This Thursday (18th Feb), if everything goes smoothly, NASA’s Perseverance Rover will touch down on the surface of Mars. The landing sequence required to get it safely to the surface is like something from science fiction. It is a complex landing sequence, made all the more difficult by the fact that real time communication with the rover is impossible given the distance and resulting 11 min time delay for a signal to reach it. This means that the entire landing sequence is automated. The sequence is known as the 7 Minutes of Terror by the NASA team.
Cruise Stage Separation and Entry
During the first stage of the landing sequence, the rover separates from the cruise stage. This is the part of the spacecraft that provided power and propulsion on the journey from Earth to Mars. Once separated, the spacecraft enters the atmosphere of Mars. Although the atmosphere is thinner than that of Earth, the friction will still cause a lot of heat so the rover is protected by a heat shield. It is expected the heatshield will reach temperatures of 1,300 degrees Celsius. In turn, this also slows down the craft.
Parachutes
Now travelling just under 1,600 km/h a huge 21.5m parachute is deployed to further slow the spacecraft down. At this point the heat shield is no longer required and is dropped. For the first time, the rover inside can see the surface. It then uses onboard radar and cameras to work out where it is and how far it is from the landing site.
Powered Descent
As stated earlier, the Marian atmosphere is much thinner than Earth’s. This means that parachutes are not as effective. Even the huge parachute sent with Perseverance can only slow it to about 320km/h. To make the final descent, the rover along with the descent stage is dropped free from the parachute. It then fires rockets on the descent stage to slow the rover down even further. The image below depicts the descent stage with the red rockets, which will fire, visible.
The Sky Crane
This is the really cool bit! The rockets on the descent stage slow the rover down to almost a hover (2.7 km/h). It then lowers the rover on cables down to the surface. Once the rover’s wheels sense it has touched the ground, a command is sent to cut the cable. With the cables cut, the descent stage flies off and crashes into Mars a safe distance from Perseverance. The following video shows an incredible animation of the landing.
Only about 40% of attempted Mars landings are successful. So although NASA has tested everything they can there is no guarantee that it will work. NASA TV will be providing coverage on Thursday (18th February) evening from 19.15 (Irish Time).
Paul Smith 