In 1972 and 1973, NASA launched two simple spacecraft, Pioneer 10 and
11, to Jupiter. They were charged with
scouting the way for the more sophisticated spacecraft that would follow. Since then, the outer solar system has been
the realm of large, expensive missions: the Voyagers, Galileo, and Cassini.
Large missions costing well over $1B have proven very successful, but
they are launched at the rate of approximately one per decade and outer solar
system destinations have had to vie with Mars for these rare slots.
At last month’s Outer Planet Analysis Group, scientists and engineers
presented three proposals in competition for selection in NASA’s cheapest
category of planetary missions, the Discovery program. One other proposal may eventually vie for a
slot in NASA’s mid-range New Frontiers program.
The road to lower costs outer planet missions has been paved by NASA’s
first two New Frontiers missions, the $700M New Horizons mission en route to Pluto and the $1.1B Juno
mission en route to Jupiter. But can the cost of a mission to the outer
solar system be cut to $450M, the limit for a Discovery mission?
The three Discovery proposals take very different approaches.
The Enceladus Life Finder (ELF) team proposes to take the tightly
focused, minimalistic path.
We now suspect that many of the icy worlds in the outer solar system
harbor oceans beneath their icy crusts. Among
those bodies, Europa and Enceladus are special because their oceans appear to
rest atop their rocky cores, providing access to elements and minerals believed
essential to life. These rocky surfaces
are also are believed to have hot hydrothermal springs that could provide the
energy needed for the complex chemistry needed to support life. For other icy worlds, the oceans are
sandwiched between layers of ice and not in contact with their rocky cores,
making Europa and Enceladus priorities for exploring potentially habitable
worlds.
Enceladus so far is unique in having plumes of water that steadily jet
from its surface, spilling the contents of its ocean into space where they can
be easily sampled by a passing spacecraft.
(Hubble Telescope observations suggest that Europa may also have plumes,
but repeat observations have failed to confirm the initial sighting. Any plumes may be episodic, repeating only
every few years.)
The Cassini spacecraft currently at Saturn has already sampled the
plumes, but its instruments were designed in the 1990s, and weren't designed to
study the highly complex molecules that could indicate life. The ELF spacecraft would narrowly focus on
sampling these plumes with modern, highly sensitive instruments.
The mission would enter orbit around Saturn and then make ten flights
through Enceladus’ plumes. ELF’s instrument payload would consist of two mass spectrometers that
“weigh” atoms and molecules to measure composition. The spectrometers would analyze particles in
the plume that are a mixture of frozen ocean water and particles from the
seafloor. One of the mass spectrometers would be optimized to study the liquids
that originate from the ocean and the other the solid particles that likely
originate from the rocky core. A third
instrument might be included that would test whether any amino acids found have
predominately left- or right-handed structures.
(Life on Earth predominately creates left-handed forms, and it’s
suspected that life that originates elsewhere will similarly favor one form
over the other instead of a random mixture likely from abiotic chemistry.) The spacecraft’s navigation camera also would
image the plumes to judge their activity at the time of each flyby.
To appreciate the simplicity of the ELF proposal, you need to consider
previous concepts for exploring Enceladus following Cassini’s last flyby late
this year. When Decadal Survey considered
this decade’s priorities for exploring the solar system, several Enceladus options
were considered. The simplest would have
been a multi-flyby spacecraft like ELF, but that would also have included two
cameras and an ice penetrating radar in addition to the mass
spectrometers. The preferred mission
would have studied Enceladus both during flybys and from orbit about the moon
with a somewhat different, but equally rich instrument compliment. These concepts would have generated volumes
of data that would have investigated the structure of Enceladus, its ocean, and
ice shell along with the chemistry of its internal ocean. Unfortunately, the estimated costs of these
missions were over $1.5B.
(Quoted mission costs often vary based on the specifics of what’s
included; in this post, I’ve tried to quote the costs for the items included
the widely-quoted $450M cap for Discovery missions. NASA’s final cost for a full mission
including launch and operations would be higher than the costs I’m including
here.)
The Io Volcano Observer would take a different tact than the ELF
mission. Like the proposed Enceladus
mission, the IVO spacecraft would observe Jupiter’s volcano-rich moon during
several flybys. Unlike the Enceladus
mission, the Io mission would carry four instruments – a two camera suite, a
thermal imager, a magnetometer, a mass spectrometer – along with a
student-built instrument to map volcanic hot spots. The spacecraft’s radio system would do double
duty by also allow precise tracking of the spacecraft’s speed during flybys to
study the distribution of matter within this moon. This would be a data rich mission, and the
spacecraft would carry an experimental high data rate optical communications
system in addition to the traditional radio system.
While ELF would focus on one investigation – the chemistry of the ocean
– IVO would perform an integrated series of studies to understand Io as a world
and a member of the Jovian system. The
mission’s goals are divided between understanding the sources and extent of its
intense volcanic activity, the effects of the injection of its volcanic plume
material into the wider Jovian system, and long term monitoring of Europa for
plumes and Jupiter’s atmosphere. The
nominal mission would last 22 months, but an extended mission might carry on
for an additional 6 years to monitor Io through time. If the extended mission occurred, then we
might have three spacecraft simultaneously studying the Jovian system in the
early 2030’s: IVO in a polar Jovian orbit with periodic flybys of Io, Europe’s
JUICE mission with broad studies of the Jovian system and a focus on the icy
moon Ganymede, and NASA’s Europa mission.
