With any luck, it all happens tomorrow!! Perseverance and its little stowaway helicopter, Ingenuity, land at Jezero Crater, on the surface of Mars!
I know a lot of people are discussing this right now, but after my post a week or so ago discussing the orbital mechanics of a Mars mission, and why launch windows turn out to be what they are, I felt like I had to summarize what’s about to happen. Not that there’s any pressure – a week ago, both the Hope Orbiter (UAE) and Tianwen-1 Orbiter (China) successfully entered orbit around the red planet. Perseverance is the last to arrive, and will undoubtedly be the most dramatic, as its headed directly for a landing at Jezero. (Tianwen-1 also has a lander, but it won’t make the attempt for a few months yet. The Chinese lander is content to orbit and check the place out for a while, first.)
Mars is not an easy spot to land. It’s got exactly the wrong amount of atmosphere – too thick to ignore completely, as you would landing on the Moon; too thin to rely on for traditional aerodynamic braking methods or aerodynamic control all the way to touchdown. As a result, landings on Mars are typically exciting – faster than desired, or overly complex (see below, and Curiosity 2012), or literally bouncing over the surface in a giant airbag (Sojourner, 1996 / Spirit and Opportunity, 2004). It’s hard enough that more missions to Mars have failed than have succeeded. Tomorrow’s landing (3:55pm Eastern, 12:55pm Pacific) of Perseverance will put the largest, most complex, most capable lander we’ve ever had on Mars’ surface, AND position Ingenuity to make the first controlled flight in an alien atmosphere.
Communication delays between Mars and Earth further complicate things. By the time we know anything happened, the news is roughly twenty minutes old. So Perseverance’s Entry, Descent and Landing (EDL) phase will have to be completely automated. It takes about 410 seconds between first encounter with the Martian atmosphere and safe touchdown – hence “Seven Minutes of Terror”. In between, we’ll just hear an occasional series of tones indicating that different stages of the complicated procedure are complete.
So what has to happen? The helpful graphic here from NASA/Jet Propulsion Laboratory helps, but let me break it down some.
First, the lander separates from its cruise vehicle. Perseverance has covered ~300 million miles travelling in excess of 50,000 miles per hour, and has been attached to a spacecraft that has allowed it to make various trajectory adjustments to line up in space and time just right to hit the desired touchdown zone. Ten minutes prior to entering the atmosphere, the lander module will detach and leave its interplanetary cruise module behind.
The lander is sandwiched between a heat shield on the Mars side, and a backshell on the space side, and looks like a classic flying saucer at this point. Entering the atmosphere, the heat shield will start to warm from friction with the thickening Martian sky, eventually reaching “peak heating” and a temperature of 2300 deg F (1300 deg C) eighty seconds after first encounter with the atmosphere.
Small thrusters on the backshell fire in response to atmospheric turbulence and buffet during this stage, in an effort to keep the lander oriented properly and on-course to its intended touchdown area.
Atmospheric drag will slow the lander dramatically, but not enough. At about 7 miles above the surface, and having slowed to just under 1,000 mph, the craft will deploy the largest supersonic parachute ever used – the ‘chute is 70.5 feet in diameter (21.5m) and will deploy at around 940mph, or Mach 1.75.
Once safely under the parachute, and having slowed to Mach 0.7, the heat shield will be jettisoned and allowed to fall to the surface. This will finally reveal the lander itself, and open up the view of its onboard radar system, which can start scanning the surface and zeroing in on the landing site.
As the craft continues to slow and descend, it begins using its radar-vision to perform Terrain Relative Navigation (TRN), which is a landing technique new to Perseverance. This allows the lander to correlate radar imagery of the surface to an onboard database, and make precise adjustments to navigate toward a very specific spot. The landing environment in Jezero crater is rocky, with several slopes, and near a significant cliff face – but the science behind this spot, in an ancient riverbed, makes it worth the attempt to navigate to this tricky location. TRN makes that possible.
Finally, at about 7,000ft altitude, at 200mph, and about one minute before touchdown, the Skycrane maneuver begins. The Skycrane rides on top of the lander, and is fitted with fuel, rockets, and a guidance system that take the lander to its final touchdown. The backshell, including the parachute, is jettisoned, and the retro-rockets enable the Skycrane-with-lander to make a powered descent while navigating to the final touchdown zone.
At about 70 feet off the ground, the Skycrane lowers the lander on a set of tether cables, so that Perseverance hangs below the crane. Slowly, gently, the Skycrane will set Perseverance on the surface. As soon as the cables go slack, Perseverance is down, the cables are cut, and the Skycrane powers up to fly up and away to deliberately crash-land somewhere far away from the lander.
Having described all this, I invite you to go back and read through this again, but think of all the things that could go wrong. A guidance problem, a radar problem, a chute deployment problem, an alignment problem that causes the wrong angle of entry into the atmosphere, a thruster that fires a tad late, any number of computer or electrical glitches. These things have been tested over and over, and every possible failure and redundancy has been considered. And despite all that, a room full of engineers and scientists at NASA’s Jet Propulsion Laboratory have to let Perseverance do all this, completely on its own, and hope that all their efforts have been successful, knowing that they are powerless to intervene.
Only after seven minutes – and then a twenty-minute communication delay – will we know whether the landing is successful, or a failure. I know I’ll be watching!
Get Out There