Imagine what you could do with a comet at your disposal. In Seveneves, Neal Stephenson's new novel (William Morrow, 2015), a Musk-like character named Sean Probst decides to go after Comet Grigg-Skjellerup. A lunar catastrophe has doomed planet Earth and humanity is in a frantic rush to figure out how to save at least a fraction of the population by living off-world. Probst understands that a comet would be a priceless acquisition: "You can't make rocket fuel out of nickel. But with water we can make hydrogen peroxide -- a fine thruster propellant -- or we can split it into hydrogen and oxygen to run big engines…. We have to act immediately on long-lead-time work that addresses what we do know. And what we know is that we need to bring water to the Cloud Ark. Physics and politics conspire to make it difficult to bring it up from the ground. Fortunately, I own an asteroid mining company…" And so on. Lest you think that was a spoiler, be advised that it's just the tip of...

I have never been exactly indignant about the demotion of Pluto to 'dwarf planet' status but I do think it's curious and in at least one respect too arbitrary for my taste. I'll buy the idea that a planet needs to be round because of its own gravity, and I'll sign off on the notion that to be a planet, an object has to be in orbit around the Sun (even though we do have apparent wandering planets in the interstellar deep, far from any star). But the International Astronomical Union also decided in its 2006 deliberations that a planet has to 'clear' its neighborhood of debris, thus sweeping out its orbit over time. That one, of course, is controversial. Assuming the Earth is a planet, why are we worried about things like Near Earth Asteroids (NEAs)? Our planet clearly hasn't swept out its neighborhood, not when we can number problematic asteroids in the thousands. Jupiter is estimated to have about 100,000 trojan asteroids in its orbital path as well, and Alan Stern, principal...

Seven years worth of Cassini images of Enceladus have told us what many have long suspected: The intriguing moon does indeed have a subsurface ocean. Not that the presence of water on Enceladus comes as a surprise: The south polar region in the area of the famous 'tiger stripes' has long been known to be venting vapor and liquid water from its fractures. The question had become, is this a regional body of water, or is the Enceladus ocean global? To find out, a team at Cornell University led by Peter Thomas, whose work was just published in Icarus, charted about 5800 surface features, contrasting images taken at different times and at different angles. Using a combination of dynamical modeling and statistical analysis, they sought to find the best values for the interior that would explain an apparent libration or 'wobble' (0.120 ± 0.014°) detectable in the imagery, a larger motion by far than would be expected if the surface of Enceladus were solidly connected with its...

Given the flow of new imagery from New Horizons, I began to realize that mission data were changing my prose. To be sure, I still lean to describing the system as Pluto/Charon, because given the relative size of the two bodies, this really seems like a binary object to me. I tend to call it a ‘binary planet’ among friends because I still think of Pluto as a planet, dwarf or not. But when New Horizons blew through the Pluto/Charon system, it was finally possible to start talking separately about Charon, because now we were seeing it, for the first time, up close. Charon as a distinct object from Pluto is a fascinating thought, one I’ve mused over since the days of the smaller object’s discovery in 1978. An enormous moon hanging in the sky, never changing its position, over a landscape unknown -- the imagination ran wild. In the event, New Horizons outdid anything I ever conceived, with imagery of both worlds we’ll be debating for a long time. But in some ways my favorite of the images...

Hard to believe it's been 55 days since the New Horizons flyby. When the event occurred, I was in my daughter's comfortable beach house working at a table in the living room, a laptop in front of me monitoring numerous feeds. My grandson, sitting to my right with his machine, was tracking social media on the event and downloading images. When I was Buzzy's age that day, Scott Carpenter's Mercury flight was in the works, and with all of Gemini and Apollo ahead, I remember the raw excitement as the space program kept pushing our limits. I had a sense of generational hand-off as I worked New Horizons with my similarly enthusiastic grandson. Carpenter took the second manned orbital flight in the Mercury program when Deke Slayton had to step down because of his heart condition, and the flight may be most remembered for the malfunction in Carpenter's pitch horizon scanner, leading to the astronaut's taking manual control of the reentry, which in turn led to overshooting the splashdown...

I'm really going to miss Cassini when it takes its plunge into Saturn's atmosphere in 2017. Having an orbiter in the outer system means that periodically we've been handed spectacular imagery and vast amounts of data for present and future analysis. Each new encounter now, such as the recent one with Dione, is a poignant reminder of how successful this mission has been, and how much we could gain with similar instrumentation around the ice giants. Meanwhile, I look at this striking view of Saturn and its rings from 20 degrees above the ring plane, a mosaic built from 75 exposures using Cassini's wide angle camera, and marvel at the view. The images were made in August of 2009, a day and a half after Saturn equinox, when the Sun was exactly overhead at the planet's equator. The result is a darkening of the rings from this perspective because of the Sun's lower angle to the ring plane, with shadows cast across the ring structure. It will be a while before we see this view again -- even...

At the Jet Propulsion Laboratory in Pasadena, Masahiro Ono has been using supercomputer simulations to model a new way of moving between small bodies in the Solar System. We've had a demonstration in the last few years of what ion propulsion can do to enable orbital operations at one asteroid (Vesta) followed by a journey to another (Ceres) and orbital insertion there. But Ono is looking at ways to simplify the process of asteroid and comet rendezvous that replaces the need for propellant during the orbital insertion and landing phases. Call it Comet Hitchhiker. "Hitchhiking a celestial body is not as simple as sticking out your thumb, because it flies at an astronomical speed and it won't stop to pick you up. Instead of a thumb, our idea is to use a harpoon and a tether," says Ono, who presents the idea today at the American Institute of Aeronautics and Astronautics SPACE conference in Pasadena. The work has intriguing implications for our investigations of the Kuiper Belt and outer...