Adam Steltzner and another Curiosity team member celebrate the successful landing. Credit: NASA/Bill Ingalls.
I admit, I’ve lost count of how many times I’ve watched the great “trailer” JPL put out showing the events in the mission control room during the Entry, Descent and Landing of the Curiosity rover, which also simultaneously provides animations of what took place on Mars. Watching it usually evokes either 1.) a fist pump, or 2.) choking up.
But if you’ve seen the video, or if you were watching live the night of the landing, just what were all those acronyms and phrases being thrown around and what did they mean? And what was the ever-pacing Adam Steltzner (EDL team leader) hearing in his headphones that made him stop, turn around and point just before spacecraft communicator Allen Chen said, “Touchdown confirmed!” … wherein bedlam ensued?
JPL has put out some info about what all was going on behind the scenes. It turns out the words “UHF Strong,” were highly anticipated by the team, plus there was a bit of acronym trickery employed by the team just so there wasn’t any doubt among them what was going on.
There had been a debate among Curiosity’s ELD team about what their first words to indicate that the rover had reached the surface should be. They knew their microphones would be “hot” and that NASA TV was beaming the landing event out live to everyone watching.*
DC Agle from JPL tells the story:
But they also knew that landing safely on Mars meant more than simply landing on Mars – which any one of the 34 engineers present at JPL’s Building 264 Room 230, also known as the “EDL War Room,” will tell you at great length is not simple at all. Their rocket-propelled backpack and rover-lowering Sky Crane system were getting their first all-up test 154 million miles (248 million kilometers) away from home, and there was still plenty that could go wrong even after the rover was gently placed on the surface… plenty.
What if the descent stage kept descending right on top of the rover? What if the bridles connecting the two did not separate? What if the algorithm used to throttle up the engines for the flyaway maneuver was not accurate?
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It was the remaining “what ifs” that made what those first words from Mars confirming the rover was on the surface so important.
“If we said ‘touchdown,’ then people not intimately familiar with EDL might infer that Curiosity was good to go,” said engineer Steve Sell. “But two more major calls had to be made before I could begin to breathe again.”
At 10:31:45 p.m. PDT, Jody Davis saw the event record, or EVR, she was looking for appear on her computer screen in the EDL War Room. She knew that the “Touchdown” EVR would only be beamed down if the rover’s descent stage had throttled down — a result which could only occur if the descent stage had offloaded half its weight. The only way the rover could offload half its weight in an instant is if it were being held up from below.
Davis, a member of the EDL team and an engineer from NASA Langley Research Center in Virginia, gave the much reviewed, pre-scripted call — “Tango Delta nominal.”
Tango and Delta are phonetic identifiers for T and D, which the team used to represent touchdown.
One down, two to go, thought Sell. The next call the EDL team was looking for was “RIMU Stable.”
“RIMU stands for Rover Inertial Measurement Unit,” said Sell. “The RIMU gives us the rover’s orientation as well as any movement it is making. If we landed on a crumbling crater wall or an unstable sand dune, or were being dragged by a still-connected descent stage across the surface, then the RIMU would show that in its data set.”
The War Room’s David Way, an engineer from JPL, was monitoring that unit’s performance. Eight long seconds after Jody’s call, he found the EVR he was looking for.
“RIMU Stable,” said Way.
One more crucial milestone to go.
This image shows Curiosity’s Entry, Descent and Landing (EDL) “war room” and its staff. On the night of Aug. 5, 2012 PDT (early morning Aug. 6 EDT), 34 engineers gathered in this room at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., to support the landing. Image credit: NASA/JPL-Caltech
Not receiving that one final call would be something of a long shot to be sure. After all, the rover was down on the ground, and RIMU indicated it wasn’t moving. Their system had been proven every step of the way so far. But everyone in the EDL War Room got as far as they did not only because they were excellent engineers, but because of their predilection for concocting unappetizing entry, descent and landing scenarios – and then figuring out how to elude them. And one ton of fuel-laden, rocket-firing descent stage climbing straight up, only to fall right back down on their factory-fresh landing site and an otherwise good-to-go, roving Mars laboratory was about as unappetizing a scenario as Sell could imagine.
That final confirmation would not come from Sell’s location. The final confirmation that Curiosity had landed clean would come 200 yards and one building away from the EDL War Room. There, in the Mission Support Area of JPL’s Building 230, Adam Steltzner, the mission’s EDL phase lead, was staring across the room at Brian Schwartz, who was not making eye contact with anyone. Schwartz, the EDL communications engineer, was staring at his screen. His task was not to check for a good-news EVR from the rover. Instead, he was waiting to see if the UHF signal became intermittent, faded away or just cut out altogether – all potential indications that the rover and descent stage had not gone their separate ways.
Eight seconds after the RIMU call – Schwartz looked up.
“UHF strong,” said Schwartz.
With that, Steltzner had all the data he needed. Seated directly in front of the pacing EDL Phase Lead, Allen Chen felt a jab in the shoulder. Chen, the mission’s (capsule communicator), knew it could only mean one thing.
“Touchdown confirmed,” said Chen.
Bedlam and joy.
Mars Science Laboratory (MSL) Entry, Descent and Landing Engineer Adam Steltzner reacts after the Curiosity rover successfully landed on Mars on Sunday, Aug. 5, 2012. Photo Credit: (NASA/Bill Ingalls)
Via Twitter, Steltner told Universe Today that when you see him in the video holding up four fingers, he was actually “counting the seconds waiting for UHF confirmation from Brian Schwartz. I think my fingers were me counting down,” he said.
How did the JPL team know what was happening on Mars? — (there was a 13.8 minute radio delay because of the distance between Earth and Mars).
MSL sent out different tones for each event that happened, and 128 distinct tones indicated when steps in the process were activated; one sound indicated the parachute deployed, while another signaled that the vehicle was in powered flight, and still another that the Sky Crane had been activated. These sounds were a series of basic, special individual radio tones.
They were simple tones, transmitted in X-band, comparable to semaphore codes, rather than full telemetry. The Deep Space Network listened for these direct-to-Earth transmissions. However, Earth went out of view of the spacecraft, “setting” below the Martian horizon, partway through the descent, so the X-band tones were not available for confirming the final steps in descent and landing. By then, the bent-pipe relay of telemetry via the Odyssey spacecraft had begun.
Also listening was the Mars Express spacecraft. It recorded about 20 minutes of the rover’s transmissions and tones. ESA engineers have now put together an audio reproduction, compressing the 20 minutes into about 19 seconds of audio that humans can hear that are a “faithful reproduction of the ‘sound’ of the NASA mission’s arrival at Mars and its seven-minute plunge to the Red Planet’s surface,” the European Space Agency team wrote. You can listen to the audio here.
And if you need to watch the landing trailer one more time, here it is:
*Estimates are that at least 3.2 million people were watching online on JPL’s UStream feed. Universe Today’s Live Hangout on Air Virtual Landing Party had a total of 30,000 viewers, with a peak of 7,000 concurrent viewers. This CNET article says that at the peak, 500,000 people were simultaneously watching the live landing on NASA’s HDTV, JPL, and JPL 2 broadcasts via Ustream. While numbers like this aren’t available for TV, Mashable cites research firm Nielsen’s ratings for the night — CNN had 426,000 viewers, MSNBC had 365,000, and Fox came in the highest with 803,000 — which overall have lower numbers than Ustream.