The paper in Nature covering an object known as 288P lays out the case in its title: “A Main Belt Comet.” But what makes this story stand out is the fact that 288P is also a binary. A team of scientists led by Jessica Agarwal (Max Planck Institute for Solar System Research) discovered when 288P neared perihelion in September of 2016 that it was not one but two objects, asteroids of roughly the same mass and size, in a binary separated by about 100 kilometers. Moreover, they have verified that the small system is not quiescent. Using the Hubble instrument, Agarwal and colleagues discovered that the increased solar heating due to perihelion was producing sublimation of water ice, in much the same way that the tail of a comet is created. Here’s how the paper describes the process on 288P: Repeated activity near perihelion is a strong indicator of the sublimation of water ice due to increased solar heating. A model of the motion of the dust under the influence of solar gravity and...

If New Horizons can make its flyby of Kuiper Belt Object MU69 at a scant 3500 kilometers, our imagery and other data should be much enhanced over the alternative 10,000 kilometer distance, one being kept in reserve in case pre-encounter observations indicate a substantial debris field or other problems close to the object. But both trajectories, according to principal investigator Alan Stern, have been moved closer following a ten-week study period, and both are closer than the 12,500 kilometers the spacecraft maintained in its flyby of Pluto. Image: Artist's concept of Kuiper Belt object 2014 MU69, which is the next flyby target for NASA's New Horizons mission. Scientists speculate that the Kuiper Belt object could be a single body (above) with a large chunk taken out of it, or two bodies that are close together or even touching. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker. Stern made the statement in early September at a...

One of the great values of the Kepler mission has been its ability to produce a statistical sample that we can use to analyze the distribution of planets. The population of asteroids in our own Solar System doubtless deserves the same treatment, given its importance in future asteroid mining as well as planetary protection. But when it comes to main belt asteroids, we're able to look up close, even though the number of actual missions thus far has been small. Thus it's heartening to see Pekka Janhunen (Finnish Meteorological Institute), long a champion of intriguing 'electric sail' concepts, looking into how we might produce just such an asteroid sampling through a fleet of small spacecraft. "Asteroids are very diverse and, to date, we've only seen a small number at close range. To understand them better, we need to study a large number in situ. The only way to do this affordably is by using small spacecraft," says Janhunen. The concept weds electric sails riding the solar wind with...

Image: A monitor shows the status of NASA's Deep Space Network as it receives data from the Cassini spacecraft, Friday, Sept. 15, 2017 in the Charles Elachi Mission Control Center in the Space Flight Operation Center at NASA's Jet Propulsion Laboratory in Pasadena, California. Credit: NASA/Joel Kowsky. Image: Cassini program manager at JPL, Earl Maize, center row, calls out the end of the Cassini mission. Credit: NASA/Joel Kowsky. Image: Cassini program manager at JPL, Earl Maize, left, and spacecraft operations team manager for the Cassini mission at Saturn, Julie Webster embrace after the Cassini spacecraft plunged into Saturn, Friday, Sept. 15, 2017 at NASA's Jet Propulsion Laboratory in Pasadena, California. At left is Cassini project scientist Linda Spilker. At right center is Jim Green, Director of NASA's Planetary Science Division. Credit: NASA/JPL-Caltech/Space Science Institute. Image: Saturn's active, ocean-bearing moon Enceladus sinks behind the giant planet in a farewell...

Cassini mission engineers are referring to its final pass by Titan as 'the goodbye kiss,' a phrase that sounds like something from a Raymond Chandler novel. Maybe it's the juxtaposition of intimacy and death that Chandler exploited so well. In any case, what counts in the last of Cassini's hundreds of passes over Titan in its 13-year exploration of the system is the gravitational nudge that is sending the spacecraft into Saturn's atmosphere tomorrow. "Cassini has been in a long-term relationship with Titan, with a new rendezvous nearly every month for more than a decade," said Cassini Project Manager Earl Maize at NASA's Jet Propulsion Laboratory in Pasadena, California. "This final encounter is something of a bittersweet goodbye, but as it has done throughout the mission, Titan's gravity is once again sending Cassini where we need it to go." Closest approach for the final pass at Titan occurred at 1504 EDT (1904 UTC) on September 12, at an altitude of 119,049 kilometers, with data...

While we track hurricane Irma and its path of devastation in the Caribbean, another kind of storm is affecting the skies over more northerly areas. A strong geomagnetic storm watch continues in effect, making it possible that the aurora borealis -- northern lights -- will be visible further south than usual, in Scotland and southern Scandinavia but perhaps into the continental United States as well, while the aurora australis could be active for those in the more southerly latitudes below the equator. All of this is due to sunspot AR2673, which is the source of a flare and coronal mass ejection hurled out of the Sun on Monday. I've seen the northern lights in Iceland on one spectacular October night, but only once -- in Iowa, in 1970 -- have I seen them in the US. The phenomenon results from electrons accelerated as they encounter the Earth's magnetosphere, following the magnetic field lines to the polar regions, where from 500 kilometers down to 80 kilometers up, they collide with...