Space Sunday: ‘Perseverance will get you anywhere’

A CGI model of the Mars 2020 rover Perseverance on the surface of Mars.  Credit; NASA

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.

Mars 2020 Entry, Decent and Landing (EDL).  Credit; NASA

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).

What TRN means for landing accuracy: superimposed over Jezero Crater, the white ellipses representing the potential landing sites for various missions. The outermost is that of Mars Pathfinder (1998) and reflects the lack of detailed data available on the proposed landing site for that mission. By 2012, and the MSL rover Curiosity, engineers had more then enough data to target a substantially smaller area for landing. Thanks to TRN, this could be reduced still further for Mars 2020 (note the InSight lander (2018) has a large landing ellipse because the amount of data available on the regions around the north and south poles of Mars is not as extensive as is the case with latitudes moving towards the planet’s equator. Credit: NASA

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.

Seconds from touchdown: this remarkable image was captured by a camera mounted on the Mars 2020 skycrane. It shows the Perseverance rover with wheels deployed and other systems (Mastcam camera systems, robot arm) still stowed, as the rover is winched away in preparation for delivery onto the surface of Mars on February 18th, 2021. The bin-like section of the rover, top right, is the shielded housing for its plutonium nuclear “battery” power source. Credit: NASA/JPL

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.

Not only was NASA’s MRO vehicle performing the role of active communications relay during the Mars 2020 landing, it was actually observing the landing using its phenomenal HiRISE camera system, which actually caught Mars 2020 suspended under its parachute as it drifts towards and inflow delta within Jezero Crater (see on the left side on the main image). Credit NASA/JPL

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.

Telemetry confirming touchdown was received with intense jubilation at the partially-staffed mission operations centre at NASA’s Jet Propulsion Laboratory in Pasadena, California (due to COVID restrictions, although many of the mission team were working remotely – a challenge in itself). Shortly after, the rover transmitted its first image from Mars.

Blurry, low-resolution and in black-and-white, it was captured by one of the vehicle’s HazCams – Hazard avoidance Cameras with the protective transparent lens cap still in place. Whilst of low quality – as noted, the intent of the images was to confirm a safe landing, little more – the image was also received with delight.  It’s worth pointing out that the HazCams on Perseverance have RGB channels, and data from these channels can be combined to give colour pictures of the images they take.

The first low-resolution image returned by Perseverance was captured by on of the rover’s six HazCams and was taken with the clear lens cap still in place. The aim of the photo was simply to show the rover was firmly on the ground. Credit: NASA/JPL

Since arriving on Mars, Perseverance has been going through a series of initial checks and other activities that will prepare the way for operations to commence. These are likely to include calibration checks of its assorted camera systems, etc., which in the case of Curiosity took some time to complete.

One of the early tests was a call to wake up the Ingenuity drone helicopter, is stored within a protective cover under the belly of the rover.

With the lens cap removed, the HazCams have been able to take more detailed shots from under the rover, with engineers using data the camera system’s RGB channels to create colour composites. This one one of the first such images to be shown, forming part of a post-landing briefing held on February 19th, 2021. Credit: NASA/JPL

Essentially a cube averaging around 15 cm on a side and with four landing legs that will extend below it when deployed, and topped by two contra-rotating rotor blades, Ingenuity is a proof-of-concept craft intended to see of flying vehicles can be made use of on Mars in support of other missions.

Once deployed – most likely some time in the next 30 days – it will operate independently of Perseverance and make around 5 short flights of up to 90 seconds duration, each to a height of between 3 and 5 metres and covering a distance of up to 50 metres. These flights will both assess the use of rotary aircraft on Mars and their ability to assist surface operations – in this case scouting the landing around the rover for points of potential scientific interest and also possible hazards the rover cannot directly see.

Following the “wake up” call, that confirmed system both on the helicopter and its communications base station contained within the rover, a command was sent for the helicopter to start charging its batteries to 30% capacity. This process will be assessed, and if the batteries are found to be functioning correctly, a series of partial charges will commence a series of charge cycles intended to give it sufficient power to heat itself and keep its systems running. Prior to deployment, the batteries will be fully charged ready for the first flight, after which the helicopter will be reliant on solar power to recharge them.

The Ingenuity Helicopter. Credit: NASA/JPL

While Perseverance – or “Percy” as some at NASA refer to it – looks to be identical to Curiosity, it is in fact a very different vehicle with a far more ambitious science mission. As such, it carries a range of systems that have been enhanced since Curiosity was built, and also a completely new suite of instruments.

The core science mission is to determine whether there was past life on Mars. To achieve this, the rover will  – somewhat similarly to Curiosity – gather samples and analyse them, although in difference to Curiosity, it will looking for direct evidence of past microbial life on Mars. In addition, Perseverance will collect samples that it will seal in special containers and deposit on the surface of Mars, which will – it is hoped – be collected by a follow-on sample / return mission that will bring them back to Earth. This latter mission, which currently appears to overly complicated, involving both a further US and a European mission, has yet to receive any official go-ahead or funding, so is unlikely to take place prior to the early 2030s.

Percy’s science payload. Credit: NASA

In addition, Perseverance will carry out tests for what is called in-situ resource utilisation (ISRU) – a fancy way of saying “using local resources to make what you need”. In this case, it involves trying to create measurable amounts of oxygen from the carbon-dioxide rich Martian atmosphere. If  successful, the experiment, called MOXIE, could pave the way for more ambitious ISRU tests – including using the Martian atmosphere to create water, which in turn could be split into oxygen and methane fuel stocks – vital elements for future human missions to Mars.

In terms of its search  of evidence of past microbial life on Mars, Perseverance has landed in what is possibly the most ideal accessible location on Mars for doing so: Jezero Crater.

While it has been fairly well established that Mars went through a period when liquid water existed on its surface, including many craters (as with Gale Crater, Curiosity’s home) exhibiting clear signs of having once been flooded, Jezero Crater is one of the few that shows it was once part of an active water system. To one side the crater wall is marked by an inflow channel, with an outflow channel cut into the wall almost directly opposite it.

A image of Jezero Crater overlaid with graphics showing how it may have appeared during the warm, wet period of Mars’ history, showing the path of water into the cater and on outwards into the Isidis Planitia. Credit NASA/JPL

This means the 49 km diameter crater is a primary location for minerals and clays suitable for kick-starting and support life to be deposited, and analyses from orbit have revealed just that, with smectite clays being much in evidence. Further, the floor of the crater shows an extensive inflow delta below the channel where water once entered it, which likely took between 1 and 10 million years to form – presenting plenty of time for life to get started, with the growing lake forming a natural protective environment as the waters increased.

Jezero had originally been under consideration as a potential landing site for the MSL Curiosity mission, before being turned over in favour of Gale Crater. So instead, Perseverance will explore now explore it in stages, with the initial focus for the primary mission being the inflow delta, the rover having landed well inside it. After this, an extended mission will see the rover move further afield.

This part of the mission will start with the selection of a “depot caching” location where the samples that are to (hopefully) be returned to Earth can all be placed, starting with those the rover collected whilst investigating the delta.

The Jezero Crater inflow delta. Credit: NASA

For now however, the focus is on getting the rover ready to start its primary mission, and NASA will be hosting an update on progress at 14:00 EST (11:00 PST, 19:00 UTC) on NASA Television, including a full recording of the landing as seen by the rover’s cameras, and which can be watched on NASA Live and You Tube, among other streaming options.

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