Three
factors make exploring Europa hard. First,
we want to explore an entire complex world, and mapping its features requires
acquiring vast amounts of data. Second,
Europa lies far from the Earth, which necessitates capable communications and
power systems (read, “expensive”) to return the data to Earth. Third, Europa lies well within the harsh
radiation fields surrounding Europa, which both requires significant radiation
hardening (again, read, “expensive”) and limits the life of any spacecraft that
explores this world. These factors can
make a mission concept that seems like less actually be more.
The limiting
factor on science for most planetary orbiters is not the time the instruments
can make observations. Rather it is the
time available to return data to Earth because many instruments can gather data
far faster than the communications system can transmit it to antennas on
Earth. (There also are a limited number
of antennas to listen to planetary spacecraft, so few missions receive
continuous coverage, and spacecraft often cannot continuously transmit either
because they must turn to observe the planet or the planet itself blocks
communication.)
To get a
sense of the challenges, compare the problems of exploring Mercury and
Europa. A mission to Mercury must deal
with the intense heat coming from both the sun and the planet surface. However, a spacecraft designed to overcome that
challenge can continue to function until its fuel is exhausted. As a result of the luxury of spending years
in orbit around Mercury and the fact that Earth is never more than 222 million
miles away, NASA’s MESSENGER mission has been able to return terabytes of data
to Earth. Between its orbital insertion
in March 2011 and March 2012, the spacecraft generated 2.3 TB of data to be archived by NASA. The mission continues to operate today, so
the total returned to date should be substantially more. The maximum data rate for this $446M (2008
dollars) mission is 104 kilobits per second.
By
comparison, it’s cold at Jupiter, but it is the intense radiation around Europa
that limits spacecraft life. Different
mission studies have assumed lifetimes in orbit between one ($1.6B estimated
cost, 2015 dollars) and nine months ($4.7B estimated cost), many times shorter
than the approximately four Earth years MESSENGER will have at Mercury. While Jupiter is never closer than approximately
2.7 times as far from Earth as Mercury, more capable spacecraft systems would
allow data rates of around 135 kilobits per second. With a lifetime of one month in orbit, the
data return would be around 334 gigabits, and with a lifetime of nine months, around
4.5 TB. (Different mission designs made
different assumptions about data return, so nine month mission data return
isn’t a simple multiple of the one month mission data return.)
 |
| Comparison of data return for different Europa mission concepts with the data archived from NASA's Mercury MESSENGER orbiter from its first terrestrial year in orbit. Credit: Kane |
These challenges
for exploring Europa have been well known since the Galileo Jupiter orbiter in
the 1990s all but proved that Europa likely has a vast ocean that could harbor
life that lies under a relatively thin icy shell. As mission planners and budget directors have
wrestled with this problem, we’ve been through at least five distinct eras of Europa
mission planning. (There have also been
various proposals by independent teams for simpler and cheaper missions, which
may or may not have been feasible for their proposed costs.)
Immediately
following the Galileo spacecraft’s discoveries, JPL conducted preliminary
mission studies that envisioned a capable spacecraft using conventional
technology to orbit Europa.
In the late
1990’s, NASA’s then Administrator redirected efforts to a mission concept that
would use yet-to-be-developed technologies (the X-2000 project) to dramatically lower
mission costs to the neighborhood of MESSENGER’s cost. By the time the program was cancelled in
2002, mission estimates had shot from around $190M to $1.4B (early 2000’s
dollars).
Not to be
outdone, the next NASA administrator proposed the Battlestar Galactica of
missions, the Jupiter Icy Moons Orbiter (JIMO) that would
orbit the moons Callisto and Ganymede in addition to Europa. This mission depended on the development of
radically new capabilities such as space-rated nuclear fission reactors to
power the spacecraft. This $16B concept
died quietly when the administrator left NASA.
 |
| NASA's concept for the Jupiter Icy Moons Orbiter. Credit: JPL/NASA. |
If the
previous two efforts were perhaps fanciful, the next effort, the 2008 Jupiter
Europa Orbiter (JEO) concept was based solidly on feasible technology. This highly capable spacecraft would have
conducted extensive studies of the Jovian system before beginning nine months
in orbit around Europa with a highly capable instrument suite. This was the mission any fan of Europa really
wanted. Unfortunately, an estimated
$4.7B price tag doomed the concept.
 |
| The bulk of the data that would have been returned by the 2008 Jupiter Europa Orbiter concept would have been produced by the ice penetrating radar, infrared spectrometer, and a trio of cameras. Credit: Kane |
Following
the JEO studies, NASA conducted studies of three missions that each would have
a firm cap of $2B: an orbiter, a multi-flyby spacecraft, and a lander. It was quickly realized that the latter would
not be feasible until a previous mission had better studied Europa’s surface to
find the best combinations of most scientifically while still safe landing
sites.
