APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Comments and questions about the APOD on the main view screen.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by alter-ego » Thu Dec 30, 2021 1:51 am

Chris Peterson wrote:
Wed Dec 29, 2021 2:04 pm
alter-ego wrote:
Wed Dec 29, 2021 6:03 am
Chris Peterson wrote:
Wed Dec 29, 2021 4:59 am


Yes, I think that is the case. There are a large number of possible halo orbits that could have been chosen. Certainly, some such path is a requirement for a metastable existence near L2. But this orbit was carefully designed because the mission is only possible if the probe is in constant sunlight. This particular orbit was designed specifically for that purpose.
In detail, I have to agree. I just don't appreciate the specific differences between one that works and one that doesn't. On the surface, it looks like a basic halo orbit. I guess every halo orbit is different and appreciating the differences in complexity means understanding those design details.
Interesting - I'd like a top-10 list of mission orbit designs, 1 being the most difficult / complex. Maybe use code optimization complexity? One thing for sure, computer evolution has impacted the design capability.
If I were to guess, I'd say that #1 was the requirement of an orbit that kept it unshadowed from the Earth and Moon, and #2 was such an orbit that minimized thruster-based station keeping (which also involves optimization of reaction wheel station keeping, since the two interact).
Thanks, for the follow-up replies. As usual, they are helpful. For some reason, I'm struggling to communicate clearly on this topic, so to avoid going round and round rephrasing questions, I'll leave any further discussion until later.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by MarkBour » Fri Dec 31, 2021 12:40 am

LagrangePointsSunEarth.jpg
I had trouble figuring out why L2 was chosen instead of another one of the 5 Lagrange points.
  • Clearly L3 would be terrible -- huge distance, never a clean line of communication.
  • And L1 would not be a good choice for this mission (though it is handy for the SOHO mission). The problem is that the Earth (and Moon) would always be in its view. Like always having a really large full moon overhead while trying to look at stars and such. And apparently earthshine would even be way too strong and overheat the JWST.
  • But the advantage(s) of L2 over L4 and L5 weren't as clear. These two points are farther away from Earth, so communication would need to be beefed up, but I don't think that's a very big issue. They would be more stable, the scope would not need to expend its energy station-keeping and avoiding the Earth/Moon shadows. Is earthshine still a big problem at those points?
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 2:49 pm

MarkBour wrote:
Fri Dec 31, 2021 12:40 am
LagrangePointsSunEarth.jpg
I had trouble figuring out why L2 was chosen instead of another one of the 5 Lagrange points.
  • Clearly L3 would be terrible -- huge distance, never a clean line of communication.
  • And L1 would not be a good choice for this mission (though it is handy for the SOHO mission). The problem is that the Earth (and Moon) would always be in its view. Like always having a really large full moon overhead while trying to look at stars and such. And apparently earthshine would even be way too strong and overheat the JWST.
  • But the advantage(s) of L2 over L4 and L5 weren't as clear. These two points are farther away from Earth, so communication would need to be beefed up, but I don't think that's a very big issue. They would be more stable, the scope would not need to expend its energy station-keeping and avoiding the Earth/Moon shadows. Is earthshine still a big problem at those points?
Good question. Found a good discussion here: https://space.stackexchange.com/questio ... propellant

The main problem seems to be that L4 and L5 are 100 times farther away than L2 is, that is, 1 AU away.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Fri Dec 31, 2021 3:01 pm

johnnydeep wrote:
Fri Dec 31, 2021 2:49 pm
MarkBour wrote:
Fri Dec 31, 2021 12:40 am
LagrangePointsSunEarth.jpg
I had trouble figuring out why L2 was chosen instead of another one of the 5 Lagrange points.
  • Clearly L3 would be terrible -- huge distance, never a clean line of communication.
  • And L1 would not be a good choice for this mission (though it is handy for the SOHO mission). The problem is that the Earth (and Moon) would always be in its view. Like always having a really large full moon overhead while trying to look at stars and such. And apparently earthshine would even be way too strong and overheat the JWST.
  • But the advantage(s) of L2 over L4 and L5 weren't as clear. These two points are farther away from Earth, so communication would need to be beefed up, but I don't think that's a very big issue. They would be more stable, the scope would not need to expend its energy station-keeping and avoiding the Earth/Moon shadows. Is earthshine still a big problem at those points?
Good question. Found a good discussion here: https://space.stackexchange.com/questio ... propellant. The main problem seems to be that L4 and L5 are 100 times farther away than L2 is, that is, 1 AU away.
  • The Earth-trailing L5 orbit has already been tried:
https://en.wikipedia.org/wiki/Spitzer_Space_Telescope#History wrote:
<<The Spitzer Space Telescope was an infrared space telescope launched in 2003 and retired on 30 January 2020. It was the first spacecraft to use an Earth-trailing L5 orbit, later used by the Kepler planet-finder.

