NASA once again has more than one rover operating on the surface of Mars. On Thursday, February 18th, the Mars 2020 mission, comprising the rover Perseverance and the aerial technology demonstrator Ingenuity, arrived in Jezero Crater in the northern hemisphere of the red planet.
The landing followed the same profile as that of NASA’s other operational rover, Curiosity, which arrived on Mars as the physical element of the Mars Science Laboratory (MSL) mission in August 2012, and which is still exploring Aoelis Mons, the huge mound at the centre of Gale Crater, although there were some notable differences.
Referred to as “the seven minutes of terror”, the landing involved the rover and its helicopter payload and landing system packed within an aerodynamic aeroshell, slamming into the upper reaches of the tenuous Martian atmosphere at 20,000 km/h, then the rover and payload touching gently down on Mars on the end of a winch just seven minutes later.
Some ten minutes prior to atmospheric entry, the mission had separated from its supporting cruise stage – the component that that provided it with power, heat and communications with Earth. Small reaction control thrusters on the aeroshell fired shortly after, slowing the spin induced to assist with stability during the 3.4 million km cruise out from Earth so that it would interfere with the vehicle’s passage through the atmosphere.
Protected by the heat shield that formed the lower part of its aeroshell, Mars 2020 passed through the searing heat of atmospheric entry, the friction of its passage helping to decelerate it. From here on in, things happened fairly rapidly.
Just under five minutes from touchdown, the vehicle used programmed control checks to align itself onto a course towards its intended landing site and entered what NASA call the “straighten up and fly right” manoeuvre – jettisoning a final group of balance masses whilst using its aerodynamic shape to steady itself on course ready for parachute deployment. This occurred with the craft just 20.8 km up-range of its landing site and still travelling at more than 2,000 km/h – or supersonic speed.
With the parachute deployed, the heat shield could be jettisoned, exposing the rover vehicle and its instruments to Mars for the first time. This meant camera and radar systems could start operating (as could the on-board microphones), and the craft could enter an entirely new mode of robotic landing.
Given the distance between Earth and Mars, two-way communications are impossible, so Martian landing have to be programmed in advance and triggered triggered by events such as velocity, atmospheric pressure, elapsed time, etc., but without any means to deviate from programming in any way. However, Mars 2020 was equipped with Terrain Relative Navigation (TRN).
This essentially took readings of the ground below and ahead of the craft as it descended under its parachute, comparing the findings with high-resolution terrain maps of the landing site and surroundings. If it noted any potential hazard, it would cause the vehicle to use its thrusters to steer itself away from the hazard whilst maintaining its overall heading towards the landing site. TRN also allowed the vehicle to identity any obstructions within its target landing area and feed the data necessary to avoid them to the rover’s skycrane system that would handle the final part of the landing.
Weighing around a tonne, Perseverance, like Curiosity before it, is too heavy to rely solely on parachutes to make a landing. Instead, both rovers relied upon a jet-powered “backpack” – the skycrane. This, with the rover strapped underneath it, fell clear of the backshell and parachute just 1.6 km above the surface of Mars. Once safely clear of the backshell, rock motors on the skycrane fired, reducing the rate of descent from around 360 km/h to just 3 km/h whilst also flying the rover directly over the ideal landing point.
Entering a hover some 21.5 metres above the landing site, the skycrane held steady as it released the rover on a winch mechanism and lowered it towards the ground. This triggered the rover’s wheels, which had been folded stowed against its body, to deploy and lock themselves into their operational position. With the rover at the extent of the cables, the skycrane eased it down to deliver it to the surface.
Once the rover was able to confirm it was firmly on Mars – a matter of a second or so using sensors in its wheel mechanisms – it sent a message up the wire to the skycrane telling it to detach. This it did before carefully piloting itself away along a course that prevented the rocket motor exhausts washing over the rover and possibly damaging / contaminating it, before crashing into the surface of Mars.
The entire EDL – Entry, Decent and Landing – phase of the mission had been watched over by three of the craft currently in orbit around Mars. The first of these was the Mars Reconnaissance Orbiter (MRO – now approaching 15 years of continuous operations in Mars orbit) that was specifically tasked to act as both observer and communications relay. Also recording the event was NASA’s MAVEN spacecraft – it would transmit the data it received some time after the landing had been completed, whilst ESA’s Mars Express orbiter (currently the longest-running operational Mars orbital mission, with 17 years under its belt in Mars orbit) acting as a back-up relay.
In addition, it had been hoped that NASA’s InSight Lander, although over 2,000 km from Jezero Crater, might be able to hear the sonic booms of Mars 2020’s passage through the Martian atmosphere. However, at the time of writing, I’m not sure if this was successful.
Continue reading “Space Sunday: ‘Perseverance will get you anywhere’”
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