Space, Spaceflight

Why is Mars so hard to land on?

This summer is chock-full of exciting missions to Mars, with China’s Tianwen-1 and the UAE’s Hope orbiter already en route to Mars, and NASA’s Mars 2020 set to launch on July 30th. ESA was also meant to launch its Rosalind Franklin Exomars rover, but that was delayed to 2022 earlier this year. I’ll talk more about these and why this summer next week!

Mars has seen a great resurgence in missions lately, as the technology to get there is proving ever-more reliable, and more countries are funding space research (yay!)

Landing Sites via Planetary Society. Mars 2020= Perseverance. Exomars= Rosalind Franklin. Unmarked Turquoise= Tianwen. White= already landed. Turquoise= Planned/Launched

But missions haven’t always been successful: Out of the 39 missions to launch before 2000, only 14 were completely successful, with both NASA and Roscosmos having a string of launch failures and many landers or rovers failing to land, or lost communication. 

Launch failures are just a fact of spaceflight, and get less frequent as the rockets we use get tried, tested and improved. There’s only been one launch failure since 1971, so most occurred back in the early days of the space age! Landing failures have also become less common, as we have engineered more ways to overcome any challenges the red planet can throw at us! 

So what are these challenges?

Mars Facts | All About Mars – NASA's Mars Exploration Program
via NASA

Mars is small- really small- which means two things. One: it’s gravity is just 38% of Earth’s, so molecules don’t have to be going as fast to escape- the general rule (according to the astrophysics/exoplanets course I took)- is that if the average velocity of a molecule is greater than ¼ the escape velocity, then it will slowly escape to space! 

Two: It’s originally liquid outer core has cooled and is now -at least partially- solid. A liquid outer core is what powers a planet’s magnetic field, as it is made of metal (a mix of nickel and iron usually, with some siderophillic elements that bond with iron). The core will still have some residual heat, so the inner edge of the outer core will be hotter than the boundary with the mantle, causing convection. Metals have free electrons (are charged), and moving charge can induce a magnetic field. A magnetic field protects the atmosphere and surface from radiation, so without it, the atmosphere is at the mercy of the solar wind, so can be slowly stripped away. 

This has left Mars with a very thin atmosphere, 100 times thinner than the earth’s, and has a surface pressure of just 610 Pa (earth’s is 101,000 Pa). But an atmosphere is rather useful, not just for astrobiology but also for exploration. If you’ve ever seen a shooting star, or a Soyuz capsule returning to earth, you’ll know that it gets engulfed in a ball of ‘fire’, making it glow. It’s not actually fire, but the air in front/beneath the entering object that has been compressed, as the object is travelling so quickly that there is no time for the air to move out the way. Compressed air heats up and becomes ionised plasma, which emits light! This is both a blessing and a curse. On the one hand it makes returning to earth from space quite dangerous, so we have to make heat shields to protect the astronauts inside, but it also reduces the number and size of asteroids hitting the surface of the earth! Additionally, the air molecules hitting the object entering slows it down. When a parachute is ejected, once it has partially slowed down enough that it won’t immediately be ripped away, there is a larger surface area for air molecules to impact on, slowing it down even more!

But on mars, there are less air molecules, so it has less of an effect. To combat this, landers enter at a shallower angle (to travel through more air before reaching the surface), might try to enter during summer (when the CO2 ice caps sublimate and thicken the atmosphere), and use heat shields, large/strong parachutes and retrorockets (thrusters pointing towards the surface to produce a force against the direction of motion). We go to all this effort to reduce the speed (and thus force) of impact, so that all of the precious instrumentation (and in the future, astronauts) remains intact.

But you may now be wondering- the moon doesn’t have any atmosphere… why can we land on the moon fairly easily? Well Mars’ atmosphere is still thick enough to heat the lander up, so it has to be covered with a thick heat shield, which makes it heavier, but moon landers don’t have to have one so can be made of much lighter material, so can be easily slowed down by firing rockets beneath it. The heavier the probe, the more force the rockets have to create to slow it down, so Mars landers would have to have incredibly powerful thrusters if that was all they relied on.

Communications issues are more unpredictable, and there is little you can do to protect it. If the landing is slightly off, the equipment might break or move about and make communication impossible, or loss of power might mean there is no energy to power up the communication device. Many rovers or landers in the past have used solar panels, which can stop working if covered by dust- and believe me, dust is everywhere on mars. Dust storms can cover huge swaths of land, and coat the solar panels with a layer of it, rendering them useless. This is probably what happened to Opportunity, which landed on Mars in 2004 and was operational until a dust storm that covered the whole planet in 2018 obstructed its solar panels or knocked it over.

wo 2001 images from the Mars Orbiter Camera on NASA's Mars Global Surveyor orbiter show a dramatic change in the planet's appearance when haze raised by dust-storm activity in the south became globally distributed.
Global Dust Storms like this one from 2001 can last for months!

There are ways to overcome this, for example Curiosity landed on August 6th 2012 (so it’s coming up to its 8th birthday!) and is powered by the decay of radioactive isotopes (in this case Plutonium-238), so isn’t affected by dust storms. Well, if the wind was strong enough then it could be knocked over, but it weighs nearly 900kg, and even though the wind on Mars can reach up to 60 mph (still only half of hurricane-speed winds on earth), the pressure is so low that even the top wind speed produces only a very small force, so we don’t really need to worry! 

NASA to Broadcast Mars 2020 Perseverance Launch, Prelaunch ...
The Perseverance Rover ready to go! via NASA

So, here’s hoping that Perseverance will have a successful launch tomorrow! You can watch it on NASA TV or NASA’s youtube, with coverage starting from 12 for launch at 12:50pm UK time! Make sure to watch, but if you can’t I will be covering it on my Instagram stories and twitter

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