One of the most important advances of this redesign was an Earth-trailing L5 orbit. Cryogenic satellites that require liquid helium (LHe, T ≈ 4 K) temperatures in near-Earth orbit are typically exposed to a large heat load from Earth, and consequently require large amounts of LHe coolant, which then tends to dominate the total payload mass and limits mission life. Placing the satellite in solar orbit far from Earth allowed innovative passive cooling. The sun shield protected the rest of the spacecraft from the Sun's heat, the far side of the spacecraft was painted black to enhance passive radiation of heat, and the spacecraft bus was thermally isolated from the telescope. All of these design choices combined to drastically reduce the total mass of helium needed, resulting in an overall smaller and lighter payload, resulting in major cost savings, but with a mirror the same diameter as originally designed. This orbit also simplifies telescope pointing, but does require the NASA Deep Space Network for communications.

Spitzer ran out of liquid helium coolant on 15 May 2009, which stopped far-IR observations. Only the IRAC instrument remained in use, and only at the two shorter wavelength bands (3.6 μm and 4.5 μm). The telescope equilibrium temperature was then around 30 K (−243 °C; −406 °F), and IRAC continued to produce valuable images at those wavelengths as the "Spitzer Warm Mission". Late in the mission, ~2016, Spitzer's distance to Earth and the shape of its orbit meant the spacecraft had to pitch over at an extreme angle to aim its antenna at Earth. The solar panels were not fully illuminated at this angle, and this limited those communications to 2.5 hours due to the battery drain.

On 1 October 2016, Spitzer began its Observation Cycle 13, a 2+1⁄2 year extended mission nicknamed Beyond. One of the goals of this extended mission was to help prepare for the James Webb Space Telescope, also an infrared telescope, by identifying candidates for more detailed observations. Another aspect of the Beyond mission was the engineering challenges of operating Spitzer in its progressing orbital phase. As the spacecraft moved farther from Earth on the same orbital path from the Sun, its antenna had to point at increasingly higher angles to communicate with ground stations; this change in angle imparted more and more solar heating on the vehicle while its solar panels received less sunlight.>>
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 3:36 pm

neufer wrote:
Fri Dec 31, 2021 3:01 pm
johnnydeep wrote:
Fri Dec 31, 2021 2:49 pm
MarkBour wrote:
Fri Dec 31, 2021 12:40 am
LagrangePointsSunEarth.jpg
I had trouble figuring out why L2 was chosen instead of another one of the 5 Lagrange points.
  • Clearly L3 would be terrible -- huge distance, never a clean line of communication.
  • And L1 would not be a good choice for this mission (though it is handy for the SOHO mission). The problem is that the Earth (and Moon) would always be in its view. Like always having a really large full moon overhead while trying to look at stars and such. And apparently earthshine would even be way too strong and overheat the JWST.
  • But the advantage(s) of L2 over L4 and L5 weren't as clear. These two points are farther away from Earth, so communication would need to be beefed up, but I don't think that's a very big issue. They would be more stable, the scope would not need to expend its energy station-keeping and avoiding the Earth/Moon shadows. Is earthshine still a big problem at those points?
Good question. Found a good discussion here: https://space.stackexchange.com/questio ... propellant. The main problem seems to be that L4 and L5 are 100 times farther away than L2 is, that is, 1 AU away.
  • The Earth-trailing L5 orbit has already been tried:
https://en.wikipedia.org/wiki/Spitzer_Space_Telescope#History wrote:
<<The Spitzer Space Telescope was an infrared space telescope launched in 2003 and retired on 30 January 2020. It was the first spacecraft to use an Earth-trailing L5 orbit, later used by the Kepler planet-finder.

One of the most important advances of this redesign was an Earth-trailing L5 orbit. Cryogenic satellites that require liquid helium (LHe, T ≈ 4 K) temperatures in near-Earth orbit are typically exposed to a large heat load from Earth, and consequently require large amounts of LHe coolant, which then tends to dominate the total payload mass and limits mission life. Placing the satellite in solar orbit far from Earth allowed innovative passive cooling. The sun shield protected the rest of the spacecraft from the Sun's heat, the far side of the spacecraft was painted black to enhance passive radiation of heat, and the spacecraft bus was thermally isolated from the telescope. All of these design choices combined to drastically reduce the total mass of helium needed, resulting in an overall smaller and lighter payload, resulting in major cost savings, but with a mirror the same diameter as originally designed. This orbit also simplifies telescope pointing, but does require the NASA Deep Space Network for communications.

