New views of three worlds: Ceres, Pluto, and Charon
Posted by Emily Lakdawalla
16-04-2015 15:17 CDT
This week we've been treated to new views of three worlds: Ceres, Pluto, and Charon. As New Horizons is still quite far away from Pluto and Charon (more than 100 million kilometers away), they are mere dots. But these dots are the first color images of Pluto and Charon from New Horizons, taken by the Ralph Multispectral Visible Imaging Camera (MVIC).
Before you get too excited about interpreting features on the surface of Pluto from this image, you need to realize that it has been enlarged by quite a lot from the original resolution of the camera. If I've done my math right, it's been enlarged by a factor of six. Just for fun, I've "de-enlarged" the image by reducing its size by a factor of six -- these don't show you the data's actual original pixels, but it should illustrate the size of the original pixels. (They would have had to enlarge the image somewhat in order to successfully overlay the separate MVIC frames taken at different wavelengths in order to make the color photo, so there really is no original-resolution version of this picture.)
In the original data, each pixel would have covered about 2300 kilometers -- just barely under the size of Pluto. So neither Pluto or Charon are actually resolved; Charon is a point source, and Pluto is basically a point source, too. All the detail that you might think you see in the publicly released image has to do with the camera instrument, not with any surface variation on Pluto. It's basically just a graphical representation of the camera's point-spread function -- a description of how light from a point source spreads out across the detector.
But there's nothing wrong with that -- it's very early in the science mission yet, and there is only better to come! The first images from MVIC that will be better than Hubble's will come at the end of June. MVIC has four times lower resolution than the monochrome camera, LORRI, so New Horizons has to be four times closer to Pluto for MVIC to achieve the "better than Hubble" landmark than for LORRI. LORRI will get better than Hubble at the end of May.
Meanwhile, Dawn is a lot closer to its target, Ceres, and spiraling closer every day. Dawn performed its sixth optical navigation observation of Ceres on April 10, just a day after New Horizons took its color picture. Dawn's perspective is down on the north pole of Ceres, so we're watching it spin beneath the spacecraft for roughly an hour out of Ceres' 9-hour day.
It's such an unusual perspective to look down on a round world's pole like this! I'm trying to think of another animation of any other world in the solar system with a similar view -- nearly at a standstill at a very high latitude, watching it spin -- and I can't really think of any, except maybe for Cassini's observations of Saturn's poles.
Here, I've split the animation out into its 20 frames.
You might notice on the right side at the beginning of the animation that there are two very bright-toned peaks jutting into space. On Twitter, someone asked me if these were Ceres' two bright spots. I couldn't figure out the answer so I asked the question at unmannedspaceflight.com and user JohnVV quickly responded to say that they were not. The territory visible in the animation is from 50 to 90 degrees north latitude; the bright spots are at 20 degrees north. But don't fear -- we'll get a closer look at the bright spots before too long!