Sen—Isn’t it amazing how space missions can keep you on the edge of your seat? The big story this week has, of course, been NASA’s New Horizons probe running the final leg of its nine-year journey to that remote world Pluto.
As the spacecraft began to complete the last few million miles of its fantastic three billion mile (five billion km) voyage, sending back fuzzy yet intriguing images of Pluto and its main moon Charon, disaster struck.
Just ten days short of closest approach, and on American Independence Day of all days, this triumph of NASA science and engineering disappeared from mission control’s computer screens.
Team members preparing to enjoy their national holiday had to hurry instead to the Mission Operations Center (MOC) at Johns Hopkins University Applied Physics Laboratory in Maryland. They included New Horizons’ principal investigator Alan Stern.
The Washington Post, in a detailed report of the events during that crisis, said that Stern later described it as “our Apollo 13” moment. The team had to work out how to recover contact with their robotic emissary to the outer Solar System and they had little time left to do that.
With just days to go until the close encounter with Pluto, and with New Horizons flying at 100 times the speed of a jet airliner, NASA was about to declare the probe’s flyby sequence of science observations to be officially under way.
The operations team’s big worry was that New Horizons might have hit something and been destroyed. The zone where Pluto sits like a gatekeeper is the start of the Kuiper Belt which holds an unknown amount of icy debris, and this had been reported well in advance as a potential hazard that would be faced by the spacecraft.
As we know from the increasingly detailed images coming back from Pluto, the probe was clearly not damaged. It turned out it had put itself into safe mode, shutting down instruments and less vital systems after its main computer had become overloaded as it struggled to carry out some scheduled commands at the same time as it compressed data that it was recording.
The mission team realised that if New Horizons had gone into safe mode, it would have switched to a backup computer on a separate communications frequency. They used NASA’s Deep Space Network of radio dishes in Australia, California and Spain to call the backup computer’s own “telephone number” and to everyone’s relief, the probe answered the call.
New Horizons had begun to go into a slow spin as it went into safe mode. MOC was able to switch control back to the main computer and stop the spin to allow the mission to get back on track. That they did so underlines the cool professionalism of the scientists and engineers on the team. One can only imagine the tension they must have felt as a sequence of many commands was sent to reconfigure the probe, especially when you remember that it is so far away that each command takes 4.5 hours to get to the spacecraft, and they then had to wait another 4.5 hours to hear back what the response had been.
Meanwhile, across the pond, another operations team was continuing to try to recover contact with one of its own probes—little lander Philae—as it sat in silence on the surface of a comet being studied by the European Space Agency’s Rosetta mission. Having lost all power just a couple of days after reaching Comet 67P Churyumov-Gerasimenko in November, Philae had delighted the ESA team by coming back to life and phoning home again on June 13, ending seven months without contact.
Oddly, however, contact proved to be intermittent and unreliable. So elation that Philae was back from the dead turned quickly to exasperation that a proper fix on communication could not be made. Until this week, nothing had been heard of from Philae since June 24, and a bid to re-establish contact via a separate antenna on one of its instruments had failed last Sunday, July 5. It was the latest drama to add to Rosetta's rollercoaster ride.
A couple of days later, I was sitting with Rosetta Chief Scientist Matt Taylor over a drink late in the evening in the resort of Llandudno, North Wales, where we both were for the National Astronomy Meeting organised by the Royal Astronomical Society. He told me what further efforts would be made, if a second attempt to switch on the CONSERT instrument also failed. Thankfully, as I reported on Friday, that second bid did end in success, with a period of contact that included 12 stable minutes of communication.
Rosetta scientists will now be poring over the data collected in that time of contact to work out just what is going on with Philae, and how they might resume sufficient regular communication to carry out proper science on the comet again. The Rosetta mission is approaching a climax of its own because the comet gets to its most active phase around perihelion, its closest approach to the Sun, which occurs on August 13.
One big difference between the New Horizons and Rosetta missions is that the European comet explorer is in a place where it is comfortably circling its target and has many months still to explore it, especially since the mission has been extended until September 2016. For the Pluto team however, everything hinges on it being “all right on the night” because their probe will zip straight past Pluto and they will then have to watch its quarry receding rapidly again in its rear-view mirror.
Thankfully, as I write, everything is back to normal and going well for the New Horizons adventure. With images of Pluto getting ever more detailed and intriguing, we will stay on the edge of our seats as we wait for the probe’s close encounter with that mysterious dwarf planet in just a couple of days time at 11.49 UTC (7.49 EDT) on Tuesday July 14. I can’t wait!