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I’m trying to figure out why the alignment was so important. Wouldn’t it be possible to get a gravity assist in any alignment - albeit just taking a little lon

I’m trying to figure out why the alignment was so important. Wouldn’t it be possible to get a gravity assist in any alignment - albeit just taking a little longer to ping-pong across the system?

ericbarrett 2y ago
The alignment greatly reduced the amount of energy/propellant/weight to launch the Voyager missions with all four of the outer planets on the menu. Alignments that allow you to reach a smaller set of the outer planets with the same budget are more common: years or decades. (Edit) Another thought, since you mentioned time—numerical computing power and the math required to exercise it have advanced greatly since the 1970s, and it's likely that some of the trajectories and maneuvers feasible (again with the same fuel budget) today, even if they took 100+ years to complete, weren't even calculable back then.
- deleted 2y ago
  [deleted]
vikingerik 2y ago
The scenario where a gravity assist doesn't work is if turning sharply enough would require your minimum planetcentric approach distance to be less than the radius of the planet - you'd crash into it instead. This is why Voyager 2 couldn't also do Pluto - it would have needed to change course by roughly 90º at Neptune, which would have required going closer to the center of Neptune than Neptune's own radius. The most unusual gravity-assist alignment that we did was for Pioneer 11 going from Jupiter to Saturn. The encounters were separated by roughly 120º of heliocentric longitude. Pioneer 11 used Jupiter to bend its path "up" out of the ecliptic plane and encountered Saturn on the way back "down". Nowadays we wouldn't bother doing that (we'd wait for a more direct launch window instead), but the purpose of this was to get preliminary Jupiter and Saturn encounters done in time before Voyager's launch window for the grand tour alignment.
- westurner 2y ago
  Could Voyager have reduced velocity, glanced off Neptune, waited for the return path on a narrow elliptical orbit, and then boosted to effectively make the 90° turn to Pluto at that time? Was that impossible given its Neptune approach trajectory, or would glancing off and waiting have been more fuel? And, why didn't this vortical model that includes the forward velocity of the sun make a difference for Voyager's orbital trajectory and current position relative to earth? https://news.ycombinator.com/item?id=42159195 https://news.ycombinator.com/item?id=42159195 : > "The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU https://youtube.com/watch?v=0jHsq36_NTU
  - vikingerik 2y ago
    Breaking that down: Voyager itself couldn't have reduced velocity, it had nowhere near enough reaction mass to do that. Hypothetically a spacecraft could but you might be talking about orders of magnitude more reaction mass. (Which means multiples more of the fuel to launch and accelerate that mass itself, which could quickly escalate beyond any chemical rocket capabilities.) It also likely wasn't possible to get to Pluto on some future Pluto orbital pass. The limiting factor is likely that Voyager's incoming trajectory to Neptune was already too far beyond solar escape velocity to get into that narrow elliptical orbit you propose. (You'd have to slingshot so close to Neptune's center that you'd hit the planet instead.) Designing from the beginning to come in slower to Neptune and adjust to encounter Pluto on some future Pluto orbital pass was probably possible, but yeah you might be talking about time scales of Pluto's entire orbit or even multiples of that. (We do similar things for inner solar system missions, like several encounters with Venus separated by multiple Venus-years, but that's on the order of single-digit years and not hundreds.) The common answer to a lot of these outlandish slingshot questions is usually, yes it's eventually possible by orbital mechanics, but it gets so complicated and lengthy that you may as well just build another separate spacecraft instead. We talk about Voyager's grand tour alignment because it's captivating, but realistically if that hadn't happened we would have just done separate Jupiter-Uranus and Jupiter-Neptune missions instead. The sun's motion relative to the galaxy doesn't matter for any of this - nothing else in the galaxy is remotely close enough to affect anything, the nearest star is still over 1000x Voyager's distance.
downrightmike 2y ago
Speed run hacking
exmadscientist 2y ago
The alignment was important because it allowed visiting four planets with one spacecraft. So you only had to launch one spacecraft. (We launched two anyway.) If you are willing to launch four spacecrafts to visit four planets, the alignment restrictions are much relaxed. You do need to be careful about your launch window to get a nice boost, but it's measured in years between windows, not so much centuries.
- khuey 2y ago
  > The alignment was important because it allowed visiting four planets with one spacecraft. So you only had to launch one spacecraft. (We launched two anyway.) IIRC the second probe was mainly intended as a backup to the first one, but visiting Titan and visiting Uranus/Neptune were mutually exclusive, and visiting Titan was higher priority, so if the first probe succeeded the backup could be (and was) sent on the four planet track.
Sharlin 2y ago
No, there's no pingponging. There's only so much momentum you can exchange with a planet, because there's only so close a flyby that you can make before it stops being a flyby and becomes a fly-into.
relistan 2y ago
Ignoring the feasibility of the physics, imagine trying to calculate all the resulting trajectories to point antennas at to talk to the probe. With computers and equipment of the 1970s.

