Every 26 months, when Mars and Earth align themselves properly, a launch window opens to allow an efficient path to the red planet. Three spacecraft are currently on their way to arrive in February of next year.
https://mars.nasa.gov/imgs/mars2020/spacecraft/Mars_Perseverance_Trajectory.jpg
One of these is the first interplanetary spacecraft from the Arab world, the United Arab Emirates (UAE) Hope spacecraft. If all goes well, it will orbit Mars and observe its global weather patterns over a full Martian year, 687 days.
The other two, China’s Tianwen-1 and NASA’s Mars 2020—both of these aim to land rovers on the surface. It is China’s first attempt to do so, a very bold attempt to send an orbiter, a lander, and a rover all in one mission, all on the first attempt. NASA is an old hand at this, having achieved the first fully successful Mars landing with the twin Viking spacecraft in 1976. (The Soviet Union landed a craft on Mars in 1971, but communications were lost less than fifteen seconds after a partial image started transmitting.)
There have been fifteen attempts to land a spacecraft on Mars, eight of them successful. The Soviet Union and the European Space Agency failed in all of their tries (four for the USSR, two for the ESA); the U.S. had one failure out of its nine attempts.
Why is it so hard? It mostly has to do with that darn atmosphere.
https://i.pinimg.com/originals/cf/68/98/cf6898138fbbb59a976bee169779c1bd.jpg
Ever wonder why the Apollo lunar lander was so ungainly looking, not sleek and aerodynamic like all those lovely science fiction crafts?
https://www.discoverspace.org/wp-content/uploads/2016/07/lem.jpg
It’s because it never had to fly in an atmosphere. The airless moon simplified matters considerably; with no atmosphere there was no need to protect the lander from the friction of atmospheric entry.
Mars’s atmosphere is considerably less dense than Earth’s, but it still must be taken into account. It is useful in some ways, as it can be used to slow a spacecraft down from thousands of miles per hour speeds to ones that can allow a soft landing. The various methods used to deliver spacecraft safely to the surface have involved some straightforward techniques and some that seem to have been the fever dreams of rocket nerds.
Mars’s low gravity is something of an advantage, as objects fall more slowly than on Earth. You might think that a parachute would allow one to gently float down, but once again, the insubstantial nature of that atmosphere thwarts you. A parachute can definitely slow you down, but not enough. The two Viking landers slowed to a descent rate of 136 miles per hour with their parachutes, then jettisoned them and started firing retro-rockets at about a mile above the surface. Touchdown occurred at a speed of about 5 mph.
NASA had a long dry spell of twenty years between those Viking landings and their next Mars mission, Mars Pathfinder. And for that mission, they pioneered a new landing technique that worked so well it was repeated for the Spirit and Opportunity rovers that landed in 2004. This video explains the sequence of events.
Did I mention that all of this has to occur out of real-time contact with Earth? The limitations of light travel time mean any signals sent between the two planets take several minutes to transit. All of these operations have to be carried out autonomously.
The next generation of NASA Mars rovers, the Curiosity rover still roaming Gale Crater and the Perseverance rover slated for Jezero Crater, are much larger and heavier than any before them (almost a ton for Curiosity, more than that for Perseverance). The airbag method simply would not work, and NASA engineers must have summoned the spirit of Rube Goldberg for what they came up with next.
But it worked! We can hope it does so again next February.
The Chinese Mars rover is lighter (530 pounds), and will use the tried and true parachute and retro-rocket method.
I’ll leave you with this historic image—the first color picture from the surface of another planet. July 21, 1976: noon on Mars.
https://www.jpl.nasa.gov/spaceimages/images/largesize/PIA00563_hires.jpg
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