Solar System geometry - Neptune in solar conjunction
August 30, 2015 10:59 AM Subscribe
I'm writing an SF story which involves something akin to a space station/industrial complex in orbit around Neptune. I need to know whether Neptune ever goes completely behind the Sun when seen from Earth and if so for how long, or whether the line of sight between Earth and Neptune just grazes the edge of the Sun.
I've looked at the three conjunctions closest in time to now, and they seem to show Neptune getting within 45-50' of the sun; is that always the case? Bonus question: what would the Shapiro Delay be for a radio signal along that path?
The setting for this story will be in an era of asteroid mining, although that's not a major part of the plot. While there's quite a lot of research/speculation published on this, and I don't intend to go into much detail in the story, I would appreciate being pointed to anything factual or fictional that does discuss the notion of living and working in orbit around an outer gas giant. They don't get enough love!
I've looked at the three conjunctions closest in time to now, and they seem to show Neptune getting within 45-50' of the sun; is that always the case? Bonus question: what would the Shapiro Delay be for a radio signal along that path?
The setting for this story will be in an era of asteroid mining, although that's not a major part of the plot. While there's quite a lot of research/speculation published on this, and I don't intend to go into much detail in the story, I would appreciate being pointed to anything factual or fictional that does discuss the notion of living and working in orbit around an outer gas giant. They don't get enough love!
I don't have answers for you, but I do have a couple things for you to think about.
First, NASA missions usually have to protect for a period before and after actual conjunction, when the probe is actually behind the sun. This is because radio antennas have beamwidth -- they are looking at a section of sky that happens to include the target of interest (probe) or the source of interference (sun). Think of a flashlight beam, and how the beam is brightest at the center and trails off away from the center, but is not off. So the probe could be well away from the sun and still be invisible, because the sun's overwhelming power was still getting into the antenna via the edge of the beam pattern. If you look into actual probe conjunction times, e.g. for the rovers/orbiters at Mars, or Cassini at Saturn, you should find that the blackout periods are much longer than the actual time behind the sun.
Second, when you ran that calculation that showed that Neptune was getting within 45-50 arc-minutes of the sun, consider that the calculation may or may not have considered the radius of the sun itself. That 45-50' might have been from the center of the sun.
posted by intermod at 1:16 PM on August 30, 2015 [1 favorite]
First, NASA missions usually have to protect for a period before and after actual conjunction, when the probe is actually behind the sun. This is because radio antennas have beamwidth -- they are looking at a section of sky that happens to include the target of interest (probe) or the source of interference (sun). Think of a flashlight beam, and how the beam is brightest at the center and trails off away from the center, but is not off. So the probe could be well away from the sun and still be invisible, because the sun's overwhelming power was still getting into the antenna via the edge of the beam pattern. If you look into actual probe conjunction times, e.g. for the rovers/orbiters at Mars, or Cassini at Saturn, you should find that the blackout periods are much longer than the actual time behind the sun.
Second, when you ran that calculation that showed that Neptune was getting within 45-50 arc-minutes of the sun, consider that the calculation may or may not have considered the radius of the sun itself. That 45-50' might have been from the center of the sun.
posted by intermod at 1:16 PM on August 30, 2015 [1 favorite]
Any two different orbital planes will have an intersection line, and if Earth and Neptune both find themselves on that line, on opposite sides of the sun, you'll have your spot. For 2 spots on either body's orbit, the sun would block direct line-of-sight between them for some period of time.
An Earth-Neptune phase angle (angle formed by Earth-Sun-Neptune) of 180 would happen almost annually (it would take slightly longer than a year, on average), so it would just be a matter of working out how often that occurs when the Earth and Neptune are on their orbital-plane-intersection line.
posted by Sunburnt at 2:27 PM on August 30, 2015 [1 favorite]
An Earth-Neptune phase angle (angle formed by Earth-Sun-Neptune) of 180 would happen almost annually (it would take slightly longer than a year, on average), so it would just be a matter of working out how often that occurs when the Earth and Neptune are on their orbital-plane-intersection line.
posted by Sunburnt at 2:27 PM on August 30, 2015 [1 favorite]
i am (well, was) as astronomer, but really for this kind of thing you're better finding a good amateur. my understanding of http://www.nakedeyeplanets.com/neptune.htm is that it's going to be pretty rare. you could maybe write to the owner of that site and ask when will be the next time neptune is hidden by the sun.
posted by andrewcooke at 3:05 PM on August 30, 2015 [1 favorite]
posted by andrewcooke at 3:05 PM on August 30, 2015 [1 favorite]
Response by poster: Thanks. I haven't read Trouble on Triton since I was a teenager - I shall go back to it, because, well, Delaney.
There is going to be a lot about radio (EM engineering, to be precise, rather than bog standard comms) in the story, because that's what I love, and it's going to be an enjoyable challenge making it anything other than dull old nerdwiffery. Beamwidth (and associated issues, of which there are many) is going to be important in the basic calculations I'm going through to see whether one of the central plot devices can be made to work without abusing the spirit of James Clerk Maxwell too outrageously. In particular, the conjunction window changes quite a lot with respect to the transmission power in the link budget. This is SF, and if I want a terawatt transmitter then by gum I'm going to order one up.