Spitzer ran out of liquid helium coolant on 15 May 2009, which stopped far-IR observations. Only the IRAC instrument remained in use, and only at the two shorter wavelength bands (3.6 μm and 4.5 μm). The telescope equilibrium temperature was then around 30 K (−243 °C; −406 °F), and IRAC continued to produce valuable images at those wavelengths as the "Spitzer Warm Mission". Late in the mission, ~2016, Spitzer's distance to Earth and the shape of its orbit meant the spacecraft had to pitch over at an extreme angle to aim its antenna at Earth. The solar panels were not fully illuminated at this angle, and this limited those communications to 2.5 hours due to the battery drain.

On 1 October 2016, Spitzer began its Observation Cycle 13, a 2+1⁄2 year extended mission nicknamed Beyond. One of the goals of this extended mission was to help prepare for the James Webb Space Telescope, also an infrared telescope, by identifying candidates for more detailed observations. Another aspect of the Beyond mission was the engineering challenges of operating Spitzer in its progressing orbital phase. As the spacecraft moved farther from Earth on the same orbital path from the Sun, its antenna had to point at increasingly higher angles to communicate with ground stations; this change in angle imparted more and more solar heating on the vehicle while its solar panels received less sunlight.>>
So this all seem like it would be good for the JWST, except, I guess, that last part about the orbit getting progressively farther away from the Earth. But I don't understand that. Don't all the Lagrange points maintain their positions in space relative to the Earth and Sun?

And another reason that L4/L5 might be bad is that those locations - and the vicinity - accumulate space debris over time and might pose a danger? Though Spitzer seemed not to have a problem with that.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Fri Dec 31, 2021 3:58 pm

johnnydeep wrote:
Fri Dec 31, 2021 3:36 pm

I don't understand that. Don't all the Lagrange points maintain their positions in space relative to the Earth and Sun?

And another reason that L4/L5 might be bad is that those locations - and the vicinity - accumulate space debris over time and might pose a danger? Though Spitzer seemed not to have a problem with that.
The chance of actually hitting trapped L4/L5 space debris like 469219 Kamoʻoalewa is pretty slim.

The chance of landing in a much more stable L4/L5 orbit than 469219 Kamoʻoalewa or Spitzer is also pretty slim.
https://en.wikipedia.org/wiki/469219_Kamo%CA%BBoalewa wrote:
<<469219 Kamoʻoalewa, provisionally designated 2016 HO3, is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately 41 meters in diameter. Currently, it is the smallest, closest, and most stable (known) quasi-satellite of Earth. Photometric observations in April 2017 revealed that Kamoʻoalewa is a fast rotator. Lightcurve analysis gave a rotation period of 0.467 ± 0.008 hours (28.02 ± 0.48 minutes) and a brightness variation of 0.80±0.05 magnitude. In 2021, a comprehensive physical characterization of Kamoʻoalewa was conducted using the Large Binocular Telescope and the Lowell Discovery Telescope, which found that the asteroid is composed of lunar-like silicates and may be an impact fragment from the Moon. Kamoʻoalewa orbits the Sun at a distance of 0.90–1.10 AU once every 366 days. Its orbit has an eccentricity of 0.10 and an inclination of 8° with respect to the ecliptic. It has an Earth minimum orbital intersection distance of 0.0348 AU (5,210,000 km) that translates into 13.6 lunar distances. As it orbits the Sun, Kamoʻoalewa appears to circle (highly elliptically) around Earth as well. The object is beyond the Hill sphere of Earth and the Sun exerts a much stronger pull on it than Earth does. Although it is too distant to be considered a true natural satellite of Earth, it is the best and most stable example to date of a near-Earth companion, or quasi-satellite.

Kamoʻoalewa was first spotted on 27 April 2016, by the Pan-STARRS 1 asteroid survey telescope on Haleakalā, Hawaii, that is operated by the University of Hawaii's Institute for Astronomy and funded by NASA's Planetary Defense Coordination Office. The name Kamoʻoalewa is derived from the Hawaiian words ka 'the', moʻo 'fragment', referring to it being a piece broken off a larger object, a 'of', and lewa 'to oscillate', referring to its motion in the sky as viewed from Earth.

Paul Chodas, manager of NASA's Center for Near-Earth Object (NEO) Studies at the Jet Propulsion Laboratory in Pasadena, California, commented on the orbit: "Since 2016 HO3 loops around our planet, but never ventures very far away as we both go around the Sun, we refer to it as a quasi-satellite of Earth. One other asteroid – 2003 YN107 – followed a similar orbital pattern for a while over 10 years ago, but it has since departed our vicinity. This new asteroid is much more locked onto us. Our calculations indicate 2016 HO3 has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's companion for centuries to come."

