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The Holy Grail of Planetary Science


After travelling for over 160 million kilometres in space, the OSIRIS-Rex spacecraft whizzed by Earth on the 24th of September to release a sample return capsule. When the lawn-mower-sized capsule hit the atmosphere, most of the energy was dissipated through its heat shield before a large parachute gently lowered it down in the Utah desert in central USA. Inside, over 250 grams of surface material from asteroid Bennu (1999 RQ36) were preserved in pristine condition; a considerable quantity for such a mission.


Sample return missions (SRMs) are not new of course, but they are rare. My last book covered over a hundred missions which sent spacecraft across the solar system to explore in situ most of the planetary bodies that populate it, and while every mission is outstanding and significant in many ways, they all suffer from budgetary, environmental, temporal, and technological limitations. Desk-size instruments of great sensibility such as gas chromatography need to be made smaller than a briefcase (e.g., the Gas Chromatograph Mass Spectrometer (GCMS) on the Viking missions) and imaging instruments get knocked out of place by grains of space rocks or ice (e.g., Voyager 2 mission at Saturn). Add to that budgetary constraints, and suddenly, the scientific instruments exploring these remote places all come with their own set of compromises and disadvantages.


This is why SRMs are crucial in improving our understanding of the solar system and beyond. These missions will return material from a planetary body which can then be analyzed with the least amount of constraints (most sensitive equipment, best laboratories, controlled conditions, etc.) and most importantly, get stored for future studies. However, the complexities inherent to such missions make them the most challenging, which is why they are the last type of missions to take place when studying a planetary body. As a reminder, the sequence of robotic exploration of a planetary body goes like this:


  • Flybys: We first send a spacecraft to whizz by the object and try to collect as many scientific measurements as possible during the short timeframe allowed. The first Mariner missions at Mars and Venus or New Horizons at Pluto are good examples.

  • Orbiters: These missions are more complex as they often involve sending a spacecraft on an orbital trajectory with all the things that can go wrong during this process and, once in orbit of an object, they will require a multi-year approach.

  • Landers/Rovers: This is where things get really interesting. After scientists believe they have well characterized a surface, they will feel confident in sending in landers and rovers to explore it in more detail (such as Philae on comet 67P or the Mars rovers).

  • Sample return missions: The hardest of them all. These missions need to capture material without contaminating it and be sent back on a trajectory back to Earth. When done well, SRMs bring the best science return out of every dollar invested.

The first robotic sample return mission to the Moon was the Soviet Luna 16 mission in 1970 (this is where, for once, humans won over robots as the Apollo astronauts brought back samples in 1969). To date though, we’ve only managed to bring back to Earth stuff from the local environment: the Moon, the near-Earth asteroids Itokawa and Ryugu thanks to the Japanese Hayabusa missions, Comet Wild 2 from NASA’s Stardust mission, and now Bennu.


As technology improves and the cost of accessing space gets reduced, more SRMs will be considered and proposed such as the NASA-ESA-led Mars Sample-Return Mission. The holy grail for me would be to bring back samples of material venting off the plumes of Enceladus. Let me know what type of SRM would you like to see.

As always, onwards and upwards

Bernard

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