From: Larry Klaes (email@example.com)
Date: Fri Apr 20 2001 - 10:24:20 PDT
The European Space Agency is informally seeking proposals for the reuse of
the Mars Express bus for a new mission, to be launch in late 2005 - Project
X. The cost ceiling for this mission would 120-150 million Euros, excluding
(Project X manuals and data available from http://sci.esa.int)
Mars Express is based on the Rosetta comet-chaser, a solar-powered
spacecraft that will fly out to 5.2 AU. Outer solar system missions are,
therefore, within the capabilities of the spacecraft.
Strict instrument and spacecraft development schedules mean that only a
"generic" (exploration rather than focussed science) payload can be carried.
The bus probably doesn't have enough fuel capacity (1500 m/s delta-v for
Mars Express) to go into orbit around a Galilean moon. The mass of radiation
shielding and solar-panel degradation mean that even an Io Volcanic Explorer
is probably out.
Ideas for getting around these problems with a Jovian orbiter are welcome.
However, launch of an unoptimised spacecraft to Io will preclude launch of a
specialised mission for decades - think Mariner 10 and Mercury.
So the best target for the mission is a world that would not otherwise be
explored. Venus is one candidate (though ther solar-sail equipped Space
Technology 7 may fly there). Another candidate is Uranus. Because Uranus is
tilted with respect to the plane of the solar system, the best time to
launch a mission is at an equinox, when both hemispheres of the moons are
visible. Voyager 2 arrived at the height of southern summer and could only
image the southern hemisphere. The next equinox is in 2006. The one after
that is in 2048. The 2008 Jupiter gravity assist opportunity is our
generation's last chance to get to Uranus. This is not a limit imposed by
poorly-constrained atmospheric models suggesting periodic freeze-out of a
tenuous planetary atmosphere, as in the case of Pluto. It is an absolute
limit imposed by celestial mechanics.
*Equinox Express science case*
Voyager 2's January 1986 flyby of Uranus allowed us to pose a large number
of questions, but answer hardly any of them. We need a follow-up mission to
carry out in-depth exploration.
It is now thought that the accretion of Uranus and Neptune took 700 million
years, and that their formation 3900 Mya caused a vast comet shower - the
Late Heavy Bombardment - to rain into the inner solar system. (This also
seeded the Oort cloud). The volatiles and organics that could only coalesce
beyond the frost line were delivered to Venus, Earth and Mars in
significant - ocean-building - quantities. Close to the Sun, reactions could
take place at high enough rates for life to form quickly, but, since much of
the heat of planetary formation had been radiated away, the volatiles did
not escape to space, nor were all the organic molecules decomposed.
So to make life, you need a late delivery of outer-system material -
"primordial ice-cream" - into the primordial broth. How common are such
serendipitious mixings, which life may well require? Ony a study of Uranus
and Neptune will tell. Currently, both planets are something of an
embarassment; nebular condensation models require very sensitive settings to
reproduce their formation. The tilt of the Uranus system is a clue to
Uranus' tilted magnetic field makes it an analogue for a class of stars
known as "oblique rotators". Is it undergoing pole reversal? Is it's
Uranus is the only gas giant without an internal heat source. We don't know
what the causes and consequences of this fact are.
Uranus' sole handicap in the public awareness stakes is it's bland methane
upper atmosphere, but Hubble images show that this has changed since the
Voyager flyby (as has Neptune's; the Great Dark Spot has disappeared). By
carrying methane filters, the deeper ammonia and water clouds, which are
expected to be turbulent and very different to those of the "hot" gas
giants, can be revealed. We need to know if giant-planet atmospheric
dynamics are significantly seasonal; superstorms on Saturn seem to occur at
three-decade intervals. This may finally allow us to choose between models
of giant-planet interiors. Certainly, the more planets we explore, the
The grainy snapshots of single hemispheres of the Uranian moons may not be
representative of the whole body; the Moon, Mars and Iapetus show enormous
hemispheric dichotomies. Study of the ice moons will allow comparison with
Saturn's satellites. What makes the Uranus system different? Parallels can
be drawn with Pluto, Charon, and the larger Kuiper Belt objects. The dark
materials on the moons are probably organic deposits. Given that these
objects are the closest analogues to the lifeseeds of the Late Heavy
Bombardment we'll explore during the next few decades, we need to know what
kinds of organics, and in what quantities. They are possibly the most
chemically primitive materials in the solar system. What caused chaos on
fracturing and cryovolcanism on Ariel and Titania, but little on Oberon and
Umbriel? Not Uranus' tides; they're too small. Are we seeing the work of
ancient orbital resonances? If so, what destroyed them? Oberon is nearly as
dark as Iapetus. Why? Do Caliban and Sycorax
have the same role as organic sources in the Uranus system that Phoebe has
in Saturn's? Do carbon oxide ices, ammonia clathrates, or exotic water ices
exist on the moons? Is the bright spot near Umbriel's terminator on the
Voyager images an ice cap, or something more exotic? Is the
expansion-cracking theory right? How was Miranda made - reassembly after
repeated disruptions or incomplete differentiation?
