John Rehling: "I think having an atmospheric isotopic analysis will eventually become a desirable feature of a long-term lander, since it is still an open possibility that Venus undergoes major volcanic outgassing on a regular basis. (Namely, Pioneer Venus seemed to have detected a major change in SO2 levels during the course of its mission.) We ought to see if Venus Express can repeat that result (of course, a negative needn't mean that there is no such phenomenon -- if there are such events, they may not be decadal in frequency). WRT volcanism, a high-resolution radar mapping could be interesting from the standpoint of seeing changes since Magellan (finally, turning the long lag between Venus missions into a virtue), and an exciting follow-on to such a finding from radar or Venus Express would be to send a mass spectrometer downwind of any identified source of the gases (this might be quite easy to localize if it were a large volcano) -- follow the lava, so to speak."
We don't necessarily need in-situ entry probes to monitor that sort of thing -- the two IR spectrometers on Venus Express will monitor SO2 (and a large number of other trace gas) levels very accurately over time and with high spatial resolution, and I believe they also have the ability to profile it at different altitudes. Nor do we need isotopic analysis, as opposed to
"The current roadmap excludes any future radar orbiter."
The White Paper on Venus exploration provided by the DPS to the Decadal Survey said that such a mission can very likely be flown within the Discovery cost cap -- and the intriguing recent notes from the Venus Exploration Analysis group on that part of the developing Solar System Strategic Roadmap ( http://www.lpi.usra.edu/vexag/meetings.html
) indicate that such a mission might be an alternative to VISE as the top-priority Venus mission for the 2010s, depending on how much evidence of volcanic activity Venus Express and the later, smaller Japanese "Planet-C" orbiter turn up. (The DPS White Paper also said that such a high-resolution radar orbiter could have a microwave radiometer and a trace-gas IR spectrometer added to it as auxiliary instruments to further monitor and locate active volcanism.)
"The other outstanding feature is that VISE has been so downscoped that I'm not sure I see the point, except as a demonstration of technology (and it's far too expensive to justify the mission on those grounds) or as a stopgap to provide an earlier quantum boost in our knowledge of Venus before the proposed geophysical network and Venusian 'MER' (rover or airborne), which would both surely duplicate any VISE findings, except to add one more surface point to spatial coverage."
I wouldn't necessarily say that. Esposito's concept would involve 2 or 3 landers on a single launch, carrying out surface composition analyses on widely different types of Venusian terrain, and including fairly detailed mineralogy. (Although -- judging from my one brief look at the list of scientific instrument PIs for SAGE that he accidentally gave me before yanking it off the public Web -- age-dating is not one of them.) It may be quite a long time before we develop the technology for any kind of long-lived or long-distance-traverse Venus mission; and even if it isn't, the detailed surface geochemical data that VISE could provide would be very valuable in further selecting the destinations for such later missions.
"If there is any way to have a nonnegligible groundtrack of a descending geophysical station, to add some panoramic coverage during a Huygens-like descent, it seems to me an all-things to all-people 'single' mission would be to build three surface stations that can last on the surface, and come away with imagery of three descent tracks, long-term atmospheric and seismic science, and upclose looks at three stationary locations."
VISE is definitely supposed to provide descent and surface imaging. (In this connection, note also the "VEVA" Discovery mission that was proposed a few years ago, which would drop four small dropsondes from a long-lasting cloud-layer balloon to provide multispectral descent imaging of different kinds of Venusian terrain before their impact -- and even had little parafoils to maximize their sideways gliding distance before impact. Moreover, one description of that mission among the JPL Technical papers says that the dropsondes might be augmented with "a laser altimeter and near-IR spot spectrometer", which I take to mean that a broadband laser might allow good detailed near-IR reflectance spectra of the local minerals from these probes as well. I've remarked elsewhere about the unique difficulties in obtaining good near-IR reflectance spectra of the dayside Venusian surface without an intermittent artificial light source, since the surface is so hot that it also glows thermally in the near-IR and the two types of near-IR spectra get mixed up together.)
"I can't see but how that would provide better science for the buck than the current plan of three huge missions before VSSR (which will almost certainly not happen before I die -- and I'm not old)."
I question whether VSSR will happen before the General Sherman Tree dies. However, there are quite a lot of possible intermediate-level Venus missions that aren't all that "huge". (I myself have always found the problem of designing scientifically cost-effective missions to explore this savage place to be a fascinating engineering problem.)