There are ‘reasons to be confident’ about faulty Artemis II heat shield ahead of 25,000 mph reentry, space expert Ed Macaulay says
THE Artemis II The Orion spacecraft should splash around later in the day (April 10), as the world waits to welcome back the mission’s record-breaking crew. So far, the Artemis II lunar mission has been a huge success, but this final leg of the journey has some experts worried.
There is a problem with Artemis II heat shieldwhich aims to protect astronauts from the scorching heat of re-entry. NASA is confident that a modification to the re-entry pathway is sufficient to alleviate the heat shield issues observed during the Artemis I test flight and to keep astronauts safe today.
Patrick Pester: What is Orion’s heat shield and why is it important?
Ed Macaulay: The heat shield is an absolutely essential part of the Orion spacecraft. When the Orion spacecraft returns and re-enters Earth, it will be traveling at a speed of about 25,000 mph (40,000 km/h) – that’s just an incredibly fast speed. For context, at this speed he would cover the distance of the London Marathon [or the New York City Marathon] in about four seconds.
Because of this speed, the capsule will approach temperatures upon re-entry about half those of the Sun’s surface. [5,000 degrees Fahrenheit or 2,800 degrees Celsius]. The heat shield is essential to protect the capsule from this torrid heat of re-entry. Without it, the capsule would completely melt and burn.
PP: Can you explain why some experts are concerned about the heat shield?
EM: Some concerns have been raised regarding the heat shield, as Artemis II is a crewed version of the Artemis I mission without a crew which flew a few years ago. This was designed to be the exact same mission profile, but without any humans on board. When the Artemis I capsule returned to Earth, it successfully passed through the atmosphere safely, but the damage and effects on the heat shield were more severe than modeling predicted.
The heat shield is designed to burn from heat; it is not designed to remain completely intact and completely pristine. You can almost think of it a bit like the crumple zone of a car, but for heat. In a car, the crumple zone is designed to be compressed upon impact so that passengers are safe. The heat shield is designed to burn, fragment and release this heat.
What was surprising about the Artemis I heat shield was that parts of the heat shield had burned into large pieces. It wasn’t an ablation [gradually eroding] uniformly. This appears to be due to hot gases trapped in the heat shield. As they heat up, they expand, breaking off pieces of the heat shield and causing damage.
For Artemis II, NASA decided to keep the heat shield identical to that of Artemis I. Instead, it decided to modify the re-entry profile to place less stress on the heat shield. So, hopefully we’ll get even ablation across the heat shield and not let it burn in big chunks.
PP: Am I right that NASA is going to take a more direct approach rather than doing a skipped re-entry, which may have caused problems the first time around?
EM: In a nutshell, that’s exactly what they do. The reentry profile of the skip was designed in principle to facilitate the reentry of the capsule and the crew. Because you enter the atmosphere at 40,000 km/h and within a few minutes all that speed has to be evacuated, transformed into heat.
The idea with the re-entry jump profile is that before going straight back into the atmosphere, the capsule simply skims the surface of the atmosphere, burns off a bit of that speed, then pops back out, then returns later for full re-entry. In principle, this will put less pressure on the capsule in terms of temperature, in terms of g load. [gravitational force].
What seems to happen in practice, however, is that as the whole process takes longer, the gases trapped in the heat shield have more time to heat up, expand and cause damage. This is why, for Artemis II, the capsule will only opt for a direct reentry profile; the same re-entry profile used in the Apollo missions.
Hopefully the gases will have less time to cause damage. The other advantage of direct reentry is that it is simpler to model. The NASA team and all the engineers involved spent an enormous amount of time doing computer simulations of these re-entry profiles, trying to determine how hot the heat shield will heat up and what the damage will be.
With skipped re-entry, the whole process is more complicated. You’re trying to model how something will bounce and then come back. With direct entry, it’s simpler. I almost think of it like, the devil is better, you know.
PP: So, maybe a little more uncomfortable for the crew, but better for the heat shield?
EM: Yeah, I think that might be a compromise. And the Artemis II crew are all real pros when it comes to experiencing G-forces. So if you’re talking about 4G g-forces or something like that, they’re not even going to bat an eyelid. They train for much higher g loads. Pulling a few G’s for a few minutes, this isn’t going to be any drama at all for the crew of Artemis II.
PP: Would you be comfortable betting your life on this heat shield?
EM: The short answer, if you ask me, I wouldn’t do it. I think there is reason to be confident, because even with the Artemis I heat shield burning to bits, the crew would still have been safe if there had been humans inside. I think this shows that there is an element of safety margin in this heat shield. Even if things are slightly, as they say at NASA, “over design” – which is not entirely normal – there is some sort of reasonable safety margin in what the heat shield is capable of withstanding. That’s not quite the same as asking myself personally: Will I be next in Artemis II?
The Artemis II crew is clearly a very extraordinary group of astronauts. They all clearly have what it takes. And what they are doing on this mission is truly extraordinary. They are go so much further than humanity has gone in more than half a century.
I’m sure they will all have studied all these details because no one is more invested in this heat shield than them. And I’m sure they will all have confidence in the work done by all the scientists and engineers involved in the heat shield.
So far, the Artemis II mission has been an extraordinary success from a technical point of view. I think this gives reason to be confident about re-entry, because it appears there is every reason to expect the trajectory to be absolutely nominal, exactly what it was designed to be. And I hope that this will give them the best possible journey until the start of the school year.
PP: But you personally wouldn’t want to take that risk, or you wouldn’t want to be an astronaut in general?
EM: Personally, I’m a nervous traveler. And I think it’s easy to forget how risky human spaceflight is and how the risk is far greater than anything we would normally experience on a daily basis. Only a few hundred people have been to space. We haven’t even sent 1,000 people into space. And, very unfortunately, even with only a few hundred manned space missions, there have been some fatal missions.
PP: Is there anything else you would like to add as we wrap up?
EM: My personal view is that this mission has been an extraordinary success so far, for all sorts of reasons. From a technical perspective, the mission was an incredible success; the space launch system [rocket]the thrusters, the entry into that initial orbit, the translunar injection – the performance of the system has been incredible.
But even more, these four astronauts were absolutely incredible. Not only accomplishing their technical tasks, but also providing that human connection, that human perspective and bring the rest of planet Earth along for the ride.
Part of the reason I’m so excited is that it’s not like it’s all going to end when Artemis II comes home. This is just the beginning of a whole new chapter: NASA recently announced a very ambitious projects for this continued human presence on the Moon and exciting and concrete projects for the next Artemis missions. So this is just the beginning of a whole new chapter.
Editor’s note: This interview has been edited and condensed for clarity.