What maintains the charcoal-black rings of Uranus? Do unseen shepherd moons
maintain their structure? Why are the ring particles so big? Has the
hydrogen exosphere draggd down smaller ring particles? How are the rings
changing with time? How can we explain the differences between planetary
The above science case is as strong as MESSENGER's, and far stronger than
PKE's (PKE was about exploration, not science).
*Equinox Express mission plan*
Spacecraft design: Mars Express. Primary modifications new solar arrays
(lightweight SARA-derived "venetian blind" 15:1 concentration ratio
aluminium-foil concentrators with a parabaloid cross-section making up a
pair of rectangular arrays supplying 100W at 19.2 AU), and a thermal control
system using small radioisotope heaters, as demonstrated on Sojourner. 20 kg
instrument package, to include combined imager and IR
spectrometer/photometer, magnetometer and plasma spectrometer. Secondary
payload occupies the 60 kg Roland/Beagle 2 lander slot. Could be a
Whipple-shielded ring- exploring miniorbiter, a Miranda lander or an
Launch in December 2005 (coincidentally, perfect for the Project X window!)
by Proton, Angara, Ariane, Delta IV Heavy demonstration launch, Atlas, or
Shuttle. ISS downsizing will free more Shuttles for science flights. Lunar
assist on Earth-system exit. Hohmann Type I to Jupiter.
Powered Jupiter flyby in mid-2008. Either very close "needle-threading"
flyby to avoid the Io torus, or a Ganymede assist. True coma mode (a.k.a.
Giotto to Gregg-Skjellerup) for the interplanetary cruise, to keep
components within their operational warranty.
Uranus Orbital Insertion into highly-elliptical orbit with two-month period
in late 2014; secondary payload release and data relay. Two-year primary
mission concentrates on global mapping of the wildly contrasting ice moons,
with detailed studies of organic deposits and cryovolcanism, and atmospheric
dynamics. Any mission extension would use Titania's gravity and mild
aerobraking to painstakingly lever the spacecraft to a higher inclination
orbit for ring, small moon and magnetosphere studies.
Nominal end of mission in 2016 or 2018, but the weak Uranian magnetosphere
will degrade the solar panels only . As an "exercise in engineering
exuberance", the spacecraft images the Verona Rupes (scarp system on
Miranda, with the steepest, tallest cliffs in the Solar System) from the
*inside* before cratering Ariel on the succeeding orbit.
Elliptical orbit with Galileo-type apocycle playback of data acquired at
pericycle. Mars Express was designed for this "store and forward" data
handling mode (another excellent coincidence!) and so has a large on-board
*The politics bit*
"Scientific planetary imperialism" - the idea that NASA has claimed Mars,
the USSR claimed Venus, and BepiColumbo will "win" Mercury for ESA - works
in Equinox Express' favour. Since no other mission will reach Uranus until
2048, this will give Europe a gas giant.(Sic).
European space science politics are as complex as NASA's; with the stakes so
small, the competition is great. However, there are no institutionalised
clashes (Ames versus Marshall type). The main alternative to any
outer-system proposal would be MASTER, a fast Mars flyby/slow Vesta flyby.
duplicates Dawn. Given that BepiColumbo already duplicates MESSENGER, common
sense should indicate that it shouldn't fly until Dawn results can be
factored into the design.
This is effectively a Discovery-class opportunity, and it's sidled up on us
with virtually no fanfare! ESA's pariochal prejudices are less strong than
NASA; it won't accept a US-led team, but 60:40 split of responsibility would
be acceptable. ESA will decide what to do with Project X in October. Even if
this idea fails for lack of community support, we need at least one
outer-system proposal in the running.
Comments, critique and constructive abuse all welcome.
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This archive was generated by hypermail 2b30 : Fri Apr 20 2001 - 10:46:53 PDT