Pioneer Spacecraft First Into The Asteroid Belt Options

[Guest_PhilCo126_*]

Launch Pioneer 10 ( 02 March 1972 )
Pioneer 10 entered the Asteroidbelt in July 1972 and emerged in February 1973 ... passing safely as we know ... during 7 months ... Jupiter Flyby in November 1973.
So we had Jovian flyby 7 months after passing Asteroid belt!


Launch Pioneer 11 ( 05 April 1973 )
Pioneer 11 entered the Asteroidbelt in March 1974 and emerged in ??? ( September 1974 ) ... encountered Jupiter in December 1974.
Already 3 months after it cleared the Asteroid belt ?
( went on for flyby of Saturn in August 1979 )

Does someone have exact dates for both Pioneers' milestones ?


Best regards,
Philip

[edstrick]

http://cfa-www.harvard.edu/iau/lists/InnerPlot.html
http://cfa-www.harvard.edu/iau/lists/InnerPlot2.html
http://cfa-www.harvard.edu/iau/lists/OuterPlot.html

linked to from
http://pdssbn.astro.umd.edu/outreach/location.html
Show plots of more or less all natural objects in the solar system, color coded as to type.

The asteroid belt is clearly defined by an instantaneous snapshot as a torus with a rather well defined inner edge a little way outside Mars orbit and a more diffuse outer edge at about 2/3 the radius of Jupiter's orbit. The presence of additional scattered objects like near-earth asteroids <in red>, asteroids in (I think) 3/2 jupiter resonant orbits <3 lobes of green dots beyond the outer belt edge out to Jupiter's orbit> and Trojan asteroids <blue bananna shaped clusters of dots on jupiter's orbit -- jupiter is at about 8 o'clock>, does not change that there is a real belt that's well defined.

BUT: The objects in the belt are so small and the belt is so big, and vertically extended into a donut, not a flat disk, that anywhere at random in the belt, you'd usually only see a few not very bright stars that are asteroids in the distance.

Mariner 6 and 7 did fast flybys of Mars in 69, as the Atlas Centaur could launch considerably heavier spacecraft than the Mariners, and they got a modest gravity assist from the posigrade flyby of Mars. I don't have an orbital plot at hand, but they undoubtably went a short ways into the inner belt.

[Bob Shaw]

I knew about Mariners 6 and 7, but did the rest just 'kiss' Mars and back off, or what? For example, the Soviet 'bad opposition' flights might have traded time and speed with vehicle mass to get none-too-reliable spacecraft there sooner, with resulting asteroid belt incursions.

I suppose a naive test would be to compare flight times to Mars - anything faster than a minimum transfer Hohmann time would have to have had at least an initial Solar orbit which would have extended beyond Mars. The Mars encounter itself would then further modify that trajectory, as would any course corrections via on-board rocketry.

Bob Shaw

[ljk4-1]

If Pioneer 11 had been sent to Uranus, what do you think it would have discovered with its limited resources? What year would Pioneer 11 have arrived? How would it have affected Voyager 2's mission there?

[ugordan]

If Pioneer 11 had been sent to Uranus, what do you think it would have discovered with its limited resources?  What year would Pioneer 11 have arrived?  How would it have affected Voyager 2's mission there?

I don't know the exact figures, but it just might be the case Pioneer 11 never could have safely gone to Uranus. Obviously, it was travelling much slower than the Voyagers did and it would have needed a more powerful slingshot at Saturn to send it towards Uranus. That would mean passing closer to Saturn, possibly within the main ring sytem which would pose a great hazard to the spacecraft. It would be interesting to work out how closer Pioneer 11 would have had to come to Saturn as it's not just a matter of gaining speed at Saturn, the more you gain, the greater the trajectory bending is and at one point your arrival point at Uranus will be ahead of the planet itself. That forces you back to select a slower trajectory (which could still have been safe) and it's quite possible (IIRC, even likely) Voyager 2 would have overtaken Pioneer 11 before reaching Uranus anyway.
Naturally, this is just arm-waving reasoning on my side.

In any case, Voyager 2 couldn't do much in terms of optimizing approach distance to Uranus as it basically had a fixed aimpoint that would slingshot it onwards to Neptune. Only possible thing would be timing the encounter to get a slightly closer approach to one of the more interesting satellites there. Then again, Pioneer's "camera" was so crappy it wouldn't see squat let alone a difference between a boring and intriguing moon...

