For Questions That Don't Merit Their Own Thread

[K^2]

13 minutes ago, Rutabaga22 said:

What are galaxies orbiting?

Galaxies are typically grouped into clusters, with galaxies roughly orbiting the center of mass of the cluster. That CoM might or might not contain a particularly massive galaxy - kind of similar to how the stars in galaxies orbit center of mass, which might or might not contain a particularly massive black hole.

Clusters are typically grouped into superclusters, which are more loosely bound. Milky Way is part of the Local Group cluster, which itself is a part of the Lniakea Supercluster.

The superclusters together form galaxy filaments, which all together form the known universe. At that scale, talking about orbiting is kind of silly, as the universe is expanding, stretching and tearing the filaments like cotton candy. The name of the filament we belong to is Pisces-Cetus Supercluster Complex.

[kerbiloid]

6 hours ago, sevenperforce said:

Well, interstellar rogue objects are orbiting Sagittarius A* and the rest of the galactic core directly

That's when are slow .

The orbital speed of the Sun (and various junk stuck to it) is ~220 km/s. So, the galactic escape speed is just ~320 km/s.

There is a lot of much faster objects which obviously are orbiting around something much heavier and farther.

I.e. who owns the relativistic particles lost by us?
There is no freedom in open space, only gravitational slavery, only change of the owner.
Everything escaping from the well just immediately finds itself in a much larger and deeper well.

6 hours ago, sevenperforce said:

Objects which are ejected from their galaxy of origin are still technically orbiting that galaxy, just on the outgoing leg of a hyperbolic trajectory.

With the same success we can say that they are orbiting Crab Nebula, or any other reference point, because there is infinite number of open trajectories and reference points.

The closed trajectory differs from them principally, as you can have only one unique reference point (letting alone the reference point own motion and orbital perturbations, let the Universe be considered static for this purpose).

So, as the Universe is kinda large, we should be always able to find a more bulky mass of matter the point is elliptically moving around if zoom out.

3 hours ago, Rutabaga22 said:

What are galaxies orbiting?

Here you go.

https://en.wikipedia.org/wiki/Observable_universe#Large-scale_structure

Edited December 1, 2022 by kerbiloid

[JoeSchmuckatelli]

Given that 'constellations' of satellites are fairly new... Has any consideration been given to seeding a constellation into lunar orbit for 'anywhere' communication with future missions? 

Six equidistant small sats should be able to cover the entire orbit, I would think... 

[sevenperforce]

11 hours ago, kerbiloid said:

17 hours ago, sevenperforce said:

Objects which are ejected from their galaxy of origin are still technically orbiting that galaxy, just on the outgoing leg of a hyperbolic trajectory.

With the same success we can say that they are orbiting Crab Nebula, or any other reference point, because there is infinite number of open trajectories and reference points.

The closed trajectory differs from them principally, as you can have only one unique reference point (letting alone the reference point own motion and orbital perturbations, let the Universe be considered static for this purpose).

The eccentricity of an orbit can be greater than 1. As long as your trajectory is still along a path which can be defined as an orbit with a periapsis and an eccentricity, it's still an orbit. That condition remains until your object gets close enough to another massive body to cause its trajectory to deviate, at which point the periapsis and eccentricity relative to the original reference point are lost.

11 hours ago, kerbiloid said:

So, as the Universe is kinda large, we should be always able to find a more bulky mass of matter the point is elliptically moving around if zoom out.

There's no static "bulky mass of matter" in a globular cluster; everything is just orbiting the stars which happen to be closer to the center than they are at any given point. The same is true of galaxies.

14 hours ago, K^2 said:

Galaxies are typically grouped into clusters, with galaxies roughly orbiting the center of mass of the cluster. That CoM might or might not contain a particularly massive galaxy - kind of similar to how the stars in galaxies orbit center of mass, which might or might not contain a particularly massive black hole.

Our own cluster, the Local Group, has essentially nothing in the center; it's basically dumbbell-shaped, with the Milky Way and a number of smaller galaxies on one end and Andromeda and a number of smaller galaxies on the other end.  At one level up from the Local Group you have the Virgo Supercluster, which includes the Local Group. The clusters in the Virgo Supercluster orbit the Virgo Cluster, which is the heaviest cluster in the Virgo Supercluster. Unlike the Local Group, the Virgo Cluster does have a single giant galaxy in the center, the supergiant elliptical galaxy Messier 87:

Spoiler

The supermassive black hole at the center of Messier 87, M87*, produces the relativistic jet shooting out of the galactic core. It was the first black hole ever imaged by the Event Horizon Telescope.

The black hole at the center of Messier 87 is probably the "most-orbited" discrete object that we orbit. Although the Virgo Supercluster is considered to be a lobe of the Laniakea Supercluster, the latter is no longer gravitationally bound and has already begun to disperse due to the dark-energy expansion of the universe. So we are not in any closed orbits above the level of the Virgo Supercluster (to answer @kerbiloid's question).

The largest gravitationally-bound object in our local universe is the Shapley Supercluster, a collection of dozens of major galaxy clusters with a total mass more than 10,000 times that of the Local Group. We are close enough to it that its gravity is still tugging the Virgo Supercluster in its general direction, but not so strongly that it will be able to overcome dark energy expansion.

