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by Jason Snell & Dan Moren

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By Jason Snell

Debugging from three billion miles away

Note: This story has not been updated for several years.

New Horizons
Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Over the weekend NASA’s New Horizons probe to Pluto stopped transmitting to Earth. A little more than a week before the mission’s high-speed flyby of one of the last unexplored major objects in our solar system, the signal just stopped.

About 80 minutes later, the spacecraft reported back in. It had gone into “safe mode,” which generally happens if there’s a major error. In fact, it happened eight years ago on this very mission.1

Debugging a computer run on a 12 MHz MIPS processor would probably be tricky enough without that computer being nearly three billion miles away. It takes a radio signal roughly 4.4 hours to reach New Horizons, and then another 4.4 hours for the results of whatever commands you’ve sent to return back to Earth.

Crazy stuff. The good news is, the entire event seems to have been resolved—and might have been the result of human error? According to NASA:

The underlying cause of the incident was a hard-to-detect timing flaw in the spacecraft command sequence that occurred during an operation to prepare for the close flyby. No similar operations are planned for the remainder of the Pluto encounter.

Adding to the degree of difficulty here, New Horizons is traveling at about 45,000 miles per hour. That means that when it passes through the Pluto-Charon system2 on July 14, it will be using every sensor to capture as much data as it can during the brief time it’s very close to these objects. The ship can only transmit at about 1 kilobit per second, so it will have to save most of the information it captures during its flyby and then slowly transmit it all back to Earth over the following two or three months.

So even though next Tuesday’s the big day for New Horizons, back on Earth we’ll be learning new things about Pluto, Charon, and the Kuiper Belt in general for weeks after the flyby is complete. Assuming that 12 MHz processor keeps working flawlessly.

  1. Yes, New Horizons has been flying to Pluto since January of 2006, meaning it left this planet before there was an iPhone. 
  2. Pluto’s not a major planet (though it was considered one at the time of the New Horizons launch) because it’s very small and very far away. The discovery of Eris, a larger object than Pluto, forced us to re-think how we view the Solar System and be a little more precise in where we categorize good ol’ Pluto. Read “How I Killed Pluto and Why It Had It Coming” for much more. 

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