Monday, January 5, 2009

ASRG Missions: Venus Balloon

In a previous post, I discussed the a new generation of plutonium-powered power sources for future planetary missions. NASA is funding mission studies to determine whether or not lightweight radioisotope power sources would enable one or more low cost (Discovery class, or ~$450M) missions.

Kevin Baines of JPL has proposed a number of Venus Discovery missions including an orbiter to study the atmosphere (whose goals were more than fulfilled by the Venus Express mission), and two balloon missions. The earlier balloon proposal, the VEVA (Venus Exploration of Volcanoes and Atmosphere) would have had been the most complex with a number of elements, while the second would have had just two balloon platforms. At least twice, to my recolleciton, Baines has been a finalist in the Discovery selection process, and both times other missions were selected.

The previous proposals suffered the limiation of being battery powered (solar cells are of questionable to no use within Venus' clouded atmosphere). Baines' current proposal would use an ASRG power source to enable much longer operation (a month instead of hours to days) and over ten times the data return. Baines and his collaborator, Tibor Balint of JPL, were kind enough to send me a copy of a poster on the mission they presented at a recent conference, and I've reproduced the abstract here:

"In situ exploration of Venus is expected to answer high priority
science questions about the planet’s origin, evolution,
chemistry, and dynamics as identified in the NRC Decadal
Survey and in the VEXAG White Paper. Furthermore,
exploration of the polar regions of Venus is key to
understanding its climate and global circulation, as well as
providing insight into the circulation, chemistry, and
climatological processes on Earth. In this paper we discuss
our proposed Nuclear Polar VALOR mission, which would
target one of the polar regions of Venus, while building on
design heritage from the Discovery class VALOR concept,
proposed in 2004 and 2006. Riding the strong zonal winds at
55 km altitude and drifting poleward from mid-latitude this
balloon-borne aerial science station (aerostat) would
circumnavigate the planet multiple times over its one-month
operation, extensively investigating polar dynamics,
meteorology, and chemistry. Rising and descending over 1
km altitude in planetary waves – similar to the two VEGA
balloons in 1985 – onboard instrumentation would accurately
and constantly sample and measure other meteorological and
chemical parameters, such as atmospheric temperature and
pressure, cloud particle sizes and their local column
abundances, the vertical wind component, and the chemical
composition of cloud-forming trace gases. As well, when
viewed with terrestrial radio telescopes on the Earth-facing
side of Venus, both zonal and meridional winds would be
measured to high accuracy (better than 10 cm/sec averaged
over an hour). Due to three factors: the lack of sunlight near
the poles; severe limitations on the floating mass-fraction
available for a power source; and the science requirements for
intensive and continuous measurements of the balloon’s
environment and movement, a long-duration polar balloon
mission would require a long-lived internal power source in a
relatively lightweight package. For our concept we assumed
an Advanced Stirling Radioisotope Generator (ASRG). In
return, this mission would provide two orders of magnitude
more science data than expected from the original batterypowered
VALOR concept, and could reduce measurement
uncertainties by a factor of five. In addition to the science
return, the secondary objective of this proposed mission would
be to space qualify ASRGs through all mission phases and in
various operating environments. Lifetime testing would be
demonstrated using a second ASRG on the carrier that would
keep operating after the in-situ element is delivered. Based on
the results of this and another eight ongoing NASA funded
studies, NASA will make a decision about the inclusion of
ASRGs in the next Discovery AO, due in the summer of 2009."