Star 48b Third-stage Motor, Leaving the solar system Options

[Guido]

I suppose the STAR 48B third-stage, which put New Horizons on its trajectory towards Jupiter, follows about the same flight-path as the New Horizons spacecraft itself. If this is the case, will it too in the end leave our solar system?
Or has it been deflected after seperation from the spacecraft?

[djellison]

Obviously retro reflectors work out to 400k km, but how far out would they work beyond that? Perhaps one could do radar reflection using Arecbo / DSN instead?

Also - there's no real way of knowing, (perhaps less so with the solid 48b than a liquid upper stage) what potential small forces are being generated by outgassing of remaining fuel, its exact mass etc.

Doug

[Comga]

Obviously retro reflectors work out to 400k km, but how far out would they work beyond that?  Perhaps one could do radar reflection using Arecbo / DSN instead?

Doug

Sorry, but this won't work. The Pioneer anomoly is a grand distance effect. 400M km is barely at Jupiter distance, but that would diminish the return from a Lunar Retroreflector Array by a factor of one trillion (1E12) as it goes as R^4. At Neptune's distance of 4G km the signal would be down by a factor of 1E16. And in order to keep the mass down, the array would have to be more than a factor of ten smaller, which could cut into both legs reducing the signal by ANOTHER factor of 100 at best. No conceovable amount of technological progress would make this signal detectable. Plus one would have to put the retro on the rocket nozzle end to face back at Earth. A radio retroreflector would have to be quite large to have any effect, and could not be carried.

A spent stage is just innert mass on an uncontrolled trajectory. It would be very hard to find a use for this mass that would not have imposed additional requirements on the mission. New Horizons survived by avoiding distractions and extraneous burdens.

Other missions have used their boosters as photographic targets, and NH knows its velocity relative to the spent third stage with high precision, and would be looking at the sunlit side. Don't know if imaging it would serve any purpose.

[AndyG]

...400M km is barely at Jupiter distance, but that would diminish the return from a Lunar Retroreflector Array by a factor of one trillion (1E12) as it goes as R^4.

Is that right? I can see that a normal return (radar, for example) from an astronomical body would obey R^4 laws, but the whole point of a retroreflector is to minimise divergence of any reflected signals. So, surely a well made retroreflector should be nearer R^2 than R^4 - a big difference at large ranges, and a more practical option than you seem to suggest?

Andy

[ugordan]

So, surely a well made retroreflector should be nearer R^2 than R^4 - a big difference at large ranges, and a more practical option than you seem to suggest?

Nope, a flat retroreflector would still follow the R^4 law. Consider that a flat reflector receives 1/4 of the power at twice the distance and it also presents only 1/4 of the angular surface area seen from Earth so that combines to 1/16 of the power received back than at the original "unit" distance.
I can imagine that if you had a spherical reflector whose curvature would follow the curvature of the sphere centered on the light source on the Earth, you could get a R^2 return power function as it would focus all received light/radio signal back at the source.
Then again, that's purely theoretical reasoning and you'd have to change the curvature of the reflector with increasing distance and keep it really precisely pointed back that it's probably undoable.

[Bob Shaw]

When the LRRR hare was first set running I simply assumed that there'd be some sort of spatially distributed array of cubical LRRR modules all over the exhausted stage, so that it's orientation was not particularly important (but which could be obtained by applying some sort of statistical process to the light curve of the stage). Such cubical LRRRs would reflect internally and would *not* need to be pointed in any more than a general direction.

The logarithmic issues still apply, of course, but imagine the possibilities inherent in the US military experiments in high power lasers, then add an array of optical telescopes (seeking to resolve only a point source, remember, where optical arrays *are* good).

Plainly, though, as Alan Stern pointed out, every bit of mass left on the booster is something left off the spacecraft. Still, let's assume that NH2 flies, and there's (for whatever reason) some spare mass budget - the experiment could be quite interesting... ...and cheap!

Bob Shaw