I’m trying to figure out why the alignment was so important. Wouldn’t it be possible to get a gravity assist in any alignment - albeit just taking a little longer to ping-pong across the system?

ericbarrett 2y ago

The alignment greatly reduced the amount of energy/propellant/weight to launch the Voyager missions with all four of the outer planets on the menu. Alignments that allow you to reach a smaller set of the outer planets with the same budget are more common: years or decades. (Edit) Another thought, since you mentioned time—numerical computing power and the math required to exercise it have advanced greatly since the 1970s, and it's likely that some of the trajectories and maneuvers feasible (again with the same fuel budget) today, even if they took 100+ years to complete, weren't even calculable back then.

deleted 2y ago
[deleted]

vikingerik 2y ago

The scenario where a gravity assist doesn't work is if turning sharply enough would require your minimum planetcentric approach distance to be less than the radius of the planet - you'd crash into it instead. This is why Voyager 2 couldn't also do Pluto - it would have needed to change course by roughly 90º at Neptune, which would have required going closer to the center of Neptune than Neptune's own radius. The most unusual gravity-assist alignment that we did was for Pioneer 11 going from Jupiter to Saturn. The encounters were separated by roughly 120º of heliocentric longitude. Pioneer 11 used Jupiter to bend its path "up" out of the ecliptic plane and encountered Saturn on the way back "down". Nowadays we wouldn't bother doing that (we'd wait for a more direct launch window instead), but the purpose of this was to get preliminary Jupiter and Saturn encounters done in time before Voyager's launch window for the grand tour alignment.

westurner 2y ago
Could Voyager have reduced velocity, glanced off Neptune, waited for the return path on a narrow elliptical orbit, and then boosted to effectively make the 90° turn to Pluto at that time? Was that impossible given its Neptune approach trajectory, or would glancing off and waiting have been more fuel? And, why didn't this vortical model that includes the forward velocity of the sun make a difference for Voyager's orbital trajectory and current position relative to earth? https://news.ycombinator.com/item?id=42159195 https://news.ycombinator.com/item?id=42159195 : > "The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU https://youtube.com/watch?v=0jHsq36_NTU
- vikingerik 2y ago
  Breaking that down: Voyager itself couldn't have reduced velocity, it had nowhere near enough reaction mass to do that. Hypothetically a spacecraft could but you might be talking about orders of magnitude more reaction mass. (Which means multiples more of the fuel to launch and accelerate that mass itself, which could quickly escalate beyond any chemical rocket capabilities.) It also likely wasn't possible to get to Pluto on some future Pluto orbital pass. The limiting factor is likely that Voyager's incoming trajectory to Neptune was already too far beyond solar escape velocity to get into that narrow elliptical orbit you propose. (You'd have to slingshot so close to Neptune's center that you'd hit the planet instead.) Designing from the beginning to come in slower to Neptune and adjust to encounter Pluto on some future Pluto orbital pass was probably possible, but yeah you might be talking about time scales of Pluto's entire orbit or even multiples of that. (We do similar things for inner solar system missions, like several encounters with Venus separated by multiple Venus-years, but that's on the order of single-digit years and not hundreds.) The common answer to a lot of these outlandish slingshot questions is usually, yes it's eventually possible by orbital mechanics, but it gets so complicated and lengthy that you may as well just build another separate spacecraft instead. We talk about Voyager's grand tour alignment because it's captivating, but realistically if that hadn't happened we would have just done separate Jupiter-Uranus and Jupiter-Neptune missions instead. The sun's motion relative to the galaxy doesn't matter for any of this - nothing else in the galaxy is remotely close enough to affect anything, the nearest star is still over 1000x Voyager's distance.

downrightmike 2y ago

Speed run hacking

exmadscientist 2y ago

The alignment was important because it allowed visiting four planets with one spacecraft. So you only had to launch one spacecraft. (We launched two anyway.) If you are willing to launch four spacecrafts to visit four planets, the alignment restrictions are much relaxed. You do need to be careful about your launch window to get a nice boost, but it's measured in years between windows, not so much centuries.

khuey 2y ago
> The alignment was important because it allowed visiting four planets with one spacecraft. So you only had to launch one spacecraft. (We launched two anyway.) IIRC the second probe was mainly intended as a backup to the first one, but visiting Titan and visiting Uranus/Neptune were mutually exclusive, and visiting Titan was higher priority, so if the first probe succeeded the backup could be (and was) sent on the four planet track.

Sharlin 2y ago

No, there's no pingponging. There's only so much momentum you can exchange with a planet, because there's only so close a flyby that you can make before it stops being a flyby and becomes a fly-into.

relistan 2y ago

Ignoring the feasibility of the physics, imagine trying to calculate all the resulting trajectories to point antennas at to talk to the probe. With computers and equipment of the 1970s.