CAESAR’s primary goal: Return to Earth a pristine sample from a comet’s nucleus to help scientists unlock the origins of planets and life.
Some of the best scientific studies involving the Apollo moon rocks are being done right now, almost half a century after they were returned to Earth. Returning samples from planetary bodies allows scientists to study them using any and all techniques available, not just with the limited instruments that can be brought on a spacecraft. Also, instead of being constrained by the relatively brief lifespan of a spacecraft, returned samples can be studied for decades to come, with instruments not yet invented. Like the moon rocks, a sample from a comet would be a gift to the whole scientific community and its legacy would extend far beyond the completion of CAESAR’s mission.
Comets are thought to be the building blocks of planets and a significant source of the Earth’s water and organic molecules. They are time capsules that preserve a record of the earliest formation of solid objects around the newly born Sun, having formed and spent most of their existence in the frigid calm of the outermost solar system. The CAESAR mission would be the first to return a sample from the surface of a comet to Earth.
While in orbit around Comet 67P near its farthest distance from th e Sun, CAESAR is being designed to use its robotic arm and custom-made sampling device to collect a sample of the comet in a brief touch-and-go maneuver. Simulated tests of this device and maneuver are being conducted. In these tests, as touch down occurs, a spring-loaded section of the arm compresses to protect the arm while keeping the sampling instrument in contact with the surface. In the few seconds that the instrument is in contact with the surface, ripper tines based on agricultural technology break up the simulated material, which mimics the expected consistency of sandy material on the comet that is rich in water and organic compounds. Finally, pressurized gas jets force the sample into a container through one-way doors.
Once the sample is stowed safely aboard the spacecraft, a process can then begin to keep it in pristine condition throughout its long journey home and landing here on Earth.
In the first step of this planned process, a valve opens to a cooled gas reservoir and the containment system slowly warms the sample to approximately the temperature it would reach at its closest distance to the Sun, which is still well below the freezing temperature of water. This warming releases water vapor, simple organics and other gases from the sample as they would on the comet when forming a tail. Next, these volatile compounds escape into the vacuum of the gas reservoir, which is several times larger than the sample container. Sensors are in place to indicate when the flow of volatiles stops and would then seal the gas reservoir off from the sample container where the remaining solid materials continues to be stored. The container with the solid materials can then be vented into the vacuum of space through the opening of another valve, which would remain open until shortly before re-entry into Earth’s atmosphere. The design of the gas reservoir includes a radiator to keep it cold while the vacuum of space keeps the solid sample cold.
The planned separation of the gas and solid portions of the sample ensures that as the spacecraft inevitably warms on its return to Earth—closer to the Sun than the comet has ever been—the gases do not remain in contact with the solids. In space, when warming releases volatiles from the comet, they are free to leave the surface. If the sample is kept in one enclosed space, contact between the gases and solids would result in significant chemical changes in the sample, ruining its pristine nature. By venting the solids to space after the gas transfer is complete, we ensure that any additional release of gases do not remain in contact with the sample.
This system is the engineering feat that would allow CAESAR to preserve and return both the volatile substances, like water and simple organic compounds, as well as the non-volatile solids, like silicates and more complex organic compounds.
The final steps in this process begin when the spacecraft arrives back at Earth. During re-entry into Earth’s atmosphere, when temperatures spike, the containment system is designed to protect the entire sample with a substance that has a melting point colder than the freezing point of water. As heat from re-entry and conditions on the surface of Earth seep into this material, it would absorb that heat by melting and not let it pass through to the sample. This process keeps the sample below freezing for hours, giving the recovery team plenty of time to locate and retrieve it.
Questions to be Tested
Does 67P contain signatures from the birth of the solar system?
Did comets deliver water to Earth?
Is 67P a remnant of a large planetesimal?
Does 67P contain organics that may have contributed to life on Earth?
The CAESAR Science Team intends to preserve and protect the returned sample to maximize science return. For sample extraction, preliminary examination, and long-term preservation, a new dedicated addition to the Johnson Space Center’s Curation Facility is planned. This would allow investigations to continue for decades after the sample is returned to Earth. Scientists could then analyze the sample materials by looking at mineralogy at an atomic scale, comparing isotopic ratios, and taking sensitive organic and inorganic chemical measurements to investigate the origin and history of Comet 67P and the solar system as a whole.
From the 2018 Lunar Planetary Science Conference.