Titan Surface and Subsurface In Situ System
What is the best architecture for a post-Cassini-Huygens mission to Titan?
Saturn's largest satellite, Titan, is the only moon in our solar system with a significant atmosphere. Larger than Mercury, it is really a planet that happens to be in orbit around a much larger planet. It would merit study in any case, but Titan is especially interesting because it appears to be hosting organic chemistry processes like those thought to have led to life on Earth.
Its atmospheric methane is believed to form organic derivatives which rain down to the surface, possibly creating lakes of liquid hydrocarbons similar to the "organic soup" from which life may have developed on Earth.
As of this writing, the Cassini-Huygens spacecraft is approaching Saturn, where Cassini will conduct a four-year orbital tour of the system, and the Huygens Probe will become the first robot to descend through Titan's atmosphere to its surface where, if it survives the landing, it will spend up to an hour before its batteries run out of power.
This study was to determine an architecture for a follow-up robotic mission to Titan in support of NASA's RASC (Revolutionary Aerospace Systems Concepts) Theme 1: Looking for Life and Resources in the Solar System. The hypothetical launch date is 2010.
Our task included determining the desired science measurements, the types of samples that would be needed, and instrument suites for collecting and analyzing the samples. In particular, we were to develop functional designs for delivering the science payload (such as an aerial platform), designs of sondes and other sample-acquisition systems, a concept for controlling a fleet of sondes, and a robust anomaly/fault management architecture. We were also to determine the best launch-and-cruise system for delivering the package to Titan.
Our science team decided that the primary science goals would be to determine what pre- and proto-biotic chemistry may be taking place at Titan, improve our understanding of Titan's surface and atmosphere, and learn about Titan's origin and evolution. To achieve these goals, we needed to design a system that could collect and analyze samples of (a) Titan's atmosphere near the surface; (b) the presumed organic-rich lakes, including the surface, subsurface, and bottom material; and (c) the presumed organic-rich ice at the rims of crater lakes.
Our principle constraints were payload mass to be delivered to the Titan surface, and time required for the mission. We had a maximum allowance of 1000 kg to be delivered to Titan (2600 kg counting the parachutes, aeroshell and backshell that would enable the science payload to descend through Titan's thick atmosphere). Our science team stipulated that the voyage to Titan should take about six-to-seven years, the minimum practical trip time with current propulsion technology.
We considered the following mission/system architectures:
- Single aerial platform (blimp) with orbiter.
- Single aerial platform with no orbiter.
- Multiple aerial platforms with no orbiter.
The question of whether to include an orbiter was a key trade, since it represents a great deal of mass. An orbiter can serve as a communications link between Earth and the aerial platform and/or sondes on the ground. We determined, however, that direct-to-Earth (DTE) communication was possible from the blimp if it were above 80° N latitude and used a 60W patch antenna. Eliminating the need for an orbiter allows a larger surface payload and/or a greater payload margin.
The multi-blimp option was similarly ruled out because of the launch vehicle's mass limitations. So option two, a single blimp with no orbiter, was determined to be the most desirable option. Since mobility for sampling of multiple surface sites appeared to be a key science driver, we determined that the blimp should be a fully-controllable aerobot, rather than a simple balloon. It would have to be capable of navigation without celestial references, since Titan's atmosphere would not permit such sightings.
Sample Acquisition Devices
While the blimp could sample Titan's atmosphere, other devices would be required for taking samples of Titan's surface and subsurface. We looked at the following options:
- Harpoon-like devices tethered to the blimp
- Tethered stationary sondes with analysis instruments in the sondes as well as in the blimp
- Mobile sondes with instruments. These could be all-tethered or all-wireless, or could consist of one tethered master sonde with wireless slave sondes.
Tethers would make the harpoons and sondes retractable, so they could be carried to various locations and reused. Tethers could also serve as communications and power conduits. However, they would limit the traveling range of mobile sondes.
We would want to take samples from solid crater rims, from the liquid presumed to fill Titan's lakes, and from the bottoms of those lakes. For taking samples from the potentially frozen crater rims, we considered 10 alternatives and concluded that a pressurized gas harpoon is the only practical micro solid-core sampler. For less dense, lower-viscosity materials, passive ingestion of liquid and/or a solid-phase microextraction system were found to be viable options.
The viscosity of Titan's presumed hydrocarbon lakes is unknown. As this graph illustrates, fully amphibious sondes would be most useful in low-viscosity environments, while harpoons and non-mobile sondes would be best in higher-viscosity settings.
A harpoon-like device would be lowered from the blimp to snatch a sample, then hauled back up for analysis by the blimp's instruments. Sondes could be mobile or non-mobile, and carry instruments for analysis or simply convey samples back to the blimp. Further, the sondes could be tethered (if they need to return to the blimp) or non-tethered (if they were to stay on the surface, conduct their own analysis of the samples, and communicate wirelessly).
The possibility of multiple mobile sondes raised the issue of managing a herd of perhaps dozens of miniature rovers. We developed systems for coordinating and controlling them, including a novel technology using field potentials. However, we determined that each sonde would need to have about 33 kg of mass which, given our mass constraints, would limit our fleet to no more than three units. The work on controlling a larger group may become useful in some other space mission farther into the future, or perhaps in different kinds of missions here on Earth, such as military or search-and-rescue operations.
Launch and Propulsion
The above graph compares three alternative combinations of propulsion to be used en route to Titan and "braking" to permit the spacecraft to stop when it reaches its destination. Depending entirely on chemical rockets would require a long trip time and a big launch vehicle. Chemical plus aerocapture (using Titan's atmosphere to slow the spacecraft) improves trip time, but still needs a big launch vehicle. A solar-electric ion propulsion engine (SEC) plus aerocapture (with the assistance of chemical rockets) offers the shortest trip time, and also allows either the use of a smaller launch vehicle or a great deal of mass margin with a large launch vehicle. We also considered nuclear-electric propulsion (NEP), but found that this technology would double the trip time due to its large mass. Further investigation into this option is warranted, however.
The previous graph applies to a net payload of about 1000 kg. Though that is the amount of mass we were asked to deliver to Titan, it is rather low for the proposed mission's requirements. This graph shows that much more mass can be delivered by using gravity-assist maneuvers at Venus, if we are willing to increase the trip time to eight years. Flying by Venus three times would enable a net payload of about 1,800 kg (enough for another two-dozen sondes!). Note that this plan would require a ballistic flight (that is, coasting without further propulsion after launch) because SEP equipment would not function properly in the heat at Venus' proximity to the Sun.
We performed similar trades for power subsystems and surface system thermal trades. We also conducted studies on how to enhance long-term survivability.
In sum, we identified an attractive Titan mission design capable of performing useful science based on a payload mass that could realistically be landed on Titan's surface. We evaluated various launch, cruise, and capture alternatives, and completed orbital insertion analysis with final mass breakdowns.
We developed sonde and harpoon surface and subsurface sampling concepts, and control architectures for the surface system; determined a way to eliminate the need for an orbiter -- at a great mass savings -- and instead provide direct-to-Earth communications from an aerial platform; and identified the tall pole technologies for mission implementation options.
We described sample collection and analysis systems consisting of an aerial platform (blimp), sondes and harpoons. And finally, we developed a thermal control concept to enable reliable surface and subsurface operations in an extremely cold environment of 80 Kelvins for minimum power.
For more information, contact: Charles.R.Weisbin@jpl.nasa.gov