A 17-year odyssey to bring Titan samples to Earth

Sample return missions from solar system bodies are all the rage. Despite the miniaturization of scientific instruments for space probes, advances in instrumentation and analysis techniques here on Earth continue to advance by leaps and bounds. As much as we shrink an instrument, sending a particle accelerator to another world to find out the composition of minerals will remain a Herculean task. We have already sent sample return missions to nearby asteroids and the Moon, while such missions to Mars and several asteroids are planned. In the future, Enceladus’s geysers and comets will also be priority targets. But what about Titan? This satellite of Saturn is one of the most fascinating worlds in the solar system thanks to its dense atmosphere and the presence of lakes and seas of liquid methane. The surface of Titan is full of complex organic substances that can never be adequately analyzed by a small space probe. NASA’s Dragonfly probe will explore Titan and its organic substances in the middle of the next decade and, although nobody doubts that we will learn a lot, the ideal would be to be able to analyze samples of this satellite on Earth.

Concept of a rocket taking off from Titan to bring samples back to Earth (NASA).

The problem with a Titan sample return mission is that, as with all these projects, you have to make your way back, and that involves enormous energy expenditure. Let’s not forget that a space probe is, in general terms, a rocket and that implies that its design must follow the Tsiolkovsky rocket equation. That is, the more fuel we carry, the more we will have to incorporate to load that extra fuel and so on. In order to avoid using gigantic probes, sample return architectures use the same ‘shortcuts’ as launchers: employ additional stages and, if possible, more efficient propulsion systems. For this reason, the NASA/ESA Mars Sample Return mission will use three probes – maybe four – and the ESA ERO orbiter that will bring the pieces of Mars back to Earth will use ion propulsion. In the case of Titan, it is necessary to overcome the gravitational well of Saturn and Titan, in addition to the enormous distance that separates them from the Earth (the distance does not affect the Delta-V, but it does affect the time of flight).

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Architecture and dates of the sample return mission to Titan (NASA/Landis et al.).

Geoffrey A. Landis of NASA’s John Glenn Research Center has spent years studying the feasibility of a Titan sample return mission to analyze the precious tholins of Saturn’s largest moon on Earth. The idea is to bring to our planet a set of surface samples with a mass of 3 kg. Although at first it was considered to use two probes —one to collect the samples and place them in orbit around Titan or Saturn, and the other to collect them and bring them to Earth—, the scheme of a single mission was finally decided on because it was simpler and avoided the risk involved in launching and operating two space missions. To achieve this, the key concept of the Landis team is the use of ISRU (In-Situ Resource Utilization), that is, local resources, which, in the case of Titan, means using the methane from the lakes as fuel and the oxygen from the rocks—mainly made of water ice—as an oxidizer.

ISRU schematic of the sample return mission (NASA/Landis et al.).
Mission masses (NASA/Landis et al.).

The mission concept, which would have a total duration of 17 years, would be as follows. The probe would take off in October 2038 by means of a Falcon Heavy and, after a flyby of Jupiter, would arrive at Titan in December 2045. The probe would be in the form of a supporting body and would enter directly at 6 km/s in the atmosphere of Titan. After parachuting down, the probe would deploy its systems on the surface. The vehicle would consist of two main parts. On the one hand, the launcher to return the samples to Earth and, on the other, a section with the plant for generating fuel and oxidant, the communication systems, a rover and a one-kilowatt radioisotope generator (the heat from the RTG would also serve to melt rocks and obtain oxygen). The mission would need about three years to synthesize 3 tons of methane and liquid oxygen for the launcher, which would introduce a novel folding design. That is, to fit into the probe’s heat shield, the rocket would use flexible propellant tanks and a deployable structure that would reach full size once the tanks were filled. Once deployed, the rocket will have a length of 11.74 meters and a diameter of 1.4 meters. The mass of the probe at landing would be approximately one ton.

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Landing probe elements (NASA/Landis et al.).
Rocket folded configuration (NASA/Landis et al.).
Inflatable cryogenic tank prototype (NASA/Landis et al.).
Deployed configuration (NASA/Landis et al.).

In order to obtain the methane, the probe would have to land near a lake or sea and deploy a suction tube with the help of the rover. The mixture of methane and ethane from the seas could be used directly or, better, distilled to obtain pure methane. Another option would be to use hydrogen resulting from the electrolysis of rock ice, which is a more efficient fuel. The bad thing is that hydrogen occupies much more volume—that is, we would have a bigger rocket—and an effective insulator would have to be used to keep it liquid once in orbit. Once the samples were collected, the three-stage launcher would use two of these stages to put the probe back with the 3 kg of samples into a 1,000-kilometer-high orbit around Titan in June 2049. The probe would then spend a year performing gravitational assist maneuvers with Titan that would culminate in a close flyby of Saturn in order to set course for Earth. The third stage of the launcher would serve in these phases to maximize the Delta-V. The return probe would have a total mass of 250 kg and would have two 3.6 square meter ROSA-type flexible solar panels. The return capsule would have the same design as the OSIRIS-Rex mission capsule that will bring samples from the asteroid Bennu to Earth. If all goes well, the capsule with the 3 kg of samples will land on Earth at a speed of 15 km/s in January 2056. Up to 1 kg of liquid oxygen would be used to keep the temperature of the samples below 100 kelvin, thus preventing them from deteriorating.

Rocket configuration and return probe at various stages (NASA/Landis et al.).
Configuration of the return probe with the capsule (NASA/Landis et al.).

And, finally, in the early 1950s we would have detailed information on Titan’s tholins, organic substances that, in addition to being abundant on this moon of Saturn, are extremely common in the outer solar system and in the Kuiper belt. Without a doubt, a mission of this type is very ambitious, although it is hoped that the Dragonfly data will help us to specify the objectives and design of such a complex project. Of course, for now a return of samples from Titan is not a priority for the scientific community, unlike the study of Enceladus or Europa.

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Lakes and seas of Titan’s northern hemisphere (NASA).
Some organic substances of the titanic atmosphere (NASA).

References:

  • https://ntrs.nasa.gov/api/citations/20210025383/downloads/Titan%20Sample%20Return_AIAA-SciTech_Finals%20(002).pdf
  • https://www.hou.usra.edu/meetings/lpsc2022/pdf/2626.pdf

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