Chodas explained how the asteroid's orbit also undergoes a slow, back-and-forth twist over multiple decades: "The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon.
The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."
[/quote]
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by Chris Peterson » Fri Dec 31, 2021 4:07 pm

johnnydeep wrote:
Fri Dec 31, 2021 3:36 pm
And another reason that L4/L5 might be bad is that those locations - and the vicinity - accumulate space debris over time and might pose a danger? Though Spitzer seemed not to have a problem with that.
Space is largely empty, even in places that our imaginations would argue otherwise for- the asteroid belt, the Oort cloud, and L4/L5.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 4:29 pm

Chris Peterson wrote:
Fri Dec 31, 2021 4:07 pm
johnnydeep wrote: And another reason that L4/L5 might be bad is that those locations - and the vicinity - accumulate space debris over time and might pose a danger? Though Spitzer seemed not to have a problem with that.
Space is largely empty, even in places that our imaginations would argue otherwise for- the asteroid belt, the Oort cloud, and L4/L5.
Ok. Excepting, or course, for low earth orbit, where the hazards are quickly multiplying (because the available volume is much smaller).
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by Chris Peterson » Fri Dec 31, 2021 4:34 pm

johnnydeep wrote:
Fri Dec 31, 2021 4:29 pm
Chris Peterson wrote:
Fri Dec 31, 2021 4:07 pm
johnnydeep wrote: And another reason that L4/L5 might be bad is that those locations - and the vicinity - accumulate space debris over time and might pose a danger? Though Spitzer seemed not to have a problem with that.
Space is largely empty, even in places that our imaginations would argue otherwise for- the asteroid belt, the Oort cloud, and L4/L5.
Ok. Excepting, or course, for low earth orbit, where the hazards are quickly multiplying (because the available volume is much smaller).
Yes, LEO is not so much a volume as a surface. We've removed an entire dimension, drastically increasing the density.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 4:40 pm

neufer wrote:
Fri Dec 31, 2021 3:58 pm
johnnydeep wrote:
Fri Dec 31, 2021 3:36 pm

I don't understand that. Don't all the Lagrange points maintain their positions in space relative to the Earth and Sun?

And another reason that L4/L5 might be bad is that those locations - and the vicinity - accumulate space debris over time and might pose a danger? Though Spitzer seemed not to have a problem with that.
The chance of actually hitting trapped L4/L5 space debris like 469219 Kamoʻoalewa is pretty slim.

The chance of landing in a much more stable L4/L5 orbit than 469219 Kamoʻoalewa or Spitzer is also pretty slim.
https://en.wikipedia.org/wiki/469219_Kamo%CA%BBoalewa wrote:
<<469219 Kamoʻoalewa, provisionally designated 2016 HO3, is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately 41 meters in diameter. Currently, it is the smallest, closest, and most stable (known) quasi-satellite of Earth. Photometric observations in April 2017 revealed that Kamoʻoalewa is a fast rotator. Lightcurve analysis gave a rotation period of 0.467 ± 0.008 hours (28.02 ± 0.48 minutes) and a brightness variation of 0.80±0.05 magnitude. In 2021, a comprehensive physical characterization of Kamoʻoalewa was conducted using the Large Binocular Telescope and the Lowell Discovery Telescope, which found that the asteroid is composed of lunar-like silicates and may be an impact fragment from the Moon. Kamoʻoalewa orbits the Sun at a distance of 0.90–1.10 AU once every 366 days. Its orbit has an eccentricity of 0.10 and an inclination of 8° with respect to the ecliptic. It has an Earth minimum orbital intersection distance of 0.0348 AU (5,210,000 km) that translates into 13.6 lunar distances. As it orbits the Sun, Kamoʻoalewa appears to circle (highly elliptically) around Earth as well. The object is beyond the Hill sphere of Earth and the Sun exerts a much stronger pull on it than Earth does. Although it is too distant to be considered a true natural satellite of Earth, it is the best and most stable example to date of a near-Earth companion, or quasi-satellite.

Kamoʻoalewa was first spotted on 27 April 2016, by the Pan-STARRS 1 asteroid survey telescope on Haleakalā, Hawaii, that is operated by the University of Hawaii's Institute for Astronomy and funded by NASA's Planetary Defense Coordination Office. The name Kamoʻoalewa is derived from the Hawaiian words ka 'the', moʻo 'fragment', referring to it being a piece broken off a larger object, a 'of', and lewa 'to oscillate', referring to its motion in the sky as viewed from Earth.