The third Discovery proposal takes an entirely different approach to
exploring the outer solar system on a budget.
The Kuiper mission would launch a space telescope dedicated to studying
the outer solar system.
The Kuiper proposal addresses two problems. First, we cannot afford to have spacecraft at
each of the major outer planets to observe their weather, their magnetospheres,
and their moons. This dedicated outer
solar telescope would be able to examine each of these worlds multiple times
each day to study these worlds as dynamic systems. Scientists will be able to observe how storm
systems in their atmospheres exchange energy, how variations in auroral
activity provides clues to the state of their magnetospheres, and how volcanic
and plume activities on the moons Io, Europa, and Enceladus vary over time.
The second problem is that we don’t understand key questions about the
formation of the outer solar system. We
suspect that the outer planets migrated during the early ages of the solar
system, but there are competing theories as to whether that migration was
smooth or more chaotic. As the planets’ orbits
shifted, they would have flung smaller bodies about. The Kuiper telescope would analyze the
spectra of thousands of small bodies ranging from Jupiter’s orbit to the
distant Kuiper belt to analyze their compositions. The mixture of compositions at different
distances from the sun would allow astronomers to distinguish between the
competing theories.
Telescopic observations have always played a crucial role in studying
the outer solar system. Earth-based
telescopes, however, have key limitations – any solar system target is visible
for only a few hours each day and our atmosphere blurs vision and blocks key
wavelengths of light. The science
proposed for the Kuiper mission could be done by the Hubble Space Telescope,
but its observing time is precious and little is allocated to solar system
studies.
The Kuiper mission would be smaller than Hubble (a 1.2 meter primary
mirror versus the Hubble’s 2.4 meter mirror) but would be dedicated to
observing the outer solar system. The
spacecraft would be parked in an orbit around the L2 Lagrange point beyond
the moon where it could observe the sky without Earth occultations and would be
beyond stray Earth light.
In addition to the three Discovery mission proposals, a fourth mission
concept was proposed, the LIFE Enceladus Sample Return. A previous incarnation of this mission was
proposed for the last Discovery competition but wasn't selected. Like the ELF mission, the LIFE mission would
make multiple flybys through Enceladus’ plumes and would use a mass
spectrometer to study their chemistry. Unlike
ELF, the LIFE spacecraft would collect dust plumes in a fashion similar to that
done by the Stardust spacecraft that collected comet dust samples in the
mid-2000s. The samples would later be
returned to Earth where far more sensitive measurements would be made than
could ever be done by instruments on a spacecraft. The cost estimates presented by the LIFE team
puts the mission outside the current scope of the Discovery program, and the
team is building support to add an Enceladus sample return mission to the New
Frontiers candidate mission list for the 2020s.
After 2017, there are no plans to have a spacecraft operating in the
outer solar system until the late 2020s at the earliest. That decade gap will exist because outer
planet missions in the past have had to be infrequent because of their
costs. The four proposals presented at
last month’s meeting represent the planetary community’s attempt to find a new
class of much lower cost missions that could fly more frequently. The Kuiper telescope would be an entirely new
approach to the problem that could begin providing data in the early
2020s. The missions to Io and Enceladus
face a tougher challenge because they propose to do missions for half the cost
of any previous outer planet mission orbiter.
Missions to both Enceladus and Io have been studied before, and the
costs were two to four times that of the cost cap for Discovery missions
($450M). Teams that propose missions are
generally fairly open about the great science their missions would do if they
are selected to fly. These teams tend to
be much more reluctant, however, to discuss the specifics of how they would
accomplish their goals within the tight cost caps of a Discovery competition –
that is their secret sauce. The teams
proposing ELF and IVO are seasoned veterans and their credibility gives me hope
that the outer solar system may open to low cost missions. We can assume that they have had a laser
focus on finding ways to reduce costs to a fraction of what previous studies
have assumed. Estimating development
cost, however, is always part art. NASA will
perform its own assessment of mission risks and costs, and its reviewers may be
more risk adverse and conservative than the mission proposers in assessing
likely costs and risks.
For this Discovery competition, NASA’s managers have changed the rules
in a key way that will help outer planet proposals. In previous competitions, the costs of
mission operations had to be included in the mission cap. A mission to Mars with an operations lifetime
of two to three years had an inherent advantage over an outer planets mission
that might take five to seven years to reach its target and then require
another year or two of operations. Now
NASA has excluded “reasonable” mission operations costs from the cost cap
(which means it picks up those costs separately). This goes a long way to leveling the playing
field between inner and outer solar system Discovery proposals.
In the last Discovery competition, a mission to land on a lake in the
north polar region of Saturn’s moon Titan made it to the list of
finalists. (The Mars InSight geophysical
lander was the winner.) If either ELF or
IVO is selected this time, then outer solar system will have been opened to
exploration by a new, low cost class of missions. If neither mission is selected, then the
experience learned from these proposals will become part of the community
experience that is likely to sharpen future Discovery proposals for the outer
solar system. I believe that eventually
an outer planets Discovery proposal will find the right formula for selection;
I hope that this happens sooner rather than later.
You can read the original presentations for these proposals as well as the other presentations from the OPAG meeting here.
