Ion Engines Drive Dawn to Asteroids Vesta and Ceres

Episode Summary

Executive Summary: This Planetary Radio episode centers on the Dawn mission’s groundbreaking journey to orbit Vesta and then Ceres using ion propulsion, with Chris Russell explaining why the two asteroids are compelling scientific targets. The show also revisits whether Voyager 1 should have skipped Titan for a Pluto flyby, covers late-breaking Mars mission and asteroid-impact news, and closes with Bruce Betts’ skywatching tips and a holiday trivia contest.

Main Topics: Dawn mission to Vesta and Ceres: Chris Russell explains Dawn’s unprecedented plan to orbit two different solar system bodies, using ion engines to reach Vesta first and later Ceres, and why both objects are scientifically rich yet very different. Ion propulsion and mission design: The episode highlights Dawn’s three ion thrusters, their long-duration gentle thrust, and the engineering challenge of ensuring reliability over years of deep-space travel. Scientific importance of Vesta and Ceres: Vesta is presented as a differentiated, rocky world-like asteroid, while Ceres appears water-rich and possibly partially icy or ocean-bearing, making both key to understanding planetary formation. Voyager 1, Titan, and the Pluto 'what if': Emily Loctawala answers a listener question about whether Voyager 1 could have been diverted to Pluto instead of making the Titan flyby, weighing the tradeoff between lost opportunity and major scientific gains. Current space news: Mars Scout delay and asteroid risk: The show reports a NASA conflict-of-interest issue delaying the Mars Scout selection and notes asteroid 2007 WD5 as a possible impact threat to Mars with uncertain odds. Holiday skywatching and trivia with Bruce Betts: Bruce Betts offers seasonal observing advice, including Mars at opposition, Saturn, Venus, and Comet Holmes, and hosts the weekly trivia contest.

Key Arguments: Dawn is the first mission designed to orbit two different bodies in deep space, making it a major step beyond one-time flybys. Ion propulsion enables the mission’s multi-target itinerary because it provides continuous low-thrust acceleration over long periods. Vesta likely differentiated early, with heavy material sinking to form a core and lighter silicates forming a mantle, making it similar in some respects to the Moon and Earth. Ceres appears roughly half as dense as Vesta, implying a large fraction of water or ice and possibly a rocky interior overlain by an ice shell or even subsurface liquid water. Voyager 1’s Titan flyby sacrificed any Pluto option, but that decision produced crucial atmospheric, ring, and planetary-system-shape data that later benefited Cassini and solar system science. The Mars Scout mission delay means the next launch window is lost, pushing the mission to 2013 instead of 2011. Asteroid 2007 WD5 is being watched because even a modest object can pose a measurable impact risk, though the odds will change as orbit calculations improve.

Data Points: Dawn mission launch phase: Cruise phase underway - Chris Russell discusses the spacecraft already on its way to Vesta First target arrival: 2011 - Projected arrival at Vesta Time at Vesta: Close to a year - Dawn will orbit/map Vesta before departing Transit to Ceres: A little more than 3 years - Travel time after leaving Vesta Time at Ceres: At least half a year, maybe longer - Planned orbital study of Ceres Mission end date: July 2015 - Official end of the Dawn mission at the time of the interview Ion thrusters on Dawn: 3 - Only one is used at a time, but three are carried for redundancy Solar panels/wingspan: 20 meters (65 feet) - Described as the spacecraft’s “wings” powering the ion engines Mars flyby: 2009 - Dawn will swing past Mars before heading to Vesta Vesta ranking: Third largest but second most massive asteroid - Russell identifies Vesta’s place among asteroids Ceres size ratio: About half as dense as Vesta - Used to infer Ceres’ higher water/ice content Ceres rocky interior estimate: About 400 kilometers in radius - Derived from Hubble-based size/shape and density analysis Ceres water layer estimate: About 90 kilometers - Possible global ocean/ice layer atop rocky interior Asteroid 2007 WD5 size: 50 meters wide - Potential Mars impactor discussed in the news segment Impact probability: 1 in 75 - Estimated chance of 2007 WD5 hitting Mars on January 30 Mars Scout delay: 2013 instead of 2011 - NASA conflict-of-interest issue pushes the mission to the next launch window Mars opposition date: December 24 - Bruce Betts notes Mars is at opposition and very bright Trivia contest deadline: December 31, 2007 at 2 p.m. Pacific - Deadline for the “longest gap since last explored” question Random space fact: Phobos 88 - Bruce notes the Soviet mission had two spacecraft, both of which ultimately failed Voyager 1 Saturn encounter: Titan flyby - Emily explains the key decision that altered Voyager 1’s trajectory

Pivotal Quotes: "“This is the day we were looking for a long time, and it's a real pleasure to see that spacecraft finally on its way to the destination it was built for.”" — Chris Russell: Reflecting on Dawn entering cruise toward Vesta "“We have basically a two-layer body, at least two layers, in which we have heavy material, a core sort of like the Earth, but probably very solid right now, and an outer mantle of silicate material.”" — Chris Russell: Explaining why Vesta is scientifically important "“The Titan flyby was worth it.”" — Emily Loctawala: Her conclusion on the Voyager 1 versus Pluto tradeoff

Implications: Dawn could transform understanding of asteroid formation and water distribution in the early solar system, while Voyager history shows how mission choices shape decades of follow-up science. The episode also underscores the fragility of mission schedules and the value of persistent skywatching.