That left the
choice of an orbiter that would spend 30 days circling the moon and a
multi-flyby spacecraft that would spend less than a cumulative 6 days close to
Europa during 34 flybys. The scientists
who reviewed the two missions solidly backed the multi-flyby concept (that has
evolved into the current Europa Clipper concept).
So how can 6
days of science be better than 30? For
the comparison that follows, I’ll use the assumptions of the 2012 studies. Since that time, the capabilities of the
multi-flyby concept have been substantially enhanced into the Europa Clipper
concept. Because the orbiter concept
didn’t have the additional two years of fine-tuning of the multi-flyby craft,
comparing their 2012 conceptions allows comparison of equally developed
concepts.
 |
| Comparison of data that would be returned by instrument for the 2012 multi-flyby and orbiter mission concepts. Credit: Kane |
Between each
of the flybys, the multi-flyby spacecraft would have seven to ten days to
transmit data stored during each brief encounter back to Earth. That would let the multi-flyby craft have up
to a year of time to transmit its data compared to just 30 days for the
orbiter. The result would be almost
three times as much data returned to Earth.
(Differing assumptions about how much of the time antennas would listen
to the spacecraft mean that the amount of data returned is not a multiple of
time.)
The larger
data return of the multi-flyby spacecraft would enable the spacecraft to carry
two high priority instruments that generate large amounts of data. The more data hungry of these, the ice
penetrating radar, would study the structure of the icy shell beneath the
surface. This would allow scientists to
study whether bodies of water are trapped within the ice between the surface and
the ocean below and fracturing of the shell.
The radar might penetrate through the shell to the top of the ocean to
measure the total depth of the icy shell.
These measurements will help scientists understand how material is
transported between the ocean and the surface.
The second
instrument, a short-wave infrared spectrometer, can identify materials exposed
on Europa’s surface and map their distribution.
Scientists believe that Europa’s surface exposes materials transported
from the ocean below, where we can easily see it and eventually study it with a
lander. The interaction of materials on
the surface with Jupiter’s radiation field creates chemicals that may be
transported to the ocean below to be available for use by any life. This spectrometer would map the presence and
distribution of these materials across Europa’s globe.
Both the
2012 orbiter and the multi-flyby spacecraft would carry a third data-hungry
instrument, a topographic imager that would map the surface.
Not all potential instruments require high
data rates. The orbiter would have
carried a trio of instruments that required measurements from around the globe:
a laser altimeter to measure surface tides to enable estimates of the thickness
of the icy shell and a magnetometer and plasma instruments that would have
enabled estimates of the volume and salinity of the underlying ocean. Unfortunately, the measurements of these
instruments are lower priority than those for a radar and shortwave infrared
spectrometer. (In the 2012 study, the
multi-flyby spacecraft also would have carried a heavy, power-intensive, but
low data rate mass spectrometer that would directly sample material sputtered
from the surface.)
The
importance of the ice penetrating radar and mid-IR spectrometer tipped the
weight of opinion in favor of the multi-flyby concept. Given a limited number of encounters that
would fly over just a tiny fraction of Europa’s surface, they key was to
distribute those flybys to fly over key locations.
With two
years of further study, the multi-flyby concept has evolved into the Europa
Clipper concept has added an additional eleven flybys (for a total of 45) and
several instruments compared to the 2012 concept. By balancing the placement and number of
encounters with many months to return data, the Europa Clipper concept would
enable a $2B mission that conducts the most crucial measurements of the $4.3B
JEO concept. The $1.6B orbiter concept
couldn’t match this feat.
However, the
Europa Clipper is not NASA’s plan for a Europa mission. White House budget analysts and NASA’s senior
management are looking for a $1B concept that wouldn’t do the job of the Europa
Clipper but would still do significant science.
Earlier this summer, they reportedly received six proposals that target
this cost cap. NASA ’s managers are
examining the proposals to ensure that they are both fiscally and technically
feasible within the budget. In the
meantime, they are not releasing any information about the types of missions
proposed.
From what I
understand, much of the scientific community and many NASA managers are
skeptical that a meaningful mission can be done within a $1B budget. Sometime in the coming months we will learn
whether NASA thinks any of the proposals have merit. If they do, then the broader scientific
community will weigh in with its assessment.
I’ve argued
in a
previous post that a $1B mission is likely technically possible, but I have
doubts about whether it could address enough high priority science to be worth
the expenditure. The coming months will
see if I’m proved wrong or not.
In the
meantime, NASA continues to refine the Europa Clipper concept, which so far has
shown the best balance between doing more with less to perform the critical
science for the next step in exploring this world.
 |
| Current concept for the Europa Clipper mission, which is an evolved version of the 2012 multi-flyby concept. Credit: JPL |
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