[Planet X]

I don't know the exact figures, but it just might be the case Pioneer 11 never could have safely gone to Uranus. Obviously, it was travelling much slower than the Voyagers did and it would have needed a more powerful slingshot at Saturn to send it towards Uranus. That would mean passing closer to Saturn, possibly within the main ring sytem which would pose a great hazard to the spacecraft. It would be interesting to work out how closer Pioneer 11 would have had to come to Saturn as it's not just a matter of gaining speed at Saturn, the more you gain, the greater the trajectory bending is and at one point your arrival point at Uranus will be ahead of the planet itself. That forces you back to select a slower trajectory (which could still have been safe) and it's quite possible (IIRC, even likely) Voyager 2 would have overtaken Pioneer 11 before reaching Uranus anyway.
Naturally, this is just arm-waving reasoning on my side.

In any case, Voyager 2 couldn't do much in terms of optimizing approach distance to Uranus as it basically had a fixed aimpoint that would slingshot it onwards to Neptune. Only possible thing would be timing the encounter to get a slightly closer approach to one of the more interesting satellites there. Then again, Pioneer's "camera" was so crappy it wouldn't see squat let alone a difference between a boring and intriguing moon...

Believe it or not, the original 1975 Ames study on the Pioneer 11 Uranus option projected December 1985 as the earliest possible arrival time at Uranus. Arrival would have been timed in such a way that closest approach to Uranus would occur either just before or sometime after superior conjunction between Sun, Earth, and Uranus (which was around December 9th of that year).

I've gotten a few different versions of how Pioneer 11 could have reached Uranus over the years. Once, I read that the Saturn encounter would have been rather distant, beyond the A Ring. Another source hinted at Pioneer 11 flying through the Cassini Division before closest approach. Finally, yet another source hinted at Pioneer 11 flying inside the D ring. Both of the latter sources had Pioneer 11 flying to within 3000 miles of Saturn's cloudtops. To me, only the latter 2 possibilities make sense.

At any rate, Pioneer 11 would have actually done pretty good at Uranus. It's particle sensors would have easily detected the "new" rings discovered by Voyager 2 and perhaps even the 2 recently discovered rings as well. Also, Pioneer 11 would have discovered Uranus's magnetic field. Since Uranus is bland anyway, fuzzy images from an old spacecraft wouldn't have been as disappointing as many might think. One last thing, chances are good that Pioneer 11 would have been sent super close to Uranus, since there's no way in hell they would have tried the Neptune option.

Finally, I should note that the Pioneer principal investigators actually voted 11 to 1 in favor of flying Pioneer 11 inside the D ring and possibly even sending it to Uranus. It was only the director of planetary programs at that time, Tom Young, who demanded Pioneer 11 to fly outside the rings. Had it not been for him, Voyager 2 just might not have been first to Uranus! Later!

J P

[ugordan]

I've gotten a few different versions of how Pioneer 11 could have reached Uranus over the years.  Once, I read that the Saturn encounter would have been rather distant, beyond the A Ring.  Another source hinted at Pioneer 11 flying through the Cassini Division before closest approach.  Finally, yet another source hinted at Pioneer 11 flying inside the D ring.  Both of the latter sources had Pioneer 11 flying to within 3000 miles of Saturn's cloudtops.  To me, only the latter 2 possibilities make sense.

Well, I don't know about the Cassini Division as it turned out to be not as empty as it was thought. It would have been interesting to watch that fly-through unfold, though

[tedstryk]

It could have gone to Uranus and Neptune, but for one thing, the bean counters didn't see the point, since it would be beaten there by Voyager. But the biggest thing that kept any big push from happening was that no one fathomed how long we would be able to track the Pioneers, and how long they would be able to send data back.

[Planet X]

It could have gone to Uranus and Neptune, but for one thing, the bean counters didn't see the point, since it would be beaten there by Voyager.

I've performed the simulation of sending Pioneer 11 on to Uranus via Gravity Simulator. Pioneer 11 ended up flying to within a distance of 2058 km from Saturn's cloud tops, with a top speed of 130,086 kmph at periapsis. I then typed out the data on an XYZ coodinate file for graphical display in Celestia. As it turned out, the ring plane crossing occurred inside the D ring just over eight minutes after periapsis, at a distance of 3282 km from Saturn's cloudtops. Pioneer 11's encounter with Uranus took place on December 2, 1985. I deliberately sent the spacecraft to within 81,000 km of Uranus's cloud tops, as this was the distance chosen for Voyager 2's January 1986 encounter. So yes, Pioneer 11 beat Voyager 2 to Uranus!

Just for fun, I also sent Pioneer 11 to Neptune in a different simulation of it's Uranus encounter. To reach Neptune, Pioneer 11's encounter with Uranus would have been more distant, beyond the orbit of Miranda. The fake Neptune encounter took place sometime in April 1990, clearly an impractical option! Later!

J P