On 30th July 2020, an Atlas V-541 rocket left the pad at Complex 41 in Cape Canaveral Air Force Base in Florida. On-board was @NASAPersevere – a rover destined for Mars. Since then, Perseverance (or Percy) has been on its way to the Red Planet #CountdownToMars (1/x)

All going well, at 11.15 am PST on Thursday 18th (that’s 8.15 am on 19th for those of us in NZ) @NASA will start livestreaming the landing broadcast. You can follow along on YouTube #CountdownToMars (2/x)

If you’d prefer to use an alternative channel, there’s a long list of options here: https://mars.nasa.gov/mars2020/timeline/landing/watch-online/ (yay @NASA for making space exploration so accessible!) #CountdownToMars (3/x)

The team behind @NASAPersevere also produced an amazing interactive animation that walks you through the various steps @PercyRover will have to take on its descent to the surface https://eyes.nasa.gov/apps/mars2020/#/home (4/x)

There are so many thing to get excited about, but I want to focus on one specific aspect of the descent, because it is related to a topic I cover in my upcoming book STICKY: the aeroshell, or really, its heatshield #stickybook #CountdownToMars (5/x)

First off, there’s a reason why the rover is packed into in a shell shaped a little like a @hersheys kiss – and that reason is drag. (All Perseverance images @nasajpl. Hershey’s image by Famartin, CC-BY-SA-4.0) (6/x) #CountdownToMars

The lack of air in space means that spacecraft face no air resistance (or drag) on their journeys, which is why they can reach such unimaginably fast speeds. But once a spacecraft hits a planetary atmosphere, air starts to pay attention to it (7/x)

Engineers use this drag to their advantage – it slows down the spacecraft so that it can land safely, rather than slam into the ground. The Apollo capsules of the 1960s & 70s hit speeds in excess of Mach 30 (30x the speed of sound) when they started their descent to Earth (8/x)

The blunt shape of @NASAPersevere’s aeroshell maximises drag, to slow it down as efficiently as possible. As it punches thru the Martian atmosphere, a bow wave will form in front of the curved heat shield, which keeps very hottest packets of gas away from the surface (9/x)

But friction between the air and heatshield will still drive temperatures way up, to somewhere around 1300 °C. The material the heatshield is made from has a job to do – to keep that heat away from Percy (10/x) #CountdownToMars

The material used on @NASAPersevere’s heatshield is called phenolic impregnated carbon ablator (PICA) which was first developed by @NASAAmes in the late 1990s. It’s gone on to feature on lots of missions (including Stardust and @spaceX) (11/x)

While the heatshield will take the brunt of the heat load, the aeroshell body will still get hot. So it has its own protective layer, SLA-561V, which was used on the Space Shuttle, Mars Pathfinder, and Viking landers, to name just a few (12/x) #CountdownToMars

But heat isn’t the only worry. Dr Priyanka Dhopade @drpriaero told me that at re-entry temperatures, air can "...no longer be treated as an ideal gas, with a simple relationship between pressure, temperature &density. At hypersonic speeds, its chemical composition changes.”(13/x)

As air breaks down, electrons get knocked out of atoms – the air around the spacecraft will turn into a bright plasma that blocks any communications and can react with its surface. Flying ultra-fast is as much about chemistry as physics! #7minutesofterror #CountdownToMars (14/x)

The materials used on the surface of @NASAPersevere’s heatshield (PICA) and aeroshell (SLA-561V) have been design to withstand these chemical and thermal attacks, but that’s not to say this will be a risk-free descent. Landing on Mars is very challenging (15/x)

Once Percy, all snug in its aeroshell, reaches an altitude of 11 km above the Martian surface, it should be travelling at a speed of about 1520 km/h. To slow down beyond this, it’ll need its parachute #CountdownToMars (16/x)

All going well, about 20 s after the huge parachute deploys, the heat shield will be ejected, having carried out its job as expected. Percy will feel the Martian air around its wheels for the first time, as sensors scan the surface to prepare for landing (17/x)

I think that’s where I’ll leave our rover. Fly safe little @PercyRover. I will be cheering you on, not just through the ‘7 mins of terror’ of descent but also in your mission beyond. Can't wait to see what you find #CountdownToMars (17/x)

Don’t forget, you can livestream the landing on YouTube. If timezone calculations are your nemesis, you can click the “Set reminder” button and it’ll do the mental maths for you  #CountdownToMars (18/x)

* A brief aside: I also interviewed @MartiniTribos for #stickybook. She and her team collaborated with @NASAJPL to develop and test a dry film lubricant (based on molybdenum disulfide, MoS2) for use on @NASAPersevere. Lubricants are fascinating! https://news.ucmerced.edu/news/2020/uc-merced-grad-student-alumnus-play-significant-role-mars-2020-mission

<Shameless > If you liked #CountdownToMars you might enjoy my upcoming book, STICKY. I talk about supersonic & hypersonic flight in Chapter 4. There are lots of other interesting topics in it too (e.g. ice, geckos, quakes, paint). Threads on those to come throughout 2021

Sticky will be published by @sigmascience in November. The cover design hasn’t yet been finalised, but hopefully it’ll be a good’un. If you’re interested in book updates (e.g. pre-order links, the aforementioned etc.), feel free to follow me #stickybook #CountdownToMars