I'll drop a line to that website owner. The intersection line model is useful and helpful, but I don't know how much the Sun deviates from being the focus of the orbits. I keep getting lost in discussions of barycenters and the like... even the simple bits of the Solar System are complicated.
posted by Devonian at 4:52 PM on August 30, 2015
There is going to be a lot about radio (EM engineering, to be precise, rather than bog standard comms) in the story, because that's what I love, and it's going to be an enjoyable challenge making it anything other than dull old nerdwiffery. Beamwidth (and associated issues, of which there are many) is going to be important in the basic calculations I'm going through to see whether one of the central plot devices can be made to work without abusing the spirit of James Clerk Maxwell too outrageously. In particular, the conjunction window changes quite a lot with respect to the transmission power in the link budget. This is SF, and if I want a terawatt transmitter then by gum I'm going to order one up.
I'll drop a line to that website owner. The intersection line model is useful and helpful, but I don't know how much the Sun deviates from being the focus of the orbits. I keep getting lost in discussions of barycenters and the like... even the simple bits of the Solar System are complicated.
posted by Devonian at 4:52 PM on August 30, 2015
yeah, you want someone who has some good planetarium software and knows how to use it. doing it from theory is going to be a pain. instead, you want to some lines on the sky and see where they cross. hopefully that's what they can do.
posted by andrewcooke at 6:04 PM on August 30, 2015
posted by andrewcooke at 6:04 PM on August 30, 2015
Best answer: Here is a wikipedia page talking about the related/similar phenomenon of Transit of Earth from Neptune. A transit happens when the Earth passes directly in front of the sun from the point of view of Neptune.
These transits in clusters about every 80 years, and during the cluster there is a transit about once a year for a period of about 6 or 8 years.
The transit clusters happen during the years when Neptune's orbit brings Neptune approximately in the plane of the Earth's orbit. The plane of Earth's orbit is called the ecliptic. So when Neptune's orbit crosses the ecliptic, you will get this series of Earth transits of the Sun.
All this is relevant to your question, because approx. 6 months after each transit of the Earth as seen from Neptune, there will be the complementary event: An occultation of Earth by the Sun as seen from Neptune. This is where the Earth disappears behind the sun, as seen from Neptune.
There were Transits of Earth as seen from Neptune in late July/early August 2001-2006. So there would be corresponding occultations of Earth as seen from Neptune in late February/early March in 2002-2006, and perhaps also in 2001 and 2007 (these will be actual occultations, or 'grazers', or just close calls depending on exact alignment of Neptune).
Similarly there will be transits of Earth in late January/early February in 2081-2088, so there will be occultations of Earth in late July/early February in 2081-2087 and perhaps in 2088 also.
Note that the transit of January 23, 2081, is a grazing transit. Similarly there will be grazing occultations, though without some calculation etc I couldn't say exactly when they will be. But very certainly they do happen.
I can't find a page that summarizes the occultations of Earth as seen from Neptune as neatly as the wikipedia page that summarizes the transits of Earth as seen from Neptune, but that is the term you are looking for: Occultations of Earth as seen from Neptune.
Also, be aware that the sun is itself a powerful radio source. We routinely lose communications with satellite that pass near the sun--doesn't have to be a total occultation, just 'near enough' will do it. And the satellite doesn't have to pass behind the sun, either--in front of it is just as bad for the purposes of radio reception. Or even just quite near it.
This would be a similar situation for Neptunians trying to communicate via radio with earth, whenever Earth is near the sun--whether in front of, behind, or just quite close to the Sun, problems could be/will be encountered. Exactly how severe the problems would be, you'd have to ask someone who is more of an expert in that area.
Summarizing: The earth will pass close enough to the Sun to cause radio communications problems with Neptune every 6 months for a period of 6 or 8 years (or maybe even 8-12 years, depending on how close to the sun we need to be before radio problems show up); these periods of near-exact alignment come in clusters every 80 years or so; during the 6-8 year period the exact alignment will vary--so you'll have Earth just missing the top of the Sun, just hitting the top, going more through the middle, passing nearer the bottom, just missing the bottom, etc, as Neptune takes on slightly different alignments WRT to Earth's orbital plane in each succeeding year.
posted by flug at 7:54 PM on August 30, 2015 [4 favorites]
These transits in clusters about every 80 years, and during the cluster there is a transit about once a year for a period of about 6 or 8 years.
The transit clusters happen during the years when Neptune's orbit brings Neptune approximately in the plane of the Earth's orbit. The plane of Earth's orbit is called the ecliptic. So when Neptune's orbit crosses the ecliptic, you will get this series of Earth transits of the Sun.
