The biggest compromise is that JET would fly just two instruments: a mid-infrared camera/thermal imager and a mass spectrometer. The most minimal of the proposed Enceladus payloads considered by the Decadal Survey would add a medium angle camera, a magnetometer, and a dust instrument. Other versions of the the Decadal Survey concepts would add up to another nine instruments beyond that minimal list.
Even flying two instruments on JET requires using an already built Rosetta mission mass spectrometer plus other hardware contributed by other nations that wouldn't be counted towards the NASA cost cap. Without those contributions, and with the mission cost estimates shared by the PI, JET would be unable to fit within a Discovery budget. Even adding a simple instrument like a magnetometer would push the JET proposal close to the cost cap, and the PI held firm against what he described as many requests to add an instrument.
Even within these limitations, the JET mission would considerably extend the measurements that the Cassini mission has been able to make in key areas. The JET camera would take advantage of spectral windows in Titan's atmosphere to image the surface at up to 25 m per pixel, 40 times better resolution than the equivalent imager on Cassini and up to 12 times higher resolution than the Cassini radar. The camera would image 15% of Titan in the nominal one year mission at resolutions of 50 m or better The cameras would take images in mid-infrared bands, possibly enabling some compositional studies if the surface materials differ in their mid-IR spectrum. At Enceladus, two of the bands would allow imaging of the distribution of heat sources associated with the tiger stripes and vents at up to 5 m resolution. The higher resolution at both moons would reveal details not seen by Cassini and allow a better understanding of the processes that may have created those structures.
The mass spectrometer would both extend the range of compounds that could be measured by sampling larger molecules and provide greater resolution within ranges of atomic weights. Combined, this mass spectrometer would allow detection of a wider range of organic molecules in the upper atmosphere of Titan and (if present) in the jets of Enceladus.
Here are some examples of the specific studies enabled by JET's instruments from Sotin's poster:
- Search for sedimentary layering in Titan valleys resulting from erosion of plateaus and mountains
- Map the distribution of solid organics and organics in Titan's small lakes
- Measure the energy output and lifetime of Enceladus' jets by high resolution mapping
- Inventory organic and heavy molecules (mass >100 Daltons) in Enceladus' plumes (if present) and Titan's upper atmosphere
- Determine what molecules (CO, N2, hydrocarbons) make up the mass 28 in Enceladus' plume that has been identified by Cassini
In the nominal one year mission at Saturn, JET would encounter Enceladus several times and encounter both the pro- and anti-Saturn facing sides of Titan on opposite sides of Saturn. This will allow mapping of both hemispheres of Titan while they are illuminated by the sun. For comparison, the Jupiter Europa Orbiter would encounter the Galilean moons on just a single hemisphere of each in its flybys, limiting studies to that hemisphere.
Two extended mission options (which would require additional funding as the prime mission nears its end) would be particularly exciting. JET would be powered by ASRG plutonium power sources, and NASA would like to have a full 14 year test of those power sources. To fulfill that desire, JET would need to continue for approximately seven years after entering Saturn orbit (although further science observations aren't required for the engineering life test). During that time, JET could act as a data relay for any in-situ craft that might land or fly above Titan. This could enhance the data return of a mission such as the Titan Aerial Explorer or AVIATR plane by many times what they could return direct to Earth using their own antennas. Complimentary to this first option (but not required for it), JET could spend three years pumping down its Saturn orbit using Titan flybys and eventually enter orbit around Titan. The orbit could be high, 2500 km, well above the atmosphere and would allow continued observations of Titan's surface for years.
Editorial Thoughts: JET is a good example of the trade offs necessary to conduct Discovery missions in the further reaches of the solar system. Assuming that the Discovery review panel agrees with the PI's cost estimates, Discovery missions to the Saturn system are possible if you can get friends to contribute some hardware and instruments and stick to minimal payloads. JET's two instruments would provide valuable science in key areas of study, though. I would prefer to see one of the more capable Enceladus missions described in the Decadal Survey studies fly over JET -- more instruments are better. (The mass spectrometer and thermal imagers of those missions should fulfill JET's Titan goals.) However, if the Survey doesn't recommend one of those missions, then I believe that JET would be an exciting mission to fly. Better a simple mission than no flight to these worlds in the coming decade.