Wednesday, June 30, 2010

Venus Mobile Explorer Concept

Note: Summer is the time for my field work, so updates to this blog will be less frequent for the next two months.

In the last blog entry, I discussed a mission concept being investigated by the Decadal Survey to place a lander on the Venus highlands (the tessera) called VITaL.  This entry explores a second mission concept, the Venus Mobile Explorer (VME).  In many ways VME is similar to the Venus tessera lander -- they both target the highlands and they both carry nearly identical compliments of instruments.  While VITaLwould land at a single site, VME would land at one site, inflate a "balloon", float in the wind a few kilometers above the surface, and then land at a second site.  This would allow imaging of a traverse of 8-16 km before settling at the second site.  The imaging traverse and landing at two sites would help determine the diversity of the highlands, which may be remnants of ancient crust.  Like the ViTAL mission, the VME mission was recently described in a paper presented at the 7th International Planetary Probe Workshop.

Since the instruments for the two missions are nearly identical (please see the VITaLdescription), I'll concentrate on the differences between the two missions in this entry.  I used the term "balloon" in quotes above because "bellows" would be a more appropriate term.  In the harsh conditions at and near the surface of Venus, traditional balloon materials cannot be used.  Instead, VME would inflate a bellows system with helium to create lift and allow the lander to lift off and drift for approximately 4 hours before the second landing.  At either a preset time or when internal temperatures within the probe rise above a preset limit, the bellows are discarded.  The bellows cannot be deflated and then reused a second time because of plastic deformation of the metal in them during their first use.

The goal of the 25 Decadal Survey mission studies  is to (1) scope out the concepts since many were ill defined at the start of the Decadal process, a problem that caused substantial cost under estimation in both the last astronomy and planetary Decadal Surveys; (2) determine the technological maturity of the concepts; and (3) provide cost estimates of the leading concept missions.  The Decadal Survey members will eventually select a small subset of the 25 missions that fit within the budget, are (or can be in the next decade) technically mature, and that are most likely to provide the greatest advance in our understanding of the planets and their histories.  

In keeping with the goals of assessing mission concepts, the authors of the VME paper list several issues with this concept:

1) The lander would need to survive the harsh conditions of the surface and near-surface for almost 5 hours (including time to transmit data up to the telecommunications relay), compared to the two hours for the VITaLconcept and three hours for the proposed SAGE Venus lander New Frontiers mission proposal.  This leads to challenges in ensuring the interior of the lander remains cool enough to operate, increasing the cost of the mission.

2) The design of the bellows, the helium tank, and other mechanisms will be challenging and these elements require additional technological maturity before they can be used, adding to mission cost and risk.

3) The estimated cost of the mission without launch vehicle is $1.7B and with launch vehicle is $1.9B.  [Editorial note: It often hard to compare mission proposal costs since details of what are included and excluded from costs often aren’t detailed.  The cost of the SAGE lander is presumably within the $650M cap for the spacecraft portion of New Frontiers missions.  However, the fully burdened cost of New Frontiers missions is approximately $1.2B, which includes the cost of the launcher and other overheads for running the New Frontiers program.]

Editorial Thoughts: Unless Venus exploration is given a high priority in the Decadal Survey process, the $1.7B price tag for this mission plus its technical immaturity would seem to make its selection unlikely.  The authors of the VME study point out that they believe that "the bellows mobility concept is likely one of the lower cost ways to visit two different landing sites, though it has a higher risk versus multiple heritage [e.g., multiple SAGE or VITaLsingle location landers]."  For the same launch mass and volume, two to three heritage landers could be delivered to Venus, although the authors do not provide a cost estimate.  I have read that additional spacecraft of identical design cost ~50-60% the cost of the first spacecraft.  So, if the SAGE spacecraft is ~$650M, then a second copy might be in the range of $325-375M.  This back of the envelope analysis suggests that two to three heritage landers could be flown within the cost envelope of VME with lower technical risk.

On another topic, the VME paper makes clear why current Venus lander concepts are proposing to use laser-based remote sensing of the surface instead of direct analysis.  To measure the surface material with any finesse, a sample needs to be brought into the lander where the instruments can be kept cool.  The sampling mechanism for bringing in the surface material would weigh ~4 kg and the analysis would require a minimum of 2 hours.  Sampling multiple locations with a laser (for illuminating the surface for Raman spectroscopy and vaporizing it for induced spectroscopy would require ~15 minutes.  Reducing surface measurement time by an hour and 45 minutes should substantially reduce mission risk.

Closing note: The VME paper provides one of the most succinct summaries of the goals for Venus landers that I've seen.  Measurements of the atmospheric composition are equally important as measurements of surface composition to meeting the full suite of goals.  I'm quoting the entire list here:

1) determine whether Venus has a secondary atmosphere resulting from late bombardment and the introduction of significant outer-solar system materials, including volatiles, 
2) characterize major geologic units in terms of major elements, rock forming minerals in which those elements are sited, and isotopes, 
3) characterize morphology and relative stratigraphy of surface units,
4) determine the rates of exchange of key chemical species (S, C, O) between the surface and atmosphere,
5) place constraints on the size and temporal extent of a possible ocean in Venus’s past, 
6) characterize variability in physical parameters of the near surface atmosphere (pressure, temperature, winds), and 
7) measure the ambient magnetic field from low- and near-surface elevations.

Link to Venus Mobile Explorer paper.  My thanks to Lori Glaze, lead author of the paper, for her assistance in writing this summary of the mission analysis.

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