Paul Chodas, manager of NASA's Center for Near-Earth Object (NEO) Studies at the Jet Propulsion Laboratory in Pasadena, California, commented on the orbit: "Since 2016 HO3 loops around our planet, but never ventures very far away as we both go around the Sun, we refer to it as a quasi-satellite of Earth. One other asteroid – 2003 YN107 – followed a similar orbital pattern for a while over 10 years ago, but it has since departed our vicinity. This new asteroid is much more locked onto us. Our calculations indicate 2016 HO3 has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's companion for centuries to come."

Chodas explained how the asteroid's orbit also undergoes a slow, back-and-forth twist over multiple decades: "The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon.
The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."
But Kamoʻoalewa is not at L4 or L5, right? It's distance from earth seems to vary way too much:
In terms of orbit, it currently is the most stable among the quasi-satellites of Earth that have been discovered and will remain in that orbit for about the next 300 years.[7] The closest Earth approach was on 27 December 1923 at 12.4 million km (32 LD).[2] By late May 2369 the asteroid will be 2.0 AU (300 million km) from Earth.[11] The Earth-like orbit may be a result of it being lunar ejecta.[7]
Or is your whole point that, contrary to what I had thought, objects at L4/L5 aren't really locked into a fairly fixed position relative to earth at all?
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Fri Dec 31, 2021 4:52 pm

johnnydeep wrote:
Fri Dec 31, 2021 4:40 pm

But Kamoʻoalewa is not at L4 or L5, right? It's distance from earth seems to vary way too much:
In terms of orbit, it currently is the most stable among the quasi-satellites of Earth that have been discovered and will remain in that orbit for about the next 300 years.[7] The closest Earth approach was on 27 December 1923 at 12.4 million km (32 LD).[2] By late May 2369 the asteroid will be 2.0 AU (300 million km) from Earth.[11] The Earth-like orbit may be a result of it being lunar ejecta.[7]
Or is your whole point that, contrary to what I had thought, objects at L4/L5 aren't really locked into a fairly fixed position relative to earth at all?
Yes...objects at L4/L5 aren't really locked into fixed positions relative to Earth.

At best, they are repelled by the unstable L3 point and kept permanently isolated to either L4 or L5.

But, in general, they bypass L3 and are only repelled by the unstable L1/L2 points.

When JWST finally loses its station keeping fuel at L2 it will slip into a Kamoʻoalewa like L4/L5 orbit.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 8:47 pm

neufer wrote:
Fri Dec 31, 2021 4:52 pm
johnnydeep wrote:
Fri Dec 31, 2021 4:40 pm

But Kamoʻoalewa is not at L4 or L5, right? It's distance from earth seems to vary way too much:
In terms of orbit, it currently is the most stable among the quasi-satellites of Earth that have been discovered and will remain in that orbit for about the next 300 years.[7] The closest Earth approach was on 27 December 1923 at 12.4 million km (32 LD).[2] By late May 2369 the asteroid will be 2.0 AU (300 million km) from Earth.[11] The Earth-like orbit may be a result of it being lunar ejecta.[7]
Or is your whole point that, contrary to what I had thought, objects at L4/L5 aren't really locked into a fairly fixed position relative to earth at all?
Yes...objects at L4/L5 aren't really locked into fixed positions relative to Earth.

At best, they are repelled by the unstable L3 point and kept permanently isolated to either L4 or L5.

But, in general, they bypass L3 and are only repelled by the unstable L1/L2 points.

When JWST finally loses its station keeping fuel at L2 it will slip into a Kamoʻoalewa like L4/L5 orbit.
Ok.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 9:12 pm

johnnydeep wrote:
Fri Dec 31, 2021 8:47 pm
neufer wrote:
Fri Dec 31, 2021 4:52 pm
johnnydeep wrote:
Fri Dec 31, 2021 4:40 pm

But Kamoʻoalewa is not at L4 or L5, right? It's distance from earth seems to vary way too much:



Or is your whole point that, contrary to what I had thought, objects at L4/L5 aren't really locked into a fairly fixed position relative to earth at all?
Yes...objects at L4/L5 aren't really locked into fixed positions relative to Earth.

At best, they are repelled by the unstable L3 point and kept permanently isolated to either L4 or L5.

But, in general, they bypass L3 and are only repelled by the unstable L1/L2 points.