All this is relevant to your question, because approx. 6 months after each transit of the Earth as seen from Neptune, there will be the complementary event: An occultation of Earth by the Sun as seen from Neptune. This is where the Earth disappears behind the sun, as seen from Neptune.
There were Transits of Earth as seen from Neptune in late July/early August 2001-2006. So there would be corresponding occultations of Earth as seen from Neptune in late February/early March in 2002-2006, and perhaps also in 2001 and 2007 (these will be actual occultations, or 'grazers', or just close calls depending on exact alignment of Neptune).
Similarly there will be transits of Earth in late January/early February in 2081-2088, so there will be occultations of Earth in late July/early February in 2081-2087 and perhaps in 2088 also.
Note that the transit of January 23, 2081, is a grazing transit. Similarly there will be grazing occultations, though without some calculation etc I couldn't say exactly when they will be. But very certainly they do happen.
I can't find a page that summarizes the occultations of Earth as seen from Neptune as neatly as the wikipedia page that summarizes the transits of Earth as seen from Neptune, but that is the term you are looking for: Occultations of Earth as seen from Neptune.
Also, be aware that the sun is itself a powerful radio source. We routinely lose communications with satellite that pass near the sun--doesn't have to be a total occultation, just 'near enough' will do it. And the satellite doesn't have to pass behind the sun, either--in front of it is just as bad for the purposes of radio reception. Or even just quite near it.
This would be a similar situation for Neptunians trying to communicate via radio with earth, whenever Earth is near the sun--whether in front of, behind, or just quite close to the Sun, problems could be/will be encountered. Exactly how severe the problems would be, you'd have to ask someone who is more of an expert in that area.
Summarizing: The earth will pass close enough to the Sun to cause radio communications problems with Neptune every 6 months for a period of 6 or 8 years (or maybe even 8-12 years, depending on how close to the sun we need to be before radio problems show up); these periods of near-exact alignment come in clusters every 80 years or so; during the 6-8 year period the exact alignment will vary--so you'll have Earth just missing the top of the Sun, just hitting the top, going more through the middle, passing nearer the bottom, just missing the bottom, etc, as Neptune takes on slightly different alignments WRT to Earth's orbital plane in each succeeding year.
posted by flug at 7:54 PM on August 30, 2015 [4 favorites]
FYI the free Celestia software is pretty good for visualizing things like this, and pretty easy to use: http://www.shatters.net/celestia/
Just for example, I set the time for Jan 23, 2081, and view from Neptune to Sun, and sure enough there is a nice grazing transit of Earth to view. July 26-28, 2081, there is a nice (and rather longer--about 30 hour-long) occultation of Earth by the Sun. Etc.
posted by flug at 8:10 PM on August 30, 2015 [1 favorite]
Just for example, I set the time for Jan 23, 2081, and view from Neptune to Sun, and sure enough there is a nice grazing transit of Earth to view. July 26-28, 2081, there is a nice (and rather longer--about 30 hour-long) occultation of Earth by the Sun. Etc.
posted by flug at 8:10 PM on August 30, 2015 [1 favorite]
Response by poster: Flug, you nailed it. In particular, thanks for unveiling the clustering, which I suspected may happen (because that's the way those sort of things tend to happen) but with no idea of the periods or duration. And that periodicity is just perfect.
I imagine that by the time we're mining asteroids and building big things in Neptunian orbit, we've probably got networks of either RF or laser comms platforms scattered around the place, or self-routing relays on ships, or whatever. Even full occultation won't disrupt comms.
posted by Devonian at 6:35 AM on August 31, 2015
I imagine that by the time we're mining asteroids and building big things in Neptunian orbit, we've probably got networks of either RF or laser comms platforms scattered around the place, or self-routing relays on ships, or whatever. Even full occultation won't disrupt comms.
posted by Devonian at 6:35 AM on August 31, 2015
Response by poster: Oh, and thanks for Celestia. I'll load that when I'm home next (currently out and about with only a Chromebook and Android phone), although the fear will be that I'll get so bogged down modelling what the solar system's doing during the period of the story I'll never write the thing.
A risk worth taking.
posted by Devonian at 8:32 AM on August 31, 2015
A risk worth taking.
posted by Devonian at 8:32 AM on August 31, 2015
we've probably got networks of either RF or laser comms platforms scattered around the place, or self-routing relays on ships, or whatever
Yes, all you would need is a couple of relay satellites in earth-leading or -trailing solar orbits--more or less like the Kepler satellite currently is--and the entire comms black-out thing would be entirely avoided.
posted by flug at 2:50 PM on August 31, 2015 [1 favorite]
Yes, all you would need is a couple of relay satellites in earth-leading or -trailing solar orbits--more or less like the Kepler satellite currently is--and the entire comms black-out thing would be entirely avoided.
posted by flug at 2:50 PM on August 31, 2015 [1 favorite]
« Older What are these strange things we found in the... | Am I doing this resume related thing wrong? Newer »
This thread is closed to new comments.
posted by moonmilk at 1:11 PM on August 30, 2015