of Jupiter’s deep atmosphere and interior. The current Europa Clipper multi-flyby proposal redefines the science goals from previous orbiter proposals to substantially cut costs. Is there another option that can cut costs substantially again?
- A proposal combines limited regional studies with flights through the possible plumes of Europa to directly measure water, and possibly life, expelled either from a lake within the shell or directly from the ocean. (However, remember that only one of several studies saw data that suggested plumes were present and those measurements were near the limit of detection. The plumes may be ephemeral or not even exist. Justifying a mission on the current plume data seems risky to me; a better strategy would seem to be to have a viable mission without the plumes but carry the already planned instruments that would also be useful to study the plumes. The JUICE mission team is already planning this.)
In a few months, we are likely to learn whether NASA received any proposals
it considers worthy of further study. They
key, though, will be whether the science community agrees that the mission meets
the core requirements for understanding Europa.
If it doesn't, then the community seems likely to recommend waiting
until budgets allow the right mission to be flown. Missions to each outer planet or their moons
occur only every couple of decades. Why
do a sub-par job on the next mission to Europa and then have to do it over a
decade or two later to get it right?
The Politics
In the introduction to this post, I said that NASA (and the President’s
budget office that writes NASA’s budget requests) and Congress disagree on
whether a Europa mission should begin now or wait to begin development several
years from now.
The root of the disagreement, as in so many relationships, is
money. Jupiter’s harsh,
electronics-frying radiation belts, make any mission that does more than a handful
of flybys a technically challenging – read expensive – proposition. More than a decade’s worth of technology
development and mission studies has provided the solutions to most of the
technical challenges. JPL’s scientists
and engineers have developed a killer proposal for a dedicated multi-flyby
Europa Clipper mission. At ~$2B, this
mission would be cheaper than the Curiosity rover mission currently exploring
Gale crater on Mars.
Unfortunately, NASA’s budget is oversubscribed. The only way to fit the Clipper mission into
the budget is to either increase NASA’s budget by several hundred million
dollars a year for several years (which I would support as a US taxpayer!) or
take the funding from other NASA programs.
We are left with this strange waltz in which Congress, which ultimately
sets NASA’s budget, has not increased the overall budget enough to fully fund
the Clipper mission, but over the last two years provided $150M for advanced
development work. This year, the House
of Representatives is proposing to put another $100M in the pot for next
year. If the Senate continues its
previous support, it is likely to substantially match this funding.
At the other side of the dance floor, the President’s budget managers
and NASA’s managers have made it clear that they don’t want to commit to any
Europa mission this decade because of the funding constraints. They also seem reluctant to commit to any $2B-class
science mission because the last two large science missions (the James Webb
Space Telescope and the Curiosity rover) went well over budget, which caused
substantial harm to the overall science program.
As a result, today NASA is spending $150M because it’s legally required
to (Federal budgets in the US are laws) to advance a mission its senior
managers don’t want to do, at least for this decade.
The House of Representatives has released details of its proposed
budget for next year. Where NASA
proposed to spend $15M to study $1B mission concepts, the House is proposing to spend $100M. Under the House’s bill none of the funding
could go towards a $1B mission (which it doesn't see as credible) but only
towards the full Europa Clipper mission.
We will have to wait for several weeks to see what the Senate
proposes. It will be several months
before we learn what the two houses of Congress ultimately compromise on for
next year.