When JWST finally loses its station keeping fuel at L2 it will slip into a Kamoʻoalewa like L4/L5 orbit.
Ok.
On second thought (and to give myself a thousandth post!), a link from today's APOD (12/31/2021) has this text:
L4 and L5 correspond to hilltops and L1, L2 and L3 correspond to saddles (i.e. points where the potential is curving up in one direction and down in the other). This suggests that satellites placed at the Lagrange points will have a tendency to wander off (try sitting a marble on top of a watermelon or on top of a real saddle and you get the idea). But when a satellite parked at L4 or L5 starts to roll off the hill it picks up speed. At this point the Coriolis force comes into play - the same force that causes hurricanes to spin up on the earth - and sends the satellite into a stable orbit around the Lagrange point.
So, I would take this to imply that you CAN have objects located around L4 and L5 long term, and thus be useful for astronomy. Though I am then not sure why Kepler had a problem staying there (that is, permanently "near" L4 or L5. Did it even start at one of those to begin with? This clearly says it's drifting severly, but I presume won't ever entirely leave this orbit:
NASA has characterized Kepler's orbit as "Earth-trailing".[59] With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). As of May 1, 2018, the distance to Kepler from Earth was about 0.917 AU (137 million km).[3] This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Fri Dec 31, 2021 9:23 pm

johnnydeep wrote:
Fri Dec 31, 2021 9:12 pm

On second thought (and to give myself a thousandth post!), a link from today's APOD (12/31/2021) has this text:
L4 and L5 correspond to hilltops and L1, L2 and L3 correspond to saddles (i.e. points where the potential is curving up in one direction and down in the other). This suggests that satellites placed at the Lagrange points will have a tendency to wander off (try sitting a marble on top of a watermelon or on top of a real saddle and you get the idea). But when a satellite parked at L4 or L5 starts to roll off the hill it picks up speed. At this point the Coriolis force comes into play - the same force that causes hurricanes to spin up on the earth - and sends the satellite into a stable orbit around the Lagrange point.
So, I would take this to imply that you CAN have objects located around L4 and L5 long term, and thus be useful for astronomy. Though I am then not sure why Kepler had a problem staying there (that is, permanently "near" L4 or L5. Did it even start at one of those to begin with? This clearly says it's drifting severly, but I presume won't ever entirely leave this orbit:
NASA has characterized Kepler's orbit as "Earth-trailing".[59] With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). As of May 1, 2018, the distance to Kepler from Earth was about 0.917 AU (137 million km). This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.
Why communicate with a satellite 1 AU away when you can have a similar satellite 0.01 AU away
and have ~10,000 times the signal to noise both to & from the satellite :?:

viewtopic.php?f=23&t=18222&p=319500#p319500
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Fri Dec 31, 2021 9:49 pm

neufer wrote:
Fri Dec 31, 2021 9:23 pm
johnnydeep wrote:
Fri Dec 31, 2021 9:12 pm

On second thought (and to give myself a thousandth post!), a link from today's APOD (12/31/2021) has this text:
L4 and L5 correspond to hilltops and L1, L2 and L3 correspond to saddles (i.e. points where the potential is curving up in one direction and down in the other). This suggests that satellites placed at the Lagrange points will have a tendency to wander off (try sitting a marble on top of a watermelon or on top of a real saddle and you get the idea). But when a satellite parked at L4 or L5 starts to roll off the hill it picks up speed. At this point the Coriolis force comes into play - the same force that causes hurricanes to spin up on the earth - and sends the satellite into a stable orbit around the Lagrange point.
So, I would take this to imply that you CAN have objects located around L4 and L5 long term, and thus be useful for astronomy. Though I am then not sure why Kepler had a problem staying there (that is, permanently "near" L4 or L5. Did it even start at one of those to begin with? This clearly says it's drifting severly, but I presume won't ever entirely leave this orbit:
NASA has characterized Kepler's orbit as "Earth-trailing".[59] With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). As of May 1, 2018, the distance to Kepler from Earth was about 0.917 AU (137 million km). This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.
Why communicate with a satellite 1 AU away when you can have a similar satellite 0.01 AU away
and have ~10,000 times the signal to noise both to & from the satellite :?:

viewtopic.php?f=23&t=18222&p=319500#p319500
Yeah, I suppose that is the ultimate problem considering the amount of data this telescope will generate. And now I think we've returned to the reason I first posted before this long detour :) [ EDIT: hmm, I think that was a different discussion on another APOD. ]
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Fri Dec 31, 2021 10:44 pm

johnnydeep wrote:
Fri Dec 31, 2021 9:49 pm
neufer wrote:
Fri Dec 31, 2021 9:23 pm

