The planet Mars has been shining brightly in the night sky for the better part of a year now, and a number of spacecraft have been speeding towards it most of that time. Among them, Mars 2020 mission, with plans to land the Perseverance rover on the Red Planet on 18th February 2021 shortly before 9pm GMT. Mars remains clearly visible in the evening sky, so when the Perseverance rover lands, you will be able to see the planet as a small but bright red dot in the sky between the crescent Moon and the Pleiades (Seven Sisters). You can follow the excitement live online. 

 

Mars as it will be seen in the south-western evening sky on 18th February 2021, with the Moon, Pleiades, Taurus, and Orion for reference. This is a screen capture from Stellarium, a free open-source planetarium software.

Missions to Mars

Earth and Mars have different orbital periods – a year on Mars is slightly less than two Earth years – so opposition – when Earth sits between the Sun and Mars in a straight line - occurs approximately every 26 monthsThe best time to send missions to Mars is just before opposition because the planets are close together, and the journey lasts for about 7 months. 

There are therefore often several missions launched to the Red Planet at once. In the summer of 2020, NASA (USA) launched the Mars 2020 mission (with Perseverance and Ingenuity on board) on 30th July 2020, the UAE launched the Emirates Mars Mission and China launched Tianwen-1 mission. The latter two have already successfully arrived to Mars on 9th and 10th February 2021, respectively 

Illustration of the route Mars 2020 mission has taken to the Red Planet, including several trajectory correction manoeuvres (TCMs) to adjust its flight path (Credit: NASA/JPL_Caltech).

 

How to land on Mars 

Even in this day and age, landing on Mars remains very difficult. Apart from the Soviet Mars 3 lander in 1971, which was the first successful lander on the Red Planet, only NASA missions have ever successfully deployed a handful of landers (Viking 1, Viking 2, Pathfinder, Phoenix, InSight) and rovers (Sojourner, Spirit, Opportunity, Curiosity), and about half of all landing attempts have been unsuccessful. ESA’s Schiaparelli lander crashed as recently as 2016. 

There are two main causes for thatThe first one is the time delay: because the distance to Mars is quite large and the time from atmospheric entry to landing is very short, all the processes have to be automated, because it is impossible for anyone on Earth to receive information and respond to it in time to avoid potential disaster. The time delay between Earth and Mars varies with their relative separation, and signal has to travel one way between 3 and 23 minutes. This is not fast enough for live response from the Earth. The second causis the thin atmosphere. Mars does have an atmosphere, but it is less than 1% the density of Earth’s. On Earth, re-entry vehicles make great use of ‘aerobraking’; the drag of the atmosphere to slow thedown – using a heat shield to prevent damage to important parts – and then deploy parachutes to slow down even further and land quite softly on the ground or in the ocean. Both aerobraking and parachutes are used on Mars, too, but they don’t slow their cargo enough for guaranteed safe landing. Therefore, more complex manoeuvres are required, and they do not always succeed.  

Mars Reconnaissance Orbiter’s HiRISE camera captured Curiosity rover during EDL on 6th August 2012: rover gliding on its parachute seen at the top, the discarded heat shield at the bottom (Credit: NASA/JPL-Caltech/Univ. of Arizona). 

Seven minutes of terror 

During the landing on 18th February 2021, the signal from the Mars 2020 mission will need more than 11 minutes to reach Earth from Mars. The atmospheric entry, descent, and landing (EDL for short) takes about 7 minutes altogether, so by the time NASA receives news that the mission has begun this procedure, the rover will have already been on the ground for over 4 minutes. This is why the EDL procedure is often called the seven minutes of terror’; when they receive that first signal, the scientists and engineers can only sit back and watch, and hope the automated procedures they designed have work as planned. 

The EDL procedure starts with aerobraking and continues with deployment of a supersonic parachute for further slowing downThroughout, thrusters are adjusting the course towards the landing site: Jezero CraterThe heat shield is discarded, radar equipment is used to adjust the course even more accurately.  

 

Curiosity rover landing area with all parts of the original spacecraft, imaged by HiRISE camera on board the NASA Mars Reconnaissance Orbiter on 17th August 2012 (Credit: NASA/JPL-Caltech/Univ. of Arizona/USGS).

