Rolling up to the crater's edge, the Opportunity rover took in a landscape unlike anything any Earthling had ever seen.
A vast, meteorite-blasted expanse of volcanic rock and iron oxide extended for 24km, ringed by rugged mountains under a dusky orange sky. In months to come, the enterprising robot would uncover signs that warm, liquid water had altered these ancient rocks - evidence that the conditions for life once existed on Mars.
"That view was one of the most spectacular things I've ever seen," recalled Ashley Stroupe, the engineer who was driving the spacecraft the day it arrived at Endeavour Crater on Mars in August 2011.
And although she was sitting a hundred million kilometres away, in Mission Control at the Jet Propulsion Laboratory in Pasadena, California, in that moment Stroupe felt like the astronaut she'd grown up always wanting to be. Opportunity had allowed her, and her fellow scientists, and her fellow humans, to experience another world.
Opportunity's historic mission, which uncovered signs of Mars' watery past and transformed our understanding of the Red Planet, has finally come to an end after 15 years, Nasa declared today.
The cause was system failure precipitated by power loss during a catastrophic, planetwide dust storm that engulfed the Mars rover last summer.
"It's going to be very sad to say goodbye," said John Callas, the mission's project manager. "But at the same time, we've got to remember this has been 15 years of incredible adventure."
Opportunity's mission was planned to last just 90 days, but it worked for 5000 Martian "sols" (which are about 39 minutes longer than an Earth day) and traversed more than 45 treacherous kilometres - two records for NASA.
"It will be a very long time," Callas predicted, "before any other mission exceeds that duration or distance on the surface of another world."
Before 2000, when Nasa announced its ambitious plan for the Mars Exploration Rover (MER) mission, just three spacecraft had ever successfully operated on the Red Planet. Of these, only one - the tiny Sojourner rover that accompanied the 1997 Pathfinder mission - moved around on the surface. It never traveled more than 100 meters and lasted less than thee months.
The images these travellers sent back were alien and bleak. Though scientists had speculated about the possibility of finding life on the Red Planet, initial investigations revealed a world with no liquid water, hardly any atmosphere and a lethal daily dose of radiation.
At that point, roughly two-thirds of all missions destined for Mars had failed, often in expensive and embarrassing ways. In 1999 alone, a unit conversion mix-up and a missing line of computer code doomed an orbiter and two landers, costing Nasa a combined US$200 million.
The agency's chief scientist, Ed Weiler, called the failures "a wake-up call." For years, Nasa had pursued a "better, faster, cheaper" exploration strategy, attempting to use a shrinking budget to send several small missions into space. But now critics began to question the merits of the Mars programme altogether. What could this desolate planet possibly teach us that would be worth the expense?
Nasa would take an US$800 million risk to find out.
Shortly after the crashes, Cornell University planetary scientist Steve Squyres got an unexpected phone call. He'd been trying to persuade Nasa to send a sophisticated robotic geologist to Mars for more than a decade. Now the agency wanted to know - could he have his idea ready to launch by 2003?
"We only had 34 months between when Nasa said, 'OK. Ready, set, go!' and when we had to be on top of the rockets in Florida," Squyres said. "People say to me, 'Oh my goodness, it's a miracle the rover lasted so long on Mars,' and I want to go, 'It's a miracle they got to the launchpad.'"
The new plan was to place a package of scientific instruments developed by Squyres and his colleagues atop two rovers called Spirit and Opportunity. The task of building these mobile robotic geologists turned out to be herculean. Dimensions changed, parachute tests failed, launches were delayed by bad weather and battery glitches.
Squyres recalled a sticky summer evening in 2003, after the scrubbing of yet another launch, when he took a walk on the beach near Cape Canaveral to clear his head.
To the East, he watched Mars - just a little red dot - rise over the glittering black Atlantic. It was hard to imagine how the rovers would ever get there, Squyres said. Mars seemed so forbidding, so alien, so impossibly far away.
