Hi All

Posted by alcaray on 12/24/2003 9:55:31 AM

In Reply to: Good news on Nexis/Jimo posted by Buck Galaxy on 12/24/2003 9:37:17 AM

of the mission profile.

Surely there's no way to use the ion thrusters to get into Jupiter orbit?

And do they intend to move from one moon to the next using only ions? Won't that take for freaking ever?

[What's the difficulty? Sure it will take time but so did bouncing Cassini and Galileo around the inner Sol System. An ion-drive will work just fine around Jupiter. For orbits under thrust if the acceleration is lower than local gravity the spacecraft will merely spiral in/out from the central body and the total delta-vee will be roughly the difference in orbital velocities between the initial and final orbits. For an escape orbit the delta-vee is the initial orbital velocity - though at some point the acceleration and gravity will finally equalise and the path becomes effectively a straight line.]

[JIMO will take ~ 8 years to reach Jupiter and the total delta-vee to Callisto is ~ 32.7 km/s, so the acceleration is ~ 1.3 x 10^-4 m/s^2 on average. From Callisto-to-Ganymede then Ganymede-to-Europa it will need another ~ 6 km/s, hence about 40 km/s dV total. Acceleration will go higher as reaction mass is used up later in the mission, though the increase will be roughly x2 at most. That's the mass-ratio.]

[All in all there are no show-stoppers for ion-drive and JIMO will test the reliability of low-thrust ion propulsion and space-reactor stability for ~ 11 years all up. I reckon it needs a good telescope [Hipparcos class] on it to use the increased parallax during the long cruise. Someone should lobby for that one!]

Adam