Why communicate with a satellite 1 AU away when you can have a similar satellite 0.01 AU away
and have ~10,000 times the signal to noise both to & from the satellite :?:
Yeah, I suppose that is the ultimate problem considering the amount of data this telescope will generate. And now I think we've returned to the reason I first posted before this long detour :) [ EDIT: hmm, I think that was a different discussion on another APOD. ]
https://en.wikipedia.org/wiki/Kepler_space_telescope#Orbit_and_orientation wrote:
<<Kepler orbits the Sun, which avoids Earth occultations, stray light, and gravitational perturbations and torques inherent in an Earth orbit. NASA has characterized Kepler's L4/L5 orbit as "Earth-trailing". With an orbital period of 372.5 days, Kepler is slowly falling farther behind Earth (about 16 million miles per annum). As of May 1, 2018, the distance to Kepler from Earth was about 0.917 AU. This means that after about 26 years Kepler will reach the other side of the Sun and will get back to the neighborhood of the Earth after 51 years.

NASA contacted the spacecraft using the X band communication link [uplink: 7.8 bit/s to 2 kbit/s: downlink: 10 bit/s – 16 kbit/s] twice a week for command and status updates. (NASA will contact JWST using an S band communication link [uplink: 16 kbit/s: downlink: 10 bit/s – 40 kbit/s].) Scientific Kepler data were downloaded once a month using the Ka band link at a maximum data transfer rate of approximately 550 kB/s [; vs. JWST Ka-band downlink of up to 28 Mbit/s]. Kepler's high gain antenna is not steerable so data collection is interrupted for a day to reorient the whole spacecraft and the high gain antenna for communications to Earth.>>
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by Chris Peterson » Fri Dec 31, 2021 10:55 pm

neufer wrote:
Fri Dec 31, 2021 9:23 pm
Why communicate with a satellite 1 AU away when you can have a similar satellite 0.01 AU away
and have ~10,000 times the signal to noise both to & from the satellite :?:
That would only be the case given an omnidirectional antenna on the satellite. With a focused beam, the S/N situation is more complex. (Not that being closer isn't better... it just isn't necessarily 10,000 times better.)
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Sat Jan 01, 2022 2:40 am

Chris Peterson wrote:
Fri Dec 31, 2021 10:55 pm
neufer wrote:
Fri Dec 31, 2021 9:23 pm

Why communicate with a satellite 1 AU away when you can have a similar satellite 0.01 AU away
and have ~10,000 times the signal to noise both to & from the satellite :?:
That would only be the case given an omnidirectional antenna on the satellite. With a focused beam, the S/N situation is more complex. (Not that being closer isn't better... it just isn't necessarily 10,000 times better.)
All antenna patterns (from simplest to most complex) expand in 2 dimensions :!:

The only question is whether (or not) the receiving antenna is
far enough away to be homogeneously filled with radiation:

The Ka band has wavelength λ of ~ 1 cm.
The JWST is at distance R ~ 1.5 Gm.

If the product of
the spacecraft high gain attenna diameter d
& the ground based high gain attenna diameter D

is less than 1.22 λ R = (4.3 km)2 then the

receiving antennae are homogeneously filled and the signal to noise
is 10,000 stronger than the result would be at a distance of 100 R.

And neither d nor D is anywhere close to 4.3 km!
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by MarkBour » Mon Jan 03, 2022 1:45 am

Thanks johnnydeep, neufer, and Chris for the links and discussion that help me understand the choice of L2. So, it seems that there were a few possible reasons to prefer L2, but this discussion centers on the difficulty of communicating large amounts of data from 1 AU away, as the biggest downside to L4/L5.

Two lingering thoughts about NASA space assets, both having to do with modularization.
  1. I wonder why it's common to have the antenna fixed (not "steerable"), so that the entire spacecraft needs to be pointed in order to point the antenna. Simplifies controls, but the accounts of Spitzer and Kepler show some serious downsides of that design. On earth. we would most likely make it easy to point the antenna freely from the telescope. Doing it on a spacecraft might be harder than I imagine. EDIT: Or, is this already now the new way? I just read that the JWST communications subsystem is "able to be articulated".
  2. Another point of modularization would be propulsion and control versus the instrument. If it were easy to detach and replace the propulsion/control unit, Kepler and the JWST missions might be economically extendable.
Last edited by MarkBour on Mon Jan 03, 2022 2:36 am, edited 1 time in total.
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by MarkBour » Mon Jan 03, 2022 1:49 am

Also, johnnydeep, welcome to the 1000-post club!
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by johnnydeep » Mon Jan 03, 2022 3:01 pm

MarkBour wrote:
Mon Jan 03, 2022 1:45 am
Thanks johnnydeep, neufer, and Chris for the links and discussion that help me understand the choice of L2. So, it seems that there were a few possible reasons to prefer L2, but this discussion centers on the difficulty of communicating large amounts of data from 1 AU away, as the biggest downside to L4/L5.

