New Horizons spots Pluto!

Yesterday the New Horizons team released a flicker animation showing the spacecraft's first sight of Pluto, using the LORRI long-range imaging instrument:

NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

First Glimpse of Pluto

The Long Range Reconnaissance Imager (LORRI) on New Horizons acquired images of the Pluto field three days apart in late September 2006, in order to see Pluto's motion against a dense background of stars. LORRI took three frames at 1-second exposures on both September 21 and September 24. Because it moved along its predicted path, Pluto was detected in all six images. The image appears pixilated because it was obtained in a mode that compensates for the drift in spacecraft pointing over long exposure times.

Pluto doesn't make a very impressive splash against the background star field. But it's impressive that LORRI managed to target it at all.

Let's do some quick calculations here. Pluto was 4.2 billion kilometers away, and it's 2,300 kilometers in diameter. That means that its angular size is (2,300 divided by 4.2 billion) = about 0.5 microradians. Now, according to this document from the New Horizons website, each LORRI pixel spans 5 microradians. In other words, Pluto is still so far away, and so small, that its entire disk covers only one tenth the width of a LORRI pixel. But consider the fact that the amount of light we see is proportional to the area of that disk. The pinpoint of light from Pluto is being spread out over a pixel that is over a hundred times the area of Pluto's disk. That's a faint spot to spot from a fast-moving spacecraft -- according to the Where Is New Horizons? page, it's moving at 21 kilometers per second right now. Congratulations to the team for the first sight of Pluto's light! We still have a long time to wait before that tiny dot spreads out over more than one LORRI pixel, but hopefully we'll keep getting brief glimpses like this from time to time to keep us hungry for more...