Last spring,
the European Space Agency (ESA) put out a call for concepts for its next two €1
billion science missions. If history
proves to be a guide, there’s a good chance that one of the selected concepts
will be a solar system mission. ESA’s
managers will announce their selection this coming November.
These large European
missions are particularly important because they have the resources both to
reach targets throughout the solar system and to carry enough instruments to conduct
wide ranging studies once there. If one
of the solar system concepts is selected, we may get our first orbiter for
Uranus, a return to Titan, or an orbiter and balloon for Venus. The range of concepts proposed shows that
planetary exploration continues to have a wealth of possible missions.
However,
don’t hold your breath. ESA believes in
long term planning and the launches of the selected missions are planned for
2028 and 2034. Add in ten to sixteen
years for, say, a flight to Uranus, and you may be looking at first science
return in the 2040s or 2050s. (Actuarial
tables suggest I won’t be around then, so for purely selfish reasons,
I like the concepts with much shorter flight times, like those to Venus.)
ESA breaks
its science missions into three classes: Large (~€1B or $1.2B), medium (~€400M
or $480M), and small (for which I couldn’t find a price target). For comparison, NASA also has three classes
of missions: Discovery ($425M likely to become $500M), New Frontiers ($750M
likely to become $1B), and Flagship (>$2B for the last three Flagship
missions).
ESA and NASA
account for mission costs differently, making direct currency conversion
comparisons difficult. For example, ESA
includes the launch but not the instruments or much of the data analysis (which
are paid for by its member states separately).
NASA does the opposite. As a
rough guide, I assume a Large ESA mission buys somewhere between the
equivalents of up to $1.5B in terms of how NASA implements missions. That mission cost target nicely fits between
NASA’s New Frontiers and Flagship mission classes. (ESA also conducts some planetary missions
out of different accounts, such as the two ExoMars missions planned for 2016
and 2018.)
Previously
selected ESA Large missions (at one time called Cornerstone missions) show what
can be done within the ESA budget. The
Rosetta mission will conduct humanity’s first rendezvous and landing on a
comet. The BepiColombo mission will be
the equivalent of the Cassini mission to Mercury with a far more capable
instrument suite than the Discovery-program MESSANGER currently at that
planet. The JUICE mission will carry out
extensive studies of Jupiter, flybys of Europa and Callisto, and will orbit
Ganymede.
ESA’s budget
allows it to fly three Large missions every 20 years. In the past, ESA has balanced its large
missions between astronomy/astrophysics and planetary missions. The first mission selection for the upcoming
20 year period, JUICE, is a solar system mission to launch in 2020. A
second solar system mission in a row for the 2028 slot seems to me
unlikely. A solar system mission for the
2034 mission seems likely, but ESA could pick a second astronomy mission in a
row and restore the balance in the following 20 year period. I’ve looked through the
astronomy/astrophysics concepts, and they are stiff competition for the solar
system concepts.
So no
guarantees, but the list of solar system mission concepts is exciting, and I’m
hopeful for one of the two slots going one of them. I’ve listed the mission concepts in order
from the sun. None of the proposed
concepts returns to the target of a previous ESA large mission. One though, returns to Mars where ESA will
send its two ExoMars missions and two would return to Titan where the joint
NASA/ESA Cassini/Huygens mission is conducted a descent and landing and
continues to make frequent flybys.
How may ESA
decide among what are a number of exciting proposals? My guess is that three criteria will be
used. First, would the mission
fundamentally enrich our understanding of an important world or class of
objects? Second, would a broad spectrum
of the European planetary science community be involved? Third, would the mission be feasible within
the budget target and with technology likely to be available?
A mission to
Uranus, for example, would greatly deepen our understanding the ice giant
worlds. By studying the atmosphere,
magnetosphere, and moons, it would involve a wide range of planetary science
disciplines. However, a Uranus mission
would need a radioactive power supply to produce electric power for the
spacecraft. U.S. law prevents supplying
plutonium 238 to other nations (and the supply is already critically low). ESA has proposed developing power supplies
based on another radioactive element, americium, but that would represent a bet
on an unproven technology.
As you read
through the list of proposals below, you might ask yourself how you would rank
each according to these three criteria. If
you decide to read the original
mission concept proposals (130MB download), be prepared for a lack of
detail in some of them. While NASA
typically selects missions after detailed design studies and then launches in
four to five years, ESA selects concepts far in advance of launch and fills in
the details after selection. In addition,
based on cost estimates from previous mission studies, some of the concepts as
presented may well bust the €1B price cap.
So it is possible that a selected concept would be scaled back as
definition progresses.
With this
background, here are the solar system concepts.
Where the concept title itself doesn’t summarize the goals, I’ve quoted
a sentence or two from the proposal that captures the essence the concept’s
goals.
Sun
SOLARIS:
SOLAR sail Investigation of the Sun
“SOLARIS from its highly inclined orbit around the Sun, aims to combine
helioseismic and magnetic observations, solar irradiance measurements and EUV
images at various latitudes.” A solar
sail would be used to place a spacecraft into an orbit close to the sun and eventually
over the solar poles.
Venus
Possible
future missions to Venus have been extensively studied, and so it’s no surprise
that the three Venus concepts propose addressing similar goals and similar
approaches. The main difference appears
to be in the ambition of the concepts.
The following summary of goals is quoted from the first of the three
concepts and summarizes the overall goals for all three (although the focus for
each is somewhat different): “A common thread for Venus and Mars is that the
atmospheres on both planets appear to have undergone catastrophic change
change—Mars may have lost almost all of its atmosphere, while Venus may have
driven off much of the water in a runaway greenhouse and perhaps increased its
atmosphere… A major part of understanding how Venus evolved as a terrestrial
planet is its [internal] thermal
evolution.”