Opportunity launched into space aboard a Delta II rocket on July 7, 2003, three weeks after its sibling, Spirit, took off. The cruise was uneventful, and seven months later, on January 25, 2004, Opportunity prepared to touch down in Mars' Meridiani Planum, a low-lying cratered expanse in Mars' southern hemisphere.
"I was in the control room" at JPL, Squyres recalled. He laughed, "Which, interestingly, is a place where we have no control whatsoever."
Because it took about 11 minutes for light signals to travel the roughly 160 million km from Mars to Earth, a spacecraft's "EDL" (entry, descent and landing) is over before scientists learn of it.
The logistics of the MER rover landings were formidable, bordering on absurd. Within six minutes of entering Mars' thin carbon dioxide atmosphere, the spacecraft had to slow from 19,310km/h to just about 0. Right before impact, a cocoon of air bags inflated around the spacecraft, allowing it to bounce safely onto the surface of the Red Planet.
For a moment, the spacecraft's radio link was lost as it shuddered to a standstill. And then a signal appeared on the computer screen in front of EDL manager Rob Manning. He flung out his arm and leaned back in his chair.
Planetary scientist Abigail Fraeman, then 16, had been invited out to JPL as part of a Planetary Society programme for high school students. She can still summon every detail of that night.
The tones that rang out as each system was found healthy. The images that Opportunity sent down from its landing site of a smooth dark plain so vivid and sharp she almost felt she could reach out and touch it. The surge of elation that swept through the science team as researchers realised what they'd landed on: layers of exposed bedrock that would reveal clues about Mars' geologic history stretching back billions of years.
"I realised I wanted to be one of those people who could jump up and down," Fraeman said. "I wanted to be someone who could understand the significance of what those images were telling us."
Fraeman wound up going to college for physics and geology, then earning her PhD in planetary geoscience. Since 2016, she has served as the deputy project scientist for the Opportunity mission.
Opportunity's first great achievement came within two months of its arrival on the Red Planet. The layered outcrop on which the rover had landed - the one that made the scientists surrounding Fraeman jump for joy - contained evidence that water once flowed through the rocks: crystals, sulfur compounds, little spherical objects that scientists likened to blueberries, and rock patterns that looked like sediments laid down by a flowing current.
This evidence constituted a "giant leap" toward determining whether Mars ever hosted life, Weiler told The Post.
That discovery was bolstered by scores more like it. Opportunity went on to find hematite, an iron mineral usually associated with water, and a vein of gypsum, which probably formed from mineral-rich water moving through rock.
"It really changed the way scientists perceive Mars," said Squyres, who has been principal investigator for the instruments aboard Spirit and Opportunity since the beginning of their mission. "It is a cold and desolate world today, but in the distant past, in the time that the rocks explored by Spirit and Opportunity were formed, it was a very different world. It was a world that was more Earthlike, a time when life was emerging on Earth."
"So it makes you seriously consider," he continued, "if it happened on Earth, which it did, could it have happened under the warmer, wetter conditions that once existed on Mars?"
Opportunity, he said, "couldn't answer that question. But we helped frame it."
Those discoveries helped build the case for subsequent missions to Mars, including the Curiosity rover, which landed in 2012 and is still exploring the Red Planet, and a 2020 mission that will collect rock samples for eventual return to Earth.
Opportunity's scientific accomplishments were only possible because it had been such an engineering success, said Nasa's acting director of planetary science Lori Glaze. The rover was adaptable, tenacious and diligent, and its drivers never failed to get it to its targets.
"Being able to really roll right up to an outcrop and examine it, to look up close with your hand lens, do the chemistry measurements . . . it allows you to really feel like you're there," she said. "That absolutely changed the way we go about doing planetary exploration."
The MER mission's cultural legacy is just as wide-reaching. The intrepid rovers, with their humanlike proportions and endearing, Wall-E-esque antics, proved phenomenal ambassadors for the Mars programme. Middle school science classes tracked the rovers' progress across the Martian landscape. A Twitter account shared selfies and snarky comments in the spacecraft's voice.
When Opportunity went silent last summer, more than 10,000 fans sent the spacecraft digital "postcards" wishing it well.