Two lingering thoughts about NASA space assets, both having to do with modularization.
  1. I wonder why it's common to have the antenna fixed (not "steerable"), so that the entire spacecraft needs to be pointed in order to point the antenna. Simplifies controls, but the accounts of Spitzer and Kepler show some serious downsides of that design. On earth. we would most likely make it easy to point the antenna freely from the telescope. Doing it on a spacecraft might be harder than I imagine. EDIT: Or, is this already now the new way? I just read that the JWST communications subsystem is "able to be articulated".
  2. Another point of modularization would be propulsion and control versus the instrument. If it were easy to detach and replace the propulsion/control unit, Kepler and the JWST missions might be economically extendable.
I'd guess that "engineering considerations" make a fully steerable antenna impractical. I'm thinking that to allow the antenna to be pointed at earth no matter where the optics are pointed would mean putting the antenna a long way away on a boom of some sort. Hmm, why not have the "antenna module" be an entirely separate "craft" simply orbiting close by and acting as a relay. Probably the engineering and orbital mechanics would be too complex.

As for keeping these mega-dollar space observatories operating indefinitely, I think it was Chris who said that at some point, the next generation of observing technology tends to make the old generation obsolete before the old scope ends its mission. But even so, there is still the repairability benefit to consider, since it would be a real shame to have to toss the 10 billion dollar JWST just because some crucial component failed after only a year's use! Or even worse, during one of the hundreds of "single points of failure" deployment steps.

PS - yeah, making it to my 1000th post at least makes it feel like I accomplished something :)
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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by Chris Peterson » Mon Jan 03, 2022 3:11 pm

johnnydeep wrote:
Mon Jan 03, 2022 3:01 pm
MarkBour wrote:
Mon Jan 03, 2022 1:45 am
Thanks johnnydeep, neufer, and Chris for the links and discussion that help me understand the choice of L2. So, it seems that there were a few possible reasons to prefer L2, but this discussion centers on the difficulty of communicating large amounts of data from 1 AU away, as the biggest downside to L4/L5.

Two lingering thoughts about NASA space assets, both having to do with modularization.
  1. I wonder why it's common to have the antenna fixed (not "steerable"), so that the entire spacecraft needs to be pointed in order to point the antenna. Simplifies controls, but the accounts of Spitzer and Kepler show some serious downsides of that design. On earth. we would most likely make it easy to point the antenna freely from the telescope. Doing it on a spacecraft might be harder than I imagine. EDIT: Or, is this already now the new way? I just read that the JWST communications subsystem is "able to be articulated".
  2. Another point of modularization would be propulsion and control versus the instrument. If it were easy to detach and replace the propulsion/control unit, Kepler and the JWST missions might be economically extendable.
I'd guess that "engineering considerations" make a fully steerable antenna impractical. I'm thinking that to allow the antenna to be pointed at earth no matter where the optics are pointed would mean putting the antenna a long way away on a boom of some sort. Hmm, why not have the "antenna module" be an entirely separate "craft" simply orbiting close by and acting as a relay. Probably the engineering and orbital mechanics would be too complex.

As for keeping these mega-dollar space observatories operating indefinitely, I think it was Chris who said that at some point, the next generation of observing technology tends to make the old generation obsolete before the old scope ends its mission. But even so, there is still the repairability benefit to consider, since it would be a real shame to have to toss the 10 billion dollar JWST just because some crucial component failed after only a year's use! Or even worse, during one of the hundreds of "single points of failure" deployment steps.

PS - yeah, making it to my 1000th post at least makes it feel like I accomplished something :)
Keep in mind that there is far more to that $10 billion than the spacecraft itself. I don't know what the replacement cost would be, once the technology has been developed and the staff and ground segments remain, but I suspect it is well under $1 billion.

Interesting fact about the Hubble: each service mission cost approximately the same as the HST itself. It was arguably a mistake to make it repairable. In the long run, it would probably have been cheaper to simply launch new ones.
Chris

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Re: APOD: James Webb Space Telescope over Earth (2021 Dec 26)

Post by neufer » Mon Jan 03, 2022 4:02 pm

Click to play embedded YouTube video.
Chris Peterson wrote:
Mon Jan 03, 2022 3:11 pm

Interesting fact about the Hubble: each service mission cost approximately the same as the HST itself. It was arguably a mistake to make it repairable. In the long run, it would probably have been cheaper to simply launch new ones.
1) The cost in a Hubble service mission was borne mostly by the human spaceflight side.

2) Launching new Hubble would not capture the public's attention. :arrow:
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