Venus: A
Natural Planetary Laboratory. Implementation: Two orbiters for atmospheric,
surface, and interior measurements, balloon or unmanned aerial vehicle, short
lifespan landers.
Venus:
Key to understanding the evolution of terrestrial planets. Single orbiter to study the atmosphere and
surface, a balloon, and an optional atmospheric descent probe to measure the
atmosphere and image the surface.
Europe
returns to Venus. Implementation:
Single orbiter to study the atmosphere and surface and a balloon or unmanned
aerial vehicle.
Lunar
Lunar Science as a
Window into the Early History of the Solar System. Implementation: multiple penetrators to
sample volatiles at the poles and a return of lunar samples.
Science from the
Farside of the Moon. Implementation: Multiple landers to conduct radio
astronomy measurements and to measure the composition of the surface and
surface impact rates.
Mars
Master: A
Mission to Return a Sample from Mars to Earth. Implementation: A single unified mission to
land, grab a 150 g surface and atmospheric sample, and return it to Earth
Asteroids
INSIDER - Interior of
Primordial Asteroids and the Origins of Earths Water. “The scientific
objectives of the proposed INSIDER mission require the exploration of diverse
primordial asteroids - possibly the smallest surviving protoplanets of our
Solar System - in order to constrain the earliest stages of planetesimal
formation.” Implementation: Spacecraft
to orbit several >100 km diameter main belt asteroids and a lander/rover to
explore the surface of a volatile-rich asteroid.
The Case for an ESA
L-Class Mission to Volatile-Rich Asteroids.
Explore one of the class of asteroids known as a main belt comet, which
are in the asteroid belt but which have been observed to emit volatile gases
like a comet. Determine if bodies like
these could have been the source for Earth’s water. Implementation: Spacecraft to orbit one or
more bodies. Possibly carry a lander or
return a sample to Earth.
Saturn,
Uranus, and/or Neptune
In situ exploration
of the giant planets and an entry probe concept for Saturn. “Comparative studies of the elemental
enrichments and isotopic abundances measured on the four giant planets would
provide unique insights into the processes at work within our planetary system
at the time of giant planet formation.”
Implementation: Put a probe into the atmosphere of Saturn, Uranus, or
Neptune, with Saturn suggested as the highest priority.
Titan/Enceladus
These two
proposals would continue the exploration of these two moons following the
Cassini mission. Fairly little detail is
provided on implementation.
The Exploration
of Titan with an Orbiter and a Lake-Probe.
Implementation: Saturn/Titan orbiter and a probe to land one of the
polar lakes.
The
science goals and mission concept for a future exploration of Titan and Enceladus.
Spacecraft delivers balloon to Titan
and then performs multiple flybys of Enceladus before entering orbit around
Titan.
Uranus
and/or Neptune
The Science Case for
an Orbital Mission to Uranus: Exploring the Origins and Evolution of Ice Giant
Planets. “The Ice Giants (Uranus and
Neptune) are fundamentally different from the Gas
Giants (Jupiter and
Saturn) in a number of ways and Uranus in particular is the most challenging to
our understanding of planetary formation and evolution.” Implementation: Orbiter to observe Uranus
remotely, explore its magnetosphere, and flyby all major moons. An atmospheric probe would study the
structure and composition of the atmosphere.
The
ODINUS Mission Concept – The Scientific Case for a Mission to the Ice Giant
Planets with Twin spacecraft to Unveil the History of our Solar System. Implementation: Two relatively modest
spacecraft that would orbit Uranus and Neptune to allow comparative studies of
these two ice giants and their systems of moons.
Neptune and Triton:
Essential Pieces of the Solar System Puzzle. “Neptune and Triton hold the keys to
paradigm-changing advances in multiple fields of planetary science: Solar
System and planetary formation, exoplanetary systems, geology and geophysics,
atmospheric science, magnetospheric physics, and astrobiology.” Implementation: Neptune orbiter to observe
that planet and perform multiple flybys of Neptune.
General
Solar System
Solar System Debris
Disk. “The dynamical and
compositional interrelations between dust, interplanetary meteoroids and their
parent objects are still largely unknown… [This mission] will shed light on all
these questions by mapping our solar system in dust, using the unique combination
of in-situ dust measurements, analyses of returned samples, and a bird’s eye view
for infrared observations of our outer “home” debris disk and beyond.” Implementation: Infrared telescope and a
spacecraft that will analyze dust in-situ and return samples to Earth.
Exploring
Planetary Origins and Environments in the Infrared. “We propose an observatory--‐class ESA mission
to provide spatially resolved infrared spectroscopy of solar system and planetary
objects in all their guises, from their origins (remaining debris in our solar system
and planet--‐forming discs around other stars) to their present--‐day appearance
(atmospheres, surfaces and interactions with their host stars for planets in our
solar system and beyond).
Implementation: Space-based thermal infrared telescope.
ExoPlanets
Several
concepts propose telescopes that could observe exoplanets in addition to other
targets. Only the concept below
addresses exoplanet studies as its focus.
Exploring Habitable
Worlds beyond our Solar System. “Among
the remarkable feats of the exoplanet community has been the ingenuity with
which new observing techniques have been invented and put into successful use
over the past twenty years. We now have a diverse set of tools at our disposal,
with which we can explore different aspects of exoplanetary systems. A number
of complementary approaches have been identified that can address habitability
from different angles. Coronographs and infrared interferometers have been studied
at some level of detail, and other more recent concepts (external occulters and
integratedlight telescopes) also show considerable promise. While none of these
is ready yet for flight, the rapid progress over the past few years in the
development of the key enabling technologies gives confidence that an exoplanet
exploration mission will become viable technically and financially in time for
implementation in the middle of the next decade.” Implementation: Several possible approaches
are given.
