An obscure instrument called a blink comparator became world famous following Clyde Tombaugh's discovery of Pluto in 1930. It was by rapidly switching between astronomical photographs that the young Tombaugh was able to compare objects in the field of view where 'Planet X' was presumed to hide. Pluto turned out to be a good deal smaller than Percival Lowell had imagined, leading to thoughts of still more distant planets, but for a time the new planet was best known as a faint dot on a series of plates, moving against a fixed field of stars. Image: Clyde Tombaugh at the Blink Comparator five years after the Pluto discovery. Credit: Lowell Observatory Archives. All of this is wonderfully told in Michael Byers' 2010 novel Percival's Planet (Henry Holt and Co.), which draws on Tombaugh's story and depicts the entire Lowell Observatory scene in his time there (see A Tour de Force of Planetary Discovery for my review of the book). Or if you want the inside view, Tombaugh's own Out of the...

Earth's axial tilt (its obliquity) is 23.5 degrees, a significant fact for those of us who enjoy seasonal change. The 'tilt' is the angle between our planet's rotational axis and its orbital axis. If we look at Earth's obliquity over time, we find a 41,000 year cycle that oscillates between 22.1 and 24.5 degrees. Here the Moon becomes useful, with recent studies showing that without it, Earth's obliquity could vary by 25° (some earlier analyses took this number much higher). Now we have new data from the Dawn spacecraft at Ceres relating the dwarf planet's axial tilt to the locations where frozen water can be found on its surface. This is interesting stuff, because it depends upon the spacecraft's ability to measure the world it orbits. "We cannot directly observe the changes in Ceres' orientation over time, so we used the Dawn spacecraft's measurements of shape and gravity to precisely reconstruct what turned out to be a dynamic history," says Erwan Mazarico, a co-author of a...

With Cassini now in the final stages of its mission, we can look forward to just one more close flyby of Titan, the 127th targeted encounter, on April 22. 'Targeted' means that Cassini has to use its thrusters to position itself optimally for the flyby. The first of the images below, by contrast, comes from a 'non-targeted' flyby, one of several anticipated for 2017. The close pass will give researchers a chance to probe the moon's northern seas one last time, which may prove useful in the investigation of the transient features some have dubbed 'magic islands.' Even as these studies proceed, Cassini will also be using the Titan flyby to alter its course enroute to the series of plunges through the gap between Saturn and its innermost rings now being called the Cassini Grand Finale. The spacecraft will plunge into Saturn's atmosphere on September 15. Image: As it sped away from a relatively distant encounter with Titan on Feb. 17, 2017, NASA's Cassini spacecraft captured this mosaic...

We've looked recently at the possibility of cryovolcanism on Ceres with regard to the unusual feature called Ahuna Mons (see Ice Volcanoes on Ceres?). Now we have further evidence that outbursts of brine from beneath the surface have been occurring over long periods of time, and that some of these eruptions have been recent. The work comes out of analysis of data from the Dawn mission by scientists at the Max Planck Institute for Solar System Research (MPS), and moves the debate to the unusual crater called Occator. Image: This view of the whole Occator crater shows the brightly colored pit in its center and the cryovolcanic dome. The jagged mountains on the edge of the pit throw their shadows on parts of the pit. This image was taken from a distance of 1478 kilometers above the surface and has a resolution of 158 meters per pixel. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Dawn's Low Altitude Mapping Orbit (December 2015 to September 2016) took the spacecraft to within 375 kilometers of the...

You wouldn't expect main belt asteroids to develop tails like comets -- their orbits are circular enough that they don't undergo the kind of temperature swings many comets experience in their plunge toward perihelion -- but we do have some twenty cases of asteroids that do exactly that. The photo below, showing imagery from the 10.4-meter Great Canary Telescope, gives us views of asteroid P/2016 G1, with a smudgy dust trail splayed out behind the object. Image: Asteroid P/2016 G1 at three different times in 2016: late April, late May and mid June. The arrow in the center panel points out an asymmetric feature that can be explained if the asteroid initially ejected material in a single direction, perhaps due to an impact. Credit: Fernando Moreno (Institute of Astrophysics of Andalusia, Spain). Moreno and team, who have specialized in the dust environment near main belt objects, have now uncovered another intriguing asteroid, this one with an even more curious tail. Asteroid P/2016 J1...