no room for error with pluto -- 10/30/19
Today's selection -- from Chasing New Horizons by Alan Stern and David Grinspoon. When NASA sent a spacecraft to Pluto, there was no room for error or after the fact corrections. So NASA did its software testing the correct, thorough way:
"As leader of the mission, Alan Stern felt as though it was part of his job to look for weaknesses in the flyby plan, ask a lot of questions of his teams, probe their assumptions, and ask for changes to fortify the planning. One of the many weaknesses he spotted and changes he asked for concerned NHOPS (New Horizons Operations Simulator).
"About the time that the seven various flyby phases were being laid out and architected, Alan became concerned that the NHOPS spacecraft simulator, which was used to test all spacecraft command sequences to weed out bugs, could become a showstopper if it failed in 2015. He just wasn't comfortable with the fact that an NHOPS failure in 2015, when there was little time for a repair, could risk the team's ability to fully test the flyby sequences. A backup, called NHOPS-2 was already in place, but it was a stripped-down version of NHOPS that lacked much of the simulation capability and fidelity of the original. So at Alan's direction, Glen put plans and budget in place to convert NHOPS-2 into a full-up spare to NHOPS, and to test it as thoroughly as had been done with NHOPS, to be sure it would be ready if there ever was a need. Little did he know then, this was a decision that would prove crucial in the final days of the approach to Pluto.
"As each of the dozens of command sequences that together comprised the entire flyby were designed and had passed their peer reviews, the MOPS team began running them on the NHOPS spacecraft simulator to see if they would work as expected. Bugs were often found, corrected, and then new NHOPS runs would be scheduled. MOPS repeated this again and again, until every sequence ran completely error-free. For the Core load -- the crucial nine-day-long chain of sequences that instructed the spacecraft how to execute all actions during the close flyby -- that took eight tries. Each of these eight NHOPS runs of the Core load took the full nine days. Version 1 was called V-1, version 2 was called V-2, and so on. Every time the team found bugs it rewrote the errant parts of the sequence and started the NHOPS run again, from scratch. When the Core finally ran bug-free on V-8, the eighth of these nine-day-long NHOPS runs, Alan celebrated by buying a couple cases of little cans of V-8 juice and handed the cans out for each team member to keep as souvenirs of the timeconsuming battle to create a completely bug-free Core load.
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| Pluto Image Sent to Earth on July 14. 2015 |
"Once that error-free milestone was achieved, the Core sequence was 'locked down' under a rigorous 'no change without careful review and approval' process, called 'configuration management' (or CM). CM's job was to ensure that an extra level of scrutiny and testing rigor went into any change, no matter how minor. Weekly meetings of a group called the Encounter Change Control Board (or ECCB), were held to evaluate change requests to the Core load and half a dozen other sequences that would run on New Horizons during the period from May to July of 2015. The ECCB was chaired by Alan and staffed by chief engineer Chris Hersman, project manager Glen Fountain, MOPS lead Alice Bowman, senior project scientist Hal Weaver, PEP lead Leslie Young, and encounter manager Mark Holdridge.
"At the same time that all the Pluto flyby command sequences were being developed, the project team also took a look at everything that could possibly go wrong during the encounter and how they or the spacecraft would have to react in order to fix any given situation. This kind of 'malfunction procedures' development is common to space missions, and it was crucial for a one-shot opportunity like a Pluto flyby.
"The largest effort to prepare for potential problems was led by spacecraft chief engineer Chris Hersman. Hersman, incredibly sharp and meticulous in his attention to detail, and incredibly knowledgeable about all aspects of the bird, made plans for each of 264 potential spacecraft, ground system, and other problems that might arise."
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