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Emily LakdawallaJuly 6, 2015

New Horizons "back in action" after safe mode event, ready to resume encounter science

NASA held a press briefing today to explain the nature and cause of the spacecraft anomaly that halted science on New Horizons for four days as it was on its terminal approach to Pluto. As of the moment that I write this post, New Horizons is not yet performing science observations, but it will begin them tomorrow. Here are the high points from the briefing:

The briefing panel consisted of Jim Green from NASA Headquarters; Alan Stern, New Horizons' principal investigator (representing the science team); and Glen Fountain, New Horizons' project manager (representing the engineering team). In the interest of getting a post up quickly, I'm just posting here my notes from the briefing, only lightly edited. I attempted to transcribe as they spoke, but in some cases I paraphrased, and I added some notes of my own in brackets []. I've interleaved the panelists' responses to questions in with their primary statements.

Jim Green:

At 2pm [I presume Eastern time] on Saturday, July 4, we lost the signal. Within 90 minutes signal was reacquired, and the spacecraft found in safe mode. We are just delighted with how New Horizons team has responded.

Glen Fountain on the nature of the anomaly and the recovery:

On July 3 we were preparing for the main event -- the command load that takes us through the flyby, which starts on the 7th. [A command load is a sequence of commands for the spacecraft, a program that it will execute autonomously.] All the observations from the 7th of July through the encounter on the 14th and on through the 16th are in a single command load. On the 3rd we loaded it onto the backup computer, and on the 4th we were loading it on to the primary computer.

At the same time, we were taking all the data we wanted to get down to the ground but wanted to save, and were compressing it into a single location on the [solid-state] data recorder. We were doing multiple things on the spacecraft at the same time, and what occurred is as we were doing the compression of all the data to save it for later download, we were burning to the flash memory the command load into the primary computer.

The computer was trying to do two things at the same time, and the two were more than the processor could handle at the same time, so the processor overloaded. The system autonomously switched to the backup computer and since it did that and we were not in encounter mode, we went to safe mode. At that point we lost the downlink from the primary side of the spacecraft because it had switched to the secondary side. [So note that while it was compressing data and loading software, New Horizons was in the middle of doing a third thing, downlinking data.]

We knew it would take about an hour for the spacecraft to transmit to Earth from the backup computer. We started looking for signal on backup side, and found it when expected. We looked at data, figured out what was happening, and started to put a plan in place to recover. The command load has now been put on the primary side so that when the 7th rolls around, we will go into the core load as planned.

Some of Fountain's responses to questions:

Alan Stern on the science impact:

New Horizons is operating flawlessly and on course. So are all the instruments in the payload. In the few days prior to this event we've been receiving tremendous data and I couldn't be happier with what we're seeing already from this great distance. Every day when the science team meets, the room is full of smiles.

Science was suspended, which is normal to do under these circumstances, making sure spacecraft was healthy, getting it configured the way it should be. What we lost was some of the science on Saturday, and then all the science for Sunday and Monday. And that was a command decision which I made and the team was in complete agreement with, that it's much more important to focus on getting ready for the flyby than to collect science 8 or 9 million miles from the target.

Precisely what we gave up:

That's 30 observations out of a total of 496 to be made between July 4 and end of close approach observations. So about 6% of observations by count, but scientifically we weight observations by how close they are to the planet. These are not nearly as important as the ones we'll make from 100 times closer. While we'd prefer this hadn't occurred, it's a speed bump in terms of total return.

Some of Alan's responses to questions:

Immediately after the briefing they shared some images composed of some of the last data that New Horizons returned to Earth before the anomaly on July 3.

Three views of Pluto, July 1-3, 2015

NASA / JHUAPL / SwRI

Three views of Pluto, July 1-3, 2015
The left image shows, on the right side of the disk, a large bright area on the hemisphere of Pluto that will be seen in close-up by New Horizons on July 14. The three images together show the full extent of a continuous swath of dark terrain that wraps around much of Pluto’s equatorial region. The western end of the swath (right image) breaks up into a series of striking dark regularly-spaced spots, each hundreds of miles in size, which were first detected in New Horizons images taken in late June. Intriguing details are beginning to emerge in the bright material north of the dark region, in particular a series of bright and dark patches that are conspicuous just below the center of the disk in the right image. In all three black-and-white views, the apparent jagged bottom edge of Pluto is the result of image processing. The inset shows Pluto’s orientation, illustrating its north pole, equator, and central meridian running from pole to pole. The Long Range Reconnaissance Imager (LORRI) obtained these three images between July 1 and 3 of 2015, prior to the July 4 anomaly that sent New Horizons into safe mode.
Pluto in color from New Horizons on July 3, 2015

NASA / JHUAPL / SwRI

Pluto in color from New Horizons on July 3, 2015
Pluto's mottled appearance began to com into focus, just 11 days before New Horizons' closest approach. A wide band of dark terrain near the equator breaks up into smaller dark islands at the bottom of the visible disk.

A questioner asked about the deconvolution being done to this data, and whether some of the features in the images are artifacts of that process. Stern replied (I paraphrase here):

I would be careful about talking about impact craters at this point, we don't have any strongly confirmed impact craters. We are still very far away. The deconvolution procedures we are using are very, very mature. There is no hocus-pocus about it. We have multiple methods and intercompare them, and look for evidence of the same features in the raw data, and we look for them to be consistent from observation to observation. Things that aren't rotating with the disk are candidates for being artifacts. We are much less subject to artifacts now that we are closer because the target is so much larger.

The last photo of Pluto that New Horizons returned was taken at 23:25 UT on July 3. The next one that it will take (if I understand the plan correctly) is at 22:41 on July 7. In that time, New Horizons will have closed more than a third of the remaining distance, increasing Pluto's apparent diameter by a factor of 1.6 and the number of pixels on Pluto by more than three times. We're so close!

Read more: trans-neptunian objects, New Horizons, pretty pictures, Pluto, mission status, dwarf planets beyond Neptune

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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