I will admit to a fascination with Pluto's moon Charon that began even before it was discovered. Intrigued by the most distant places in the Solar System, I had always imagined what the view would be like from a tiny moon circling Pluto. At the time, we didn't know about Charon, so my vantage point was more like what we now know Kerberos or Styx to be. Then my interest tripled when the sheer size of Charon became known. A large moon was truly a world of its own, and Charon rose in my estimation to rival my other most intriguing moon, Neptune's Triton. Now we have word that Charon may once have had an internal ocean, still further evidence of the intricacy of objects in or near the Kuiper Belt. In Charon's case, something intriguing is shown by a study of the surface, one side of which New Horizons saw during the July 2015 flyby. What appears to be a series of tectonic faults that show up in the form of ridges, scarps and valleys reveals a surface that has to have been stretched over...

Landing on a small object in the Solar System isn't easy. Witness the Philae lander, which traveled to Comet 67P/Churyumov-Gerasimenko as part of the European Space Agency's Rosetta mission. Philae 'landed' on November 12, 2014, having to deal with a malfunctioning thruster along the way. Upon arrival at the surface of the comet, Philae was to have fired anchoring harpoons to steady itself on the surface, but after a dramatic seven-hour descent, the harpoons failed to fire. Thus the lander did touch down at the initial landing site, called Agilkia, but then bounced to a new site -- Abydos -- about a kilometer away. Even now, we're not sure just where Philae is despite imagery from the orbiting Rosetta. What ESA has told us, however, is that the lander evidently made contact with the comet four times during an unplanned for two-hour additional flight across the surface. During the process it grazed the rim of a depression called Hatmehit on its way to its resting place at Abydos....

Knowing what comets are made of -- dust and ice -- only begins to answer the mystery of what is inside them. A compact object with this composition should be heavier than water, but we know that many comets have densities much lower than that of water ice. The implication is that comets are porous, but what we'd still like to know is whether this porosity is the result of empty spaces inside the comet or an overall, homogeneous low-density structure. For answers, we turn to the European Space Agency's continuing Rosetta mission. In a new paper in Nature, Martin Pätzold (Rheinische Institut für Umweltforschung an der Universität zu Köln, Germany) and team have gone to work on the porosity question by analyzing Comet 67P/Churyumov-Gerasimenko, around which Rosetta travels. It's no surprise to find that 67P/Churyumov-Gerasimenko is a low-density object, but an examination of the comet's gravitational field shows that we can now rule out a cavernous interior. Image: These images of comet...

If there's one thing Pluto turned out to have beyond all expectation, it's geological activity. New Horizons is now showing us what researchers are calling 'hills of water ice' floating in a sea of frozen nitrogen, much like icebergs moving through our own Arctic Ocean. The isolated hills are thought to be fragments of the water ice in the surrounding upland regions. Measuring several kilometers across, they are found in Sputnik Planum, a plain within Pluto's 'heart.' Image: The image shows the inset in context next to a larger view that covers most of Pluto's encounter hemisphere. The inset was obtained by New Horizons' Multispectral Visible Imaging Camera (MVIC) instrument. North is up; illumination is from the top-left of the image. The image resolution is about 320 meters per pixel. The image measures a little over 500 kilometers long and about 340 kilometers wide. It was obtained at a range of approximately 16,000 kilometers from Pluto, about 12 minutes before New Horizons'...

Given that we have four planets in our Solar System with rings, it's a natural thought that if so-called Planet Nine does exist, it might likewise show a system of rings. After all, Caltech's Konstantin Batygin and Mike Brown are talking about a planet with a mass on the order of ten times that of the Earth. Neptune is about 17 Earth masses, while Uranus is 14.5 as massive. If Planet Nine is an ejected ice giant, perhaps it joins Uranus, Neptune, Jupiter and Saturn in having a ring system of its own, along with a thick atmosphere of hydrogen and helium. Of course, we have to discover Planet Nine first, a process that may take some time if, indeed, it is successful. Meanwhile, we have interesting developments in the Solar System's most intriguing ring system. As compared with those of other planets, Saturn's rings are visually stunning. The B ring is the brightest and most opaque of the planet's rings, but now we're finding out that brightness and opacity have little correlation with...

Newly interpreted data from the New Horizons spacecraft tells us that Pluto has more water ice on its surface than we once thought. The image below tells the tale, a false-color view derived from observations by the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument. Here we're at infrared wavelengths and can see areas showing the spectral signature of water ice. Note the sharp contrast between the left and right sides of the image below. Image: This false-color image is derived from observations in infrared light by the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument. It is based on two LEISA scans of Pluto obtained on July 14, 2015, from a range of about 108,000 kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. The two scans, as this JHU/APL news release explains, were taken about fifteen minutes apart and merged into a 'data cube' -- a three-dimensional array covering the hemisphere New Horizons...