P-5 days: Navigating Pluto

Midnight beginning Thursday, July 9th on the U.S. east coast: New Horizons is 6.5 million kilometers from it’s destination, still doing 1.2 kilometers per day.

Seen from the spacecraft, Pluto is only two-hundredths of a degree across. To put that in perspective, that’s only 4 percent of the diameter of the full moon seen from your back yard. However, to the LORRI camera, that amounts to 74 pixels, which is starting to be a respectable size in terms of picking up the hitherto unseen Plutonian details. It will continue to get much better as we continue this high-speed approach.

But I digress; this is about the trip, not the destination. Today we’ll process Crit 33 OpNav data. That’s short for the critical 33rd delivery of the precious images used for optical navigation. It’s the images of Pluto, Charon, and the rest of the satellites against a starry background.

It’s critical for a couple of reasons. First, image files are big, even after data compression, and there’s a lot to download for each critical delivery from a spacecraft 4.8 billion kilometers from Earth. It takes a long time to download the images at the low data rate. Second, the arrival time is still unknown by about a minute, and that doesn’t get knocked down to the few-second level until the last few days, beginning now. The time pressure on all the spacecraft teams, especially the Navigators, is high. Every new delivery of optical navigation data is critical to determining the encounter timing so that the onboard sequence can be adjusted by that improved timing knowledge. Image mosaics are already planned and onboard the spacecraft, but they may need tweaking by whatever amount of time change we get from the OpNav data.

As I write, several hours have flown by since the previous sentence and we’ve processed the OpNav data, merged it with the radiometric tracking data, and gotten multiple solutions. It looks like we’re arriving early by roughly a minute, but we have to be cautious about that because the uncertainty is still almost the same size as the time shift, meaning that the shift is only barely significant.

For that reason, we’re not going to take the chance of tweaking the onboard sequence yet for fear of inadvertently screwing up something else in the spacecraft computer. Editing data on the computer on your desk is one thing—editing data on a spacecraft 4.8 billion kilometers from Earth is quite another. A little fat-fingered “oops” on your laptop won’t cost much to fix compared to the hundreds of millions of dollars spent on this spacecraft and the fact that this is probably the only time in our lives for this opportunity.

This is history, and a little oops that might turn the antenna away from Earth, never to be turned back again, never to be heard from again, never to send back those first-time detailed pictures from the last planet of our traditional solar system … well, that would be a disaster of the first magnitude. We don’t want to take the least unnecessary risk of that happening, and while the timing uncertainties are still large, that’s an unnecessary risk.

Tomorrow we tackle Crit 34 to see if we can improve on the arrival time even more.

Meanwhile, why do we make this extraordinary effort to explore this tiny, almost insignificant body at the edge of our solar system?

Because we have to. Because it defines us. Because billions of years of evolution have shaped us into the curious, never-satisfied beings that we are.

Because it is part of our souls.


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