"Wake up little buddy!" one read. "We miss you!"
Even the scientists who operated the spacecraft couldn't help but anthropomorphise them. Stroupe, the JPL engineer, jokes that Spirit and Opportunity had "the dynamic of being rival siblings." Spirit, which landed on Mars first, faced much tougher terrain and suffered several breakdowns, culminating in the rover's eventual loss of contact in 2010.
As the "younger child," Stroupe said, "everything kind of came easy to Oppy." The engineer laughed. "I mean, she found signs of water before we even drove off the lander!"
The charmed rover barely escaped becoming trapped in a sand dune in 2005, survived a global dust storm in 2007, and undertook the longest-ever traverse performed by a rover - the three-year journey from its landing site at Victoria Crater to Endeavour Crater, 20km away.
"It's been a privilege," Stroupe said, "to see Mars through Opportunity's eyes."
She calls Spirit and Opportunity "the first Martians" - the first things to live and work longer on another planet than they ever did on Earth.
And as a systems and operations engineer for Nasa's Mars missions, responsible for driving robots across unforgiving alien terrain, "I do feel a bit like I have naturalised dual citizenship," Stroupe added.
A sticker in her office declares, "My other vehicle is on Mars." She uses an app on her phone to track the 24-hour, 39-minute Martian day. When she closes her eyes to sleep, rusty landscapes and dust-filled skies are the background to her dreams.
In May 2018, scientists at JPL received a worrying weather report from Nasa's Martian satellites: A large dust storm was brewing just a few hundred miles away from Opportunity, blocking out the solar-powered rover's view of the sun.
The spacecraft had survived such storms before. But at more than 14 years old, it was no longer as hardy as it had once been. A fault in one of Opportunity's memory banks resulted in loss of all long-term memory. Problems with the rover's wheels and robotic arm looked like spacecraft arthritis. If Opportunity experienced another prolonged power loss, it might not recover so easily.
By June, the dust storm had grown into a planet-encircling event, one of the most ferocious Nasa had ever seen. It looked likely that Opportunity would experience a low-power fault, putting itself to sleep until the skies cleared. Efforts to make contact with the spacecraft went unanswered.
When the storm finally began to subside, in September, Nasa adopted a "sweep and beep" strategy for waking the rover, sending commands multiple times per day. Except for a few false alarms from other spacecraft - Nasa's Mars Reconnaissance Orbiter transmits on a similar frequency - scientists heard nothing back.
Still, the team held out hope. If the storm had deposited dust on Opportunity's solar panels, the coming windy season - which runs from November to January - might help sweep them clean.
"The hardest part was the not knowing," Stroupe said. "It takes a real toll."
The robot's 15th birthday, on January 24, passed without so much as a ping from the Red Planet.
After sending more than 835 recovery commands to the spacecraft, including a last-ditch program that would completely reboot Opportunity's clock, hope began to dwindle. Every day that passed, Callas said, it became less likely that Nasa would ever get a response to its frantic calls.
The very last signal was sent from JPL yesterday. It was met with only silence.
"We've reached the end of the road," said Nasa's associate administrator for science, Thomas Zurbuchen. "We've exhausted all the good ideas [for waking the rover] . . . and now we declare the mission as being complete."
A meeting with the mission's scientists and engineers this week felt almost like a funeral, Zurbuchen said. Researchers cried not just for the death of their rover, but for the disintegration of a 15-year-old team.
Still, Squyres was resolute as the mission drew to a close.
"I always knew it was going to end," he said. "And boy, if this is the end . . . getting killed by one of the most ferocious storms we have ever seen. Well, you can walk away from that with your head held high."
The rover is survived at Mars by Curiosity, the InSight lander and six orbiting spacecraft. Nasa's next rover mission, which will seek out signs of ancient life, will launch in 2020.
As for Opportunity, its metal shell will remain in the spot where it sent its last message, on the rim of Endeavour Crater. "It's always going to be there," Zurbuchen said, "like a monument, or a shipwreck."
It is a marker of where humanity has been. And a beacon for whatever comes next.