# How will Bennu's volatile compounds stay cool and unmodified by the heat of re-entry (OSIRIS-REx)?

Watching the video in this NYTimes article NASA Aims at an Asteroid Holding Clues to the Solar System’s Roots I realized for the first time how the samples actually make it back to Earth.

The chemical composition of the organic and volatile compounds is of great interest for this mission for many reasons. So I would think that it is important to avoid thermally induced chemical reactions, migration, and redistribution of the compounds within the sample matrix. If it gets hotter than it does while on Bennu, the science may be compromised.

note: According to the text in this excellent Asteroid Missions animation near the end, the re-entry velocity will start at about 12.4 km/sec. See also OSIRIS-REx Press Kit.

Return and analyze a sample of pristine carbonaceous asteroid regolith in an amount sufficient to study the nature, history, and distribution of its constituent minerals and organic material.

Is the re-entry kept shallow to avoid peak external temperatures, or is it deep and rapid to "get it over with" before heat can diffuse through (presumably) a lot of insulation?

Screen shots from the NYTimes video:

• Bear in mind that the asteroid is revolving, with its surface in direct sunlight for half of its 4.3 hour rotation period. Depending on latitude, the collected surface sample may have already cycled many times between -100 and +100 degrees C. Commented Sep 8, 2016 at 23:25
• @RussellBorogove yep, thus the cautious wording "If it gets hotter than it does while on Bennu, the science may be compromised." Ballparking: $T^4 = TSI_{max} / \sigma \epsilon$ where $TSI_{max} = 1368/ 0.9AU^2 \ W/m^2$, $\epsilon \approx 0.96$ and $\sigma \approx 5.67x10^{-8} \ W/m^2K^4$ gives about 420K or 150C, but material down a few cm may never get nearly that hot since the packing will be very loose in low gravity making thermal conductivity quite low. Since temperatures associated with reentry are of order 3000K for say 100 sec, the capsule is quite a feat of engineering!
– uhoh
Commented Sep 9, 2016 at 1:20

The sample return capsule is designed to keep its contents below 75 °C. It is a flight-proven design, reusing technology developed for the Stardust mission.

The return capsule’s structure consists of a graphite-epoxy material covered with a Thermal Protection System making use of NASA’s PICA heat shield technology – Phenolic-Impregnated Carbon Ablator.

PICA is a lightweight material designed to withstand high temperatures and mechanical stress. The technology was developed at NASA’s Ames Research Center and currently is in its third generation after undergoing gradual improvements. SpaceX modified the PICA technology for the company’s Dragon and Dragon 2 spacecraft, enabling re-use of heat shields.

The principle behind ablative heat shield technology is to create a boundary layer between the shield’s outer wall and the extremely hot shock layer gas by allowing the heat shield material to slowly burn away and, in the process, generate gaseous reaction products that flow out of the heat shield and keep the shock layer at a separation distance, reducing the overall heat flux experienced by the outer shell of the spacecraft.

The processes occurring at the heat shield material include a charring, melting and sublimation on the one hand and pyrolysis on the other.

Pyrolysis creates the product gases that are blowing outward and create the desired blockage of convective and catalytic heat flux. Radiative heat flux is reduced by introducing carbon compounds into the boundary layer gas which make it optically opaque.

The backshell of the SRC is also covered with thermal protection material, but since it resides in the wake of the hot gas flow, it will not need as much protection as the forward facing side. Its Thermal Protection System is comprised of a cork-based material known as SLA 561V, originally developed for the Viking missions to Mars in the 1970s and in use on a number of missions including the Mars Pathfinder, Genesis and MER rover missions. Also facilitated within the backshell is the parachute mechanism with secure attach points for the parachutes to the capsule.

The Sample Canister itself is an aluminum enclosure mounted on a composite equipment deck and residing between the backshell and heat shield. The overall performance of the SRC will maintain the sample at a temperature below 75°C in order to avoid pyrolysis (the thermochemical decomposition) of organic material at higher temperatures.

• OK! It's a lot bigger than I imagined, so there is more room for insulation. I added an answer below with some images from your link which are helpful. If you want to include them in your answer, I'll delete it later. I think the key point is in these two sentences - you might consider highlighting them: "Pyrolysis creates the product gases that are blowing outward and create the desired blockage of convective and catalytic heat flux. Radiative heat flux is reduced by introducing carbon compounds into the boundary layer gas which make it optically opaque."
– uhoh
Commented Sep 8, 2016 at 12:36

This is to supplement the accepted answer by @Hobbes, and this information is from this link from that answer.

The return capsule is a lot larger than I imagined, 81cm diameter, 50cm tall, so there is plenty of room for insulation.

According to the OSIRIS-REx website run by NASA the heat shield on the sample return capsule removes over 99% of the initial kinetic energy:

That should be enough to prevent any chemical/biological changes and preserve the regolith for further scientific study.

• Thanks, but have you calculated how big the other 1% is in Joules? "That should be enough" seems to be an opinion. Can you find something quantitative you can link to?
– uhoh
Commented Sep 8, 2016 at 4:36
• Agreed, it is an opinion. The issue is really dependent on the type of insulation used to hold the samples. I may delete the answer as insufficient Commented Sep 8, 2016 at 6:46
• You could also say something like "OK I'll see what more I can find out" no need to delete. The heat shield interacts with the atmosphere, and together they might get rid of 99% of the kinetic energy. If that thing weights 10kg for example, then the 1% is still 7.7 MegaJoules. The heat shield and the plasma in contact with it (and radiating back to it) are going to be really really hot for say 100 sec. I think the interesting thing is the sample is within centimeters of something almost white hot - how does it stay cool, and how cool does it actually stay? It must be amazing insulation!
– uhoh
Commented Sep 8, 2016 at 7:19
• The rest should be in the parachute mostly. Commented Sep 8, 2016 at 12:01
• @Andy one does not "get rid" of energy. The heat shield's interaction with the atmosphere transfers most of the initial kinetic energy of the shuttle to kinetic energy of the gas molecules (in the form of heat (and also some ionization)) but it doesn't just 'go away; some of that is transferred right back to the capsule through contact and through radiation. Even 0.1% transferred would be enough to modify the sample and reduce its usefulness. There's a lot of science in these two sentences!
– uhoh
Commented Sep 11, 2016 at 1:59

In the same way re-entering astronauts remain cool and unmodified, by using a heat shield.

• All the other answers already mentioned heat shields, so can you elaborate on this more? Commented Sep 10, 2016 at 18:00
• It is called humour, there is a good article on wikipedia about it. Commented Sep 10, 2016 at 18:46
• On Stack Exchange we delete humor (and everything else) that doesn't help the goal of answering questions. This isn't a site to show off sparkling wit or engage in interesting discussions; it's just questions and their answers. Commented Sep 10, 2016 at 20:16
• Heat shields are wonderful, but they are not "magic". You can't just say there's a heat shield, so everything is OK and therefore the sample will stay cold. Scientists have agreed to a max acceptable temperature transient of something like +75C for the sample, which is good because the sample will not be wearing a spacesuit.
– uhoh
Commented Sep 11, 2016 at 3:20
• I think it's possible to have a well written answer that is also humorous. Commented Sep 11, 2016 at 18:20