One of the more memorable moments from yesterday’s teleconference on the New Horizons mission was Alan Stern’s comment that the latest pixelated images of Pluto/Charon constituted his ‘meet Pluto moment.’ If anyone has an interest in meeting Pluto, it’s Stern (Southwest Research Institute), who serves as principal investigator and whose unflagging efforts made it possible. As for those pixelated views, well, they’re a glimpse of what is to come, but even now, they’re telling us helpful things about the target. The animation below speaks volumes, with the first showing Charon’s rotation with the center of Pluto fixed in the frame. The images were acquired with the Long Range Reconnaissance Imager (LORRI) camera.
Image: A series of LORRI images of Pluto and Charon taken at 13 different times spanning 6.5 days, from April 12 to April 18, 2015. During that time, the spacecraft’s distance from Pluto decreased from about 111 million kilometers to 104 million kilometers. Pluto and Charon rotate around a center-of-mass (also called the “barycenter”) once every 6.4 Earth days, and these LORRI images capture one complete rotation of the system. The direction of the rotation axis is shown in the figure. In one of these movies, the center of Pluto is kept fixed in the frame, while the other movie is fixed on the center of mass (accounting for the “wobble” in the system as Charon orbits Pluto). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
The second image shows the same view with the motion around the barycenter clearly revealed, a spectacular teaching tool for those trying to explain rotation around the center of mass.
Image: The 3x-magnified view of Pluto highlights the changing brightness across the disk of Pluto as it rotates. Because Pluto is tipped on its side (like Uranus), when observing Pluto from the New Horizons spacecraft, one primarily sees one pole of Pluto, which appears to be brighter than the rest of the disk in all the images. Scientists suggest this brightening in Pluto’s polar region might be caused by a “cap” of highly reflective snow on the surface. The “snow” in this case is likely to be frozen molecular nitrogen ice. New Horizons observations in July will determine definitively whether or not this hypothesis is correct. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
The zoomed image is particularly notable because it clearly shows different brightness patterns as Pluto rotates (and notice that Charon, even though much smaller, still stays relatively the same, an indication that the surface is more uniform than Pluto, or as Stern said in the news conference, more ‘muted’). An apparent oddity here actually is not — If you look closely at the view of Pluto in the center, it seems to show an uneven surface. This is only an optical effect caused by the fact that dark areas on the dwarf planet are rotating in and out of view.
These albedo changes on the Plutonian surface are striking. Stern commented that it was unusual that we should be seeing such marked effects from such a distance. The possibility of a polar cap — probably made of highly reflective frozen molecular nitrogen ice — seems strongly enhanced by the bright area that persists in the zoomed image. JHU/APL has used a mathematical technique called deconvolution to improve the resolution of the raw LORRI images, giving us about the best resolution the camera and detector can provide at this distance.
And speaking of raw images, be aware of the raw image archive being assembled on the LORRI Images from the Pluto Encounter page. The imagery is, of course, going to keep getting better. We’ll get another sequence from an observing run scheduled for May 8 through May 14, producing another ‘movie’ in mid-May, followed by a lengthy pause to downlink the data that lasts until the 27th. The spacecraft will then begin observing Pluto/Charon daily at ever higher resolution. Fasten your seat belt.