Starship Asterisk*

Chris Peterson wrote: ↑Thu Nov 30, 2023 9:41 pm

johnnydeep wrote: ↑Thu Nov 30, 2023 9:40 pm

Chris Peterson wrote: ↑Thu Nov 30, 2023 9:27 pm
It's clear what you mean, which is what's most important.

My (rigorous?) definition would be: orbit A is more circular than orbit B if e(A) < e(B), where e is the eccentricity.

That's certainly how I interpreted your wording.

johnnydeep wrote: ↑Thu Nov 30, 2023 9:40 pm

Chris Peterson wrote: ↑Thu Nov 30, 2023 9:27 pm

johnnydeep wrote: ↑Thu Nov 30, 2023 9:10 pm

Ok, but I guess I'll just have to be too colloquial regarding this particular terminology.

It's clear what you mean, which is what's most important.

My (rigorous?) definition would be: orbit A is more circular than orbit B if e(A) < e(B), where e is the eccentricity.

Chris Peterson wrote: ↑Thu Nov 30, 2023 9:27 pm

johnnydeep wrote: ↑Thu Nov 30, 2023 9:10 pm

Chris Peterson wrote: ↑Thu Nov 30, 2023 12:10 am
It's just too colloquial for my taste. Orbital elements are well defined. There is no concept of "circular" or "diameter". Every orbit is elliptical, where that ellipse is defined by an eccentricity and a semimajor axis. It happens that if the eccentricity is zero, the orbit is circular, and the semimajor axis is the same as the radius. But those aren't the best terms.

Ok, but I guess I'll just have to be too colloquial regarding this particular terminology.

It's clear what you mean, which is what's most important.

johnnydeep wrote: ↑Thu Nov 30, 2023 9:10 pm

Chris Peterson wrote: ↑Thu Nov 30, 2023 12:10 am

johnnydeep wrote: ↑Wed Nov 29, 2023 9:02 pm

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

It's just too colloquial for my taste. Orbital elements are well defined. There is no concept of "circular" or "diameter". Every orbit is elliptical, where that ellipse is defined by an eccentricity and a semimajor axis. It happens that if the eccentricity is zero, the orbit is circular, and the semimajor axis is the same as the radius. But those aren't the best terms.

Ok, but I guess I'll just have to be too colloquial regarding this particular terminology.

Chris Peterson wrote: ↑Thu Nov 30, 2023 12:10 am

johnnydeep wrote: ↑Wed Nov 29, 2023 9:02 pm

Chris Peterson wrote: ↑Wed Nov 29, 2023 4:23 pm
I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

It's just too colloquial for my taste. Orbital elements are well defined. There is no concept of "circular" or "diameter". Every orbit is elliptical, where that ellipse is defined by an eccentricity and a semimajor axis. It happens that if the eccentricity is zero, the orbit is circular, and the semimajor axis is the same as the radius. But those aren't the best terms.

johnnydeep wrote: ↑Wed Nov 29, 2023 9:02 pm

Chris Peterson wrote: ↑Wed Nov 29, 2023 4:23 pm

johnnydeep wrote: ↑Wed Nov 29, 2023 3:48 pm

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

Chris Peterson wrote: ↑Wed Nov 29, 2023 4:23 pm

johnnydeep wrote: ↑Wed Nov 29, 2023 3:48 pm

Chris Peterson wrote: ↑Wed Nov 29, 2023 12:23 am
The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

Eclectic Man wrote: ↑Wed Nov 29, 2023 4:33 pm

Chris Peterson wrote: ↑Wed Nov 29, 2023 2:04 pm

Eclectic Man wrote: ↑Wed Nov 29, 2023 12:48 pm Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

We don't know how far the camera is from the Moon, so we don't know what percentage of a full hemisphere we're seeing here. But how do you conclude that more than half of the surface is lit? The most we can see is half (and probably much less), and if the spacecraft is between Ganymede and the Sun, all of the visible surface would be lit.

Actually we do know how far the camera is from the moon, according to the data for the three images used to create the composite, the spacecraft altitude was just shy of 1,000,000km, so I would have thought that was far enough away to show the poles, as compared to the view of the Moon from the Earth, however, the terminator in the image does not reach to opposite edges of the disc, so it appears that significantly more than half of Ganymede is illuminated. I guess this is due to the fact that the image is a composite.

See: https://www.flickr.com/photos/kevinmgill/51238659798/
https://www.missionjuno.swri.edu/junoca ... C00001_V01
https://www.missionjuno.swri.edu/junoca ... C00002_V01
https://www.missionjuno.swri.edu/junoca ... C00003_V01

Even in the separate images 34C00001/2/3 the terminator does not reach opposite points of the disc.

Chris Peterson wrote: ↑Wed Nov 29, 2023 2:04 pm

Eclectic Man wrote: ↑Wed Nov 29, 2023 12:48 pm Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

We don't know how far the camera is from the Moon, so we don't know what percentage of a full hemisphere we're seeing here. But how do you conclude that more than half of the surface is lit? The most we can see is half (and probably much less), and if the spacecraft is between Ganymede and the Sun, all of the visible surface would be lit.

johnnydeep wrote: ↑Wed Nov 29, 2023 3:48 pm

Chris Peterson wrote: ↑Wed Nov 29, 2023 12:23 am

johnnydeep wrote: ↑Tue Nov 28, 2023 3:58 pm

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

bls0326 wrote: ↑Tue Nov 28, 2023 10:59 pm I noticed Ganymede bulge about the 11 o'clock position. One link I found https://phys.org/news/2015-04-jupiter-m ... bulge.html
includes "As noted by National Geographic, the bulge, or protuberance, is approximately the size of Ecuador and about half the height of Mount Kilimanjaro." It also notes "The researchers proposed at the conference that the bulge came about due to polar wandering—where ice built up at one of the poles and then slid over the top of an ocean below, to the equator"

Chris Peterson wrote: ↑Wed Nov 29, 2023 12:23 am

johnnydeep wrote: ↑Tue Nov 28, 2023 3:58 pm

jimbo48 wrote: ↑Tue Nov 28, 2023 2:03 pm

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

Eclectic Man wrote: ↑Wed Nov 29, 2023 12:48 pm Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

Chris Peterson wrote: ↑Wed Nov 29, 2023 12:23 am

johnnydeep wrote: ↑Tue Nov 28, 2023 3:58 pm

jimbo48 wrote: ↑Tue Nov 28, 2023 2:03 pm

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

johnnydeep wrote: ↑Tue Nov 28, 2023 3:58 pm

jimbo48 wrote: ↑Tue Nov 28, 2023 2:03 pm

Ann wrote wrote:
Really? How is that possible?
Ann

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

Whiskybreath wrote: ↑Tue Nov 28, 2023 12:03 pm An interesting detail:
Clipboard01.jpg

jimbo48 wrote: ↑Tue Nov 28, 2023 2:03 pm

Ann wrote wrote:

APOD Robot wrote:
The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.

Really? How is that possible?
Ann

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

APOD Robot wrote:
The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.

The featured image was captured in 2021 by NASA's robotic Juno spacecraft when it passed by the immense moon. The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.