Hi All

Early some of the guys discussed JIMO and its long ion-driven flight. Based on what I've read at the JIMO web-site the vehicle will take ~ 1041 days to achieve Jupiter capture, and 1210 days to enter Callisto orbit. There's not a lot on the JIMO web-page about its proposed trajectory aside from a single graphic.

1041 days is almost 3 years, but is better than Galileo's long "spiral" - which wasn't really that necessary except for a reluctance to use cryogenic upper-stages after "Challenger" blew - like it mattered since the Main Tank blew-up not the payload.

Ion-drives are tricky to calculate orbits for in the low-thrust range - too low for straight-line calculations. From the graphic JIMO uses about 2.2 kg/day of propellant and change of 3 x 10^-4 per day. Essentially low-thrust. A good approximation for the delta-vee [dV] between orbits is the difference in orbital velocities - hence to escape requires a dV of the initial orbital velocity, and likewise to arrive. Callisto's orbital velocity is ~ 8.2 km/s, and initial capture is achieved at ~ 12 million km from Jupiter so from the graphic the dV is ~ 5 km/s. The mass ratio of before and after implies the Isp is ~ +10,000 seconds, which is high for an ion-drive.

Actually looking at the graphic the plotted vector arrows imply the orbit might have been plotted iteratively, but the difference in dV is probably minor.

Even so old Pioneer 10 and 11 are still the fastest travellers to Jupiter - have we advanced then in 30 years?

A more adventureous Prometheus mission would have been a mixed-mode NTR-powered probe with landing and self-fuelling capabilties. Self-fuelling on any of the target moons would've been simple since they all have all icy crusts, and landing would not have required more than a thrust/mass ratio of 3, since the moons' gravities are all under 1.5 m/s^2. The total initial delta-vee would have been less than the ~ 40 km/s or so for JIMO. About 10 km/s to Callisto's surface for a probe launched towards Jupiter by giving itself a perigee kick after a fall from HEO.

The Mixed-Mode Multi-Moon Explorer [4MX] would be way sexier than JIMO and deliver some real chemical data from three icy moons - heck, it might even detect microfossils on Europa, and a landing in Europa's radiation shadow would extend its operational life-time indefinitely.

Adam