P–15 days: Navigating Pluto

As of today, Monday June 29th, I find myself airborne headed for the Applied Physics Laboratory in Maryland while 13 hours earlier, at midnight leading into this morning on the east coast, New Horizons found itself 18.5 million kilometers from Pluto and moving 1.2 million kilometers closer every day.

The Dreaded Target-plane Drift has come and gone. For about a week, the solutions drifted downward and leftward on the target plane—that big dart board in the sky—each time more tracking data was added, until finally it departed the bottom of the acceptable target box, a 300 x 200 kilometer rectangle centered about 14,000 kilometers down (Ecliptic south) and left (eastward) of Pluto.

The cause of the Dreaded Target-plane Drift is not yet completely understood, although it’s almost certainly related to earlier errors in locating the centers of Pluto and Charon, and this was most likely caused by inadequate knowledge of the albedos of those bodies, the dark and light patterns covering their surfaces. It’s hard to model the surface brightness of a body you’ve never seen up close before.

The DTpD, while it was in play, was a very worrisome thing because the Navigators didn’t know if it was real or—much worse—caused by a hidden problem, maybe serious dynamic mis-modeling that’s been overlooked to this point or even an unknown bug in the software. This caused some very restless activity and sleepless nights for several days in a row. It now appears that the drift has stopped and the current position, just outside the box, is correct and the earlier positions were not. There was probably not a dynamic mis-modeling problem or software bug. The moving finger of Navigation writes, and having writ, moves on.

In any event, the Dreaded Target-plane Drift may be a thing of the past as the Navigators rely more on the pin-prick images of Styx, Nix, Kerberos, and Hydra. Those bodies are small enough that no appreciable diameters are yet seen and the albedos of their surfaces doesn’t even enter the problem. So … while the DTpD may never rear its ugly head again, the Navigation Team will not hold its collective breath. Eternal vigilance is the norm.

At this red hot moment, if the current dart in the dart board is the correct one, surrounded by it’s little cloud of Gaussian-shaped probability, then the science observations won’t be completely optimal at closest-approach time, and the intended occultation of the Sun and Earth by Charon may not happen.

Thus in the wee hours of tomorrow morning the penultimate maneuver, TCM17B1, will execute and move the drifted solution back into the box, the happy return of the prodigal son. It will cost a few centimeters per second change in velocity, and then all will be well.

We hope.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s