This continues a series of posts inspired by a similar set of posts at Future Planetary Exploration blog selecting the 5 most compelling missions from the Planetary Science Decadal Survey list. This presents my personal selection for the 5th and last most compelling mission from the list.
I'd like to select a mission that fits well with one of the Mars missions I selected as my 3rd and 4th most compelling mission: a 2-part Mars Climate mission and a Mars Geophysical Network. The obvious choices to me were the Venus Climate Mission and the Lunar Geophysical Network. First, let me described these 2 contenders for the 5th spot as presented by the Decadal Survey mission concept studies.
The Lunar Geophysical Network includes 4 similar landers that would arrive at different locations on the Moon. These landers would have goals that are not very different from those of the Mars Geophysical Network. They would be expected to determine information about the lunar crust, mantle layers, and core (e.g.: size, state, composition, temperature), assess lunar heat flow, and measure moonquakes. Each lander would include a seismometer, magnetometers, electric field sensors, a Langmuir probe, retroreflectors, and a heat flow sensor. As with the Mars Geophysical Network seismometers, the 4 seismometers on 4 landers in the lunar network concept would work together simultaneously to produce results that are much more useful than measurements from a single seismometer. The heat flow sensor would be deployed under the regolith up to 3 meters, possibly delivered by a "mole". The retroreflectors are targets for Earth-based lasers that precisely measure the distance from the laser to the retroreflector.
The lunar day/night cycle encourages use of ASRGs, which in turn encourages use of an Atlas V variant for launch. The Falcon 9 is not certified for launch of ASRGs, but a less capable mission variant is depicted using Falcon 9 to launch 2 solar-powered landers. There is a tradeoff between expensive ASRGs (and related certification) and heavy, less capable solar power and batteries. My mission selection for the top 4 most compelling missions is already ASRG-heavy, so the ASRG option might be problematic in that context.
Clive Neal presents more justification for a mission like this one in The Rationale for Deployment of a Long-Lived Geophysical Network on the Moon.
One of the nice things about this proposal is that it can start to produce results quickly. The mission could launch and begin to return data in FY16. Compare that to my second most compelling mission choice, the Enceladus Orbiter, which might launch in the mid-2020's and arrive at Saturn in the 2030's.
Another attraction compared to many other missions in the Decadal Survey list is the estimated cost, $903M in FY15 dollars with reserves included.
On the other hand, if I were selecting a lunar mission, I would put some thought into selecting a second Lunar Polar Volatiles Explorer rover (which I suspect would be quite affordable assuming the first is built) sent to another region (perhaps the other lunar pole), or a lunar sample return mission like MoonRise, before this geophysical mission. However, I bent my own rules enough by choosing 2 of the Decadal Survey's Mars Climate Orbiter concepts. I didn't include carbon copies of earlier choices or current New Frontiers mission contenders as possible choices in the first place - I only want to select unique missions from the Decadal Survey's mission list.
Now let's take a look at the ambitious Venus Climate Mission. This mission is intended to study the origin, variability, suspected major ancient climate change, and interaction with the surface of the mostly carbon dioxide atmosphere of Venus. One angle of this study is to learn about potential climate change on Earth by comparison, and to test terrestrial General Circulation Models using Venus as a model test scenario. The mission includes several distinct pieces of Venus hardware.
There is a Venus orbiter spacecraft that serves as a carrier and telecommunications rely for the other components. The orbiter also includes a "Venus Monitoring" camera that gives context for the measurements from the elements of the mission that reach Venus itself.
There is a balloon that itself serves as a carrier and deployer for other mission elements. The balloon is intended to last at least 3 weeks, floating 55km in the Venusian atmosphere and going around Venus up to 5 times during its journey. The balloon has instruments that sample the atmosphere and clouds of the planet. For example, it includes a Neutral Mass Spectrometer that can carefully measure noble gas isotopes. A Tunable Laser Spectrometer measures trace gases. The NMS and TLS should give even better results together than they would do individually. A Nephelometer studies cloud particles. Clues on atmospheric circulation are revealed by tracking the balloon as the atmosphere moves it about the planet.