When the parachute has done its job, the back shell with it separates from the rest, leaving a powered descent stage – sometimes called a ‘sky crane’ – which is simply a scaffolding with rocketon the sides (like a rather oversized jetpack), with the rover cradled within it on a winch. The landing can’t be completed in that configuration, because the rockets would raise too much material from the surface and likely damage the rover. The sky crane instead descends until it safely hovers 15-20 metres (50-66 ft) above the ground. It then gently lowers the rover to the surface, after which it flies away from the landing site with its remaining fuel. Touchdown! Only a day before, the whole mission was still hurtling through space at mind-boggling 77,000 km/h (48,000 mph)!  

While we will not be able to see this live, due to the time delay and slow data speeds between Mars and Earth, the Mars 2020 mission carries 25 cameras with it – the most ever used in deep-space exploration. 19 of them are on the Perseverance rover, three on the back shell looking at the parachute, one on the sky crane looking down at the rover, and two on the Ingenuity helicopter. Many of these will be operational during landing, and will send us spectacular footage from landing in due time. Five Mars orbiters will also be tracking the progress of the EDL procedure and help send data to Earth faster than Mars 2020 mission could achieve on its own. The precision of the landing will also be better than ever before, with the area in which the rover is expected to land, called the ‘landing ellipse’, being ten times smaller than it was for Curiosity rover back in 2012. 

Perseverance rover's Entry, Descent and Landing (EDL) profile (Credit: NASA/JPL-Caltech).

 

The mission of Perseverance 

Comparisons are often made between the Perseverance and Curiosity rovers. This is because Perseverance, while not quite its twin, is very much Curiosity’s younger, slightly larger, and more sophisticated brother: they look quite similar at first glance, but Perseverance is equipped for the new decade of Mars exploration with many different and more sophisticated instruments. 

Perseverance rover with its seven main instruments (Credit: NASA/JPL-Caltech). 

The main scientific goals of all NASA missions to Mars in the last couple of decades are to help determine whether Mars could ever have supported life, as well as determining the role of water, and to study the climate and geology of Mars. Additionally, the missions are meant to pave way for human exploration of Mars. Curiosity has been investigating whether Mars could have been habitable. Perseverance, however, will take the next step by looking for the signs of past life itself. To help with future human exploration, Perseverance will attempt to synthesise oxygen from Mars’s predominantly carbon dioxide atmosphere, using the MOXIE instrument suite (Mars Oxygen In-Situ Resource Utilization Experiment). It will also find and collect promising samples of Mars rock and soil that could be brought back to Earth in the future by a sample-return mission. The rover – like Curiosity – is powered by a plutonium power generator. Its primary mission will last for one Martian year, but it is hoped it will – like rovers that came before it – greatly exceed expectations. 

Perseverance will land in Jezero crater, named thus because it is – much like Gale crater where Curiosity is continuing its research – an ancient lakebed. Jezero is named after a lakeside town in Bosnia and Herzegovina, and the name means ‘lake’ in a number of Slavic languages. There is therefore great hope of finding interesting geological formations and perhaps even evidence of ancient life, if there is any to be found.

Artist's impression of NASA's Perseverance rover operating on the surface of Mars (Credit: NASA/JPL-Caltech)

Ingenious Ingenuity - a helicopter on Mars 

One of the most exciting parts of the Mars 2020 mission is the inclusion of a little autonomous helicopter. Its name is Ingenuity, it weighs only 1.8 kg (4.0 lb), and it has few scientific instruments. It will, however, attempt to make the first ever powered flight in an atmosphere outside Earth’s; a mighty feat! If all goes well, it will fly five times, up to 90 seconds, some 3–5 metres (10–16 ft) above the ground, and as far as 50 metres (160 ft) away from the starting position and then back again. Though flights will be autonomous due to the time delay, they will be planned ahead from Earth. Mars’ thin atmosphere makes this a very tricky proposition, so it is possible things will not go as planned. Any success will be built upon to design future aircraft that could scout ahead of rovers and help track the best paths for them, or perhaps help collect rock samples left by missions like Perseverance for delivery back to Earth. 

Artist's impression of Ingenuity in flight, with Perseverance monitoring its progress (Credit: NASA/JPL-Caltech).


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