'Tango Delta nominal' first words of safe Curiosity landing
Washington: There had been a debate amongst Curiosity’s entry, descent and landing team about what their first words to indicate that the rover had reached the surface should be.
The EDL team at NASA’s Jet Propulsion Laboratory knew their microphones would be “hot” and that NASA TV was beaming the landing event out live to anybody with the desire and wherewithal to watch.
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.
“If we said ‘touchdown,’ then people not intimately familiar with EDL might infer that Curiosity was good to go,” said Steve Sell, one of the engineers.
“But two more major calls had to be made before I could begin to breathe again,” said the 42- year-old engineer from Gettysburg, Pa.
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 that 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,” he stated.
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.
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.