Data comes in three streams: telemetry to report temperature, power and other onboard conditions; computer process reporting (of the type you'd see in a DOS or UNIX computer booting); and the actual science data, a pipeline that will grow only slowly.
It is slow enough that NASA measures bandwidth in bits rather than bytes (8 bits = 1 byte). Curiosity can transmit up to two megabits per second to Mars Reconnaissance Orbiter; the older Odyssey radio can only handle 256 kilobits, one-eighth as much. Right now the downlinks reported at press conferences are on the order of 10s of megabits of data a couple of times per day.
“We are operating the vehicle very cautiously and we want to see the health data first,” says the scientist. “There was a lot of data transmission to reconstruct from entry, descent and landing and that had the highest priority by far.”
Transmission rates are partly a function of power, and Curiosity’s nuclear battery puts out a steady 100 watts, just a fraction of what’s needed to run a blow dryer. So, solar panels charge other batteries, but every day’s planning has to carefully account for activity.
“If we are going to run the [robotic] arm, god forbid, we have to calculate the power it takes to heat the mechanisms that move it, to use an instrument the arm is providing samples for plot all this out,” Devereaux says. “When you get to a certain battery state you have to stop and recharge and recalculate.”
All the variables that made the successful beginning of the Curiosity mission so remarkable are based on sound engineering, science and mathematics. It could not have been done with wholesale stacking and layering of infrastructure as happens in mainstream IT and data management.
“Compared to most IT development you have to program in very clever ways to work on Mars. The best result is that you end up with just the key crisp points of data.” Devereaux compares her task to an email that might read, “Hi, hope you are well, I wanted to tell you X,Y and Z and remind you of a couple of things we are working on…”
”All we get to say is, ‘Hi’ and we’re done,” Devereaux laughs. Her analogy is that anyone can generate large amounts of numbers and load them to a server farm, “but it takes a clever person to do Sudoku where everything is tiny and has to fit in boxes.”
NASA’s mission is also to inspire the public to future scientific exploration and it prioritized activities to be able to deliver that first grainy shot of Curiosity’s wheels on the Martian surface almost immediately. It was a bit of fortunate timing that also lifted the whole project team.
“It was funny because we’d tested entry, descent and landing hundreds of time in the test bed and dozens of times in dress rehearsals where the computer says everything is working,” says the scientist. “We knew it was real but it was a weird experience to see the data you’ve looked at dozens of times and then you see that picture and it’s really sitting on Mars and it hits home.”
Devereaux, third from left, celebrates at JPL.
Now, she says, an impatient consumer audience is expecting streaming video, but like everything else, only in due time. “That video stream would be kind of a killer app that people want to do, but the storage and bandwidth will dictate when it’s possible. Plus, I’ve got to tell you, the surface of Mars doesn’t change that much so the rover drivers might go crazy for it but you might not see much new.”
Also, compressed video can’t give scientists the accuracy they want. It might be fine “for watching ‘Battleship’ on NetFlix,” but scientists want lossless compression because they want to see every shadow and detail that uses the sun to estimate position and reveal textures.
The audience engagement part she gets personally. “I will tell you when we were doing EDL and there was this pause and blank screens until the UHF radio kicked in once we’d gotten rid of the parachute that was covering the antennas. Someone called out, ‘We’re getting Odyssey back,' which meant UHF data through Odyssey to the ground, all the screens started populating and I jumped up and screamed oh yeah, Electra! This is what we waited eight years for from the time we built this radio, the first time we’d used it and seen it work. The whole team felt this way.”