At 2 different times during the balloon mission, it deploys is a pair of small Drop Sondes. These measure pressure, temperature, acceleration, and wind speed as they fall from the balloon to the surface over the course of 45 minutes using "Atmospheric Structure Instrumentation". The balloon and Mini-Probe (which I will describe momentarily) also include similar instrumentation. The Drop Sondes also include a Net Flux Radiometer to measure solar and Venus-based radiation. Again, the balloon and Mini-Probe host similar instruments. The Drop Sondes are tracked by the balloon to gain more data about winds at the various levels the Drop Sondes fall through.
The other element of the mission is a Mini-Probe that is larger and more capable than the 2 Drop Sondes. It is released by the entry system at the same time as the balloon system, and falls for 45 minutes. In addition to instruments like those the Drop Sondes carry, like the balloon system, it carries a Neutral Mass Spectrometer to measure aspects of Venus's atmospheric chemistry. In this case the profile is taken vertically (i.e. the probe falls through the atmosphere as it takes measurements), whereas the balloon profile is generally horizontal.
The Venus Flagship Reference Mission has some commonalities with the Venus Climate Mission, but it's even more ambitious. It includes 2 landers that last for several hours on the surface, 2 balloons, and a much more capable orbiter able to map Venus at a much higher resolution than Magellan did. That mission is also much more expensive, and was not one of the ones on the Decadal Survey list, so I'm not considering it.
The Venus Mobile Explorer, another concept on the Decadal Survey list, also includes a Neutral Mass Spectrometer, Tunable Laser Spectrometer, and pressure/temperature/wind sensors for analysis of the atmosphere at different altitudes. It's able to land and later float to one other location on the surface. It has fewer climate/atmosphere capabilities than the Venus Climate Mission and may cost a bit more, but it gains surface capabilities and imaging.
The Venus Intrepid Tessera Lander, another mission studied by the Decadal Survey, includes similar atmosphere instrumentation on a lander mission, but at a projected cost ($1.3B in FY15 dollars with reserves) that is lower than either the Venus Mobile Explorer or the Venus Climate Mission.
When finding a partner for the Mars Climate Mission, the Venus Climate Mission comes to mind first, but these other missions should also be considered, since they have some climate capabilities mixed in with surface analysis.
As with the lunar geophysical mission, I would consider the SAGE (Surface and Atmosphere Geochemical Explorer) Venus mission in the current New Frontiers competition as a strong alternative to the Venus Climate Mission. SAGE consists of a lander that would survive for at least 3 hours on a Venus volcano. It can dig and analyze samples, and also includes a number of instruments to study the climate and atmosphere of Venus (including the Atmospheric Structure Investigation, Tunable Laser Spectrometer, and Neutral Mass Spectrometer, which I assume are similar to the ones of the climate mission). However, SAGE is not on the Decadal Survey list, so I'm not including it as a possible choice. Allowing the 3 current New Frontiers competitors could have taken a lot of the fun out of this survey of Decadal Survey options since I could very well have given 3 of the top 5 spots to them.
The combination of a Venus Climate mission, various climate studies of Earth (including those based on satellite data), and surface and orbiter Mars Climate missions (as I already selected for the 3rd most compelling mission) should give us a lot of practical data to allow us to compare climate at these planets. Of course learning about implications for Earth from the other 2 planets is the immediately practical aspect, and it's a compelling one. However, I'm concerned about the cost of the Venus Climate mission, estimated at $1.577B in FY15 dollars with reserves. I already selected the Enceladus Orbiter as a Flagship class mission, and I'm inclined to limit the number of flagship missions to allow a greater number of less costly (but, it should be admitted, less capable) missions to fly. As a result, even though in one sense I consider the Venus Climate mission to be the more compelling of the 2 missions by a hair, when factoring in cost, the Lunar Geophyiscal Network wins. I select LGN as the 5th and last of my "top 5 most compelling missions" from the Decadal Survey. The LGN folks shouldn't rest easy, though, because if an international partner picks up the costs for one or two of the significant components of the Venus Climate mission, thereby lowering the cost of the mission to NASA (which I think could be done given the several distinct parts of the mission), the Venus mission would probably bump LGN off the list and into spot #6.
Now that I've selected my personal top 5 selections from the Planetary Science Decadal Survey, in the next post I'll take a look at some ideas for the rest of NASA's Planetary Science mission budget. I've come up with a top 5 list that only gets to 3 destinations, so it would nice to see where else we can go. I may also discuss how my 5 most compelling selections fit with the theme of this blog.