The details of the entry, descent and landing, almost preposterous on the surface, make it no surprise that many years of planning, calculations and building led up to the daring touchdown of Curiosity. The 14 minute transmission delay between Mars and Earth meant the landing would meet its fate before the signal arrived back at Earth. It inspired a viral movie JPL dubbed “7 Minutes of Terror.”
But to any follower of computing technology and especially to a computing or information professional, the data now flowing back and forth between Mars and Earth is a telling study in IT engineering minimalism using technology as simple as could be practical to do many things very reliably.
Ann Devereaux was deputy lead for entry, descent and landing (EDL) stage of Curiosity’s mission. She also built the Electra-Lite radios that are on the Mars Science Laboratory (MSL) and Mars Reconnaissance Orbiter (MRO), a project she started in 2004. Her job up through EDL was to make sure that computers would be doing what they were supposed to and that all the data interfaces on the rover and orbiters would be working.
Image courtesy FlordaToday and Tom Walters
With EDL accomplished she’s turning to tactical duties with the same mission equipment that took Curiosity to Mars, a slim stack of working gear that all but the earliest Internet users would find quaint. The central processor aboard the MRO, an IBM PowerPC 750 hardened to deflect radiation, clocks at 133Mhz and comes from a generation of chips introduced in the 1990s. The code footprint is 20 MB with 128 MB of total execution space. The entire file storage for telemetry and science data storage is 4.5 GB.
Put another way, the computing that powers NASA’s most sophisticated space exploration mission ever is pretty much the dead opposite of the technology you’d use to make a Hollywood movie about space exploration.
“When people saw the data volumes we process and the memory space they’d inevitably ask, ‘You're taking this to Mars?’” Devereaux says. “Why isn’t the latest gear? My memory stick has more memory than your whole computer.”
It Has to Last
As Devereaux explains, NASA’s big design constraint is finding parts that are “space qualified,” a term that figuratively translates to “bulletproof.” Everything on board including the radios has some computing inside just as an iPhone does, but there is no getting around the use of older technologies for a few reasons.
For starters, she began building mission components eight years ago, which predetermined the lineage. And, newer electronic circuit designs have some drawbacks in space. Before microcircuits were commonplace, components ran at higher voltages and were less sensitive to radiation bombardment and extremes of temperature. For military or NASA use, manufacturers spend a lot of time and money to be approved as space qualified; it takes a long history and a big database of performance reliability.
“You need to have super controlled processes to know parts are still going to work in 10 years,” says Devereaux. “If you are Dell or Apple, and 10 percent of a chip run fails, you just throw it on the floor. And if one out of 10,000 iPads goes bad because of a memory chip or a video driver, you just replace it.” NASA, she confirms, has no service centers on the Red Planet.
Like the CPU and memory, the bandwidth of data coming from the rover is modest and arrives from two transmitters. One points directly to Earth and operates at about half the speed of a home telephone modem. The other handles UHF data relays between Curiosity, the orbiters and JPL in Pasadena, and is Devereaux’s mission focus.
The rover’s UHF transmitter sends data to either the Mars Reconnaissance Orbiter or Odyssey Orbiter, an older space craft with an older radio. All the transmissions have to take place in 15 minute windows three or four times a day depending on geometry “when the orbiter goes screaming overhead,” in Devereaux’s words.
The orbiters act like cell towers with more power and storage than the rover and a longer look at Earth’s horizon. “They suck up whatever data Curiosity can give them every day and when have a view they can dump all the data back to JPL at once,” says Devereaux. “It makes for a very chunky sort of day where you plan everything around when you uplink orders to Curiosity to do stuff during the day and when you downlink and assess all the work you did that day and how it went.”
Even the 15-minute windows are unreliable with signal strength rising as the orbiter moves overhead and falling as it regains the horizon, says Devereaux. “We get a lot of junky links until the orbiter is straight above, so on the newer Reconnaissance Orbiter we use adaptive data rates where the orbiter monitors the signal strength and can tell the rover to speed up or slow down its transmission.” The aim is to get the absolute most out of 15 minutes, and NASA is starting very slow in order to optimize and capture the most important information cleanly.