P-3 days: Navigating Pluto

Where? 4.0 million kilometers out, getting 1.2 million kilometers closer each day, as of midnight this Saturday morning of July 11th in Columbia, Maryland. Pluto is about 120 pixels across in the LORRI camera, but to the naked eye it’s only 7 percent the size of the full moon.

Where to? Pluto! At 7:49:02 AM, July 14th Columbia time, plus or minus 47 seconds or so.

Where from? Earth.

Yesterday afternoon the Navigators recommended to the project that an update to the onboard sequence be made, since the current solution location in the target plane (think of it as the throw of a dart) and its corresponding cloud of uncertainty has drifted towards the top center of the 65 x 130 kilometer box that represents the acceptable region for the flyby.

It’s actually a cube rather than a box, since there’s also a limit to the acceptable arrival time of 200 seconds centered on 7:49:49 AM Columbia, Maryland time. If the trajectory goes through that box and the spacecraft closest approach is within 100 seconds of the desired time, all is well and the current design is good. If it goes outside that box/cube, then either some science data may be lost, or the sequence will need tweaking to make the observations optimal again.

The Navigation Team recommendation was to upload a tweak to the canned-in spacecraft sequence design to guard against a drifting solution, essentially re-centering the cube around the current solution. The independent Navigation Team agreed. The Science Team agreed. The Mission Operations Team agreed.

The mission leader didn’t agree. Alan Sterns’ reasoning was that there’s too much risk of missing the whole flyby if the spacecraft should go into safe mode again, like it did on July 4th, an unanticipated event. If that happened, it’s quiet possible that the spacecraft couldn’t be returned to normal operation in time for the flyby, resulting in a loss of almost all of the mission objectives. Better to risk some suboptimal science observations than the whole mission.

Nobody wants that, not the scientists, not the engineers, and not the Navigators, so there has not been any grumbling about the decision. It was justified.

Today we processed Crit 35 optical navigation data.

The good news is that the mild solution drift we’ve seen in the last few days in both the target plane and the arrival time seems to have abated, and the error bounds continue to shrink up around the solution.

There is no bad news.

My wife just landed in Baltimore.

Life is good!

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