[sevenperforce]

51 minutes ago, JoeSchmuckatelli said:

Given that 'constellations' of satellites are fairly new... Has any consideration been given to seeding a constellation into lunar orbit for 'anywhere' communication with future missions? 

Six equidistant small sats should be able to cover the entire orbit, I would think... 

Constellations have been around for quite a while now. The GPS constellation was launched beginning in 1993 and the Iridium communications constellation was launched beginning in 1997.

We haven't had much in the way of a need for a lunar relay constellation. All crewed landings took place on the near side of the moon, as well as all robotic landers up until Chang'e 4 (which had a dedicated communications link to Earth via a separately-launched relay, Queqiao, at L2). The orbiters we have (like the LRO) are out of communications with Earth half the time, but that's fine since they can just uplink whenever they come back around.

You wouldn't need six smallsats; you can get away with three. That's because you're on the "Earth" side of the moon half the time anyway, so as long as one of the satellites has line-of-sight to a satellite that has line-of-sight to Earth, you're fine. One problem with constellations around the moon, at least in low lunar orbit, is the distribution of mascons that make it hard to maintain orbit. There are a handful of frozen orbits, though, or you could put the constellation in a more distant orbit.

For Artemis II, the crew in Orion will be isolated from Earth briefly during the flyby, but not for very long. It's a free-return after all. Artemis III will place Orion in NRHO which has constant communications with Earth, and the landing will be on the south lunar pole with line-of-sight to Earth. In subsequent missions, Gateway will be in NRHO, and it will have relay capability if NASA wants to do landings on the far side of the moon.

[JoeSchmuckatelli]

34 minutes ago, sevenperforce said:

Constellations have been around for quite a while now

I did not hear the term being used, at least not regularly, until SX started dumping multiple satellites per mission.  I know that GPS and Iridium can be called 'constellations' - but my impression was that each of those were large satellites that were (effectively) individually placed by different rocket launches.  So - is there a different term for that?  Or does 'constellation' simply refer to a group of related satellites?  (My intent was to suggest a single mission that could dump a bunch of satellites into lunar orbit for low cost and high return.

38 minutes ago, sevenperforce said:

You wouldn't need six smallsats; you can get away with three.   ...you're on the "Earth" side of the moon half the time anyway

I should have thought about that!

[kerbiloid]

2 hours ago, sevenperforce said:

The eccentricity of an orbit can be greater than 1.

And this makes the trajectory open. There is a infinite number of them for any object.

But there is only one (two, but its obvious) point which the object is moving on a closed trajectory around. And it's only a question of scale when there is enough large mass to be that point.

When you escape the near-Earth orbit, you are moving hyperbolically, but just relative to the Earth.
At the same time now you are on elliptic heliocentric orbit.

Once you add 12 km/s and escape from the Sun, now you are orbiting the Galaxy CoM, at 200+ km/s speed.

Once you add 100 km/s more, you escape from Galaxy, but now you are orbiting the galactic cluster CoM,

Every time you have left the previous closed trajectory, you just pass to the next scale level, and you are on some ellipse again.

And ad infinitum.

At the same time you may have a quantillion of open hyperbolic orbits, relatie to quasar #76547, pulsar #596865, and other quantillion of celestial bodies.

[Entropian]

11 minutes ago, kerbiloid said:

Every time you have left the previous closed trajectory, you just pass to the next scale level, and you are on some ellipse again.

Only up until you run into the dark energy expansion of the universe.  After that, it is impossible to reach any further gravitationally-interacting object even if you're traveling at the speed of light.

[kerbiloid]

Just now, Entropian said:

Only up until you run into the dark energy expansion of the universe.  After that, it is impossible to reach any further gravitationally-interacting object even if you're traveling at the speed of light.

That's conceptually true, but a real object has a real trajectory, and thus a real orbit focus with real coordinates among the real celestial objects.

[sevenperforce]

1 hour ago, kerbiloid said:

That's conceptually true, but a real object has a real trajectory, and thus a real orbit focus with real coordinates among the real celestial objects.

A real object does have a real trajectory but you can have a real object with a real trajectory that is on an escape trajectory such that it will never again form a closed orbit relative to any object, due to the expansion of the universe.

[kerbiloid]

2 minutes ago, sevenperforce said:

A real object does have a real trajectory but you can have a real object with a real trajectory that is on an escape trajectory such that it will never again form a closed orbit relative to any object, due to the expansion of the universe.

In the curved, non-Euclid world any straight line is just a zoomed curve.

[sevenperforce]

3 hours ago, JoeSchmuckatelli said:

4 hours ago, sevenperforce said:

Constellations have been around for quite a while now

I did not hear the term being used, at least not regularly, until SX started dumping multiple satellites per mission.  I know that GPS and Iridium can be called 'constellations' - but my impression was that each of those were large satellites that were (effectively) individually placed by different rocket launches.  So - is there a different term for that?  Or does 'constellation' simply refer to a group of related satellites?

The Iridium constellation launch campaign delivered between two and seven satellites to LEO per launch, depending on the launch vehicle capabilities. They used a variety of launch vehicles, including Delta II, Proton-K, and Chang Zheng 2C. Iridium's replacement constellation, Iridium-NEXT, began launching on Falcon 9 rockets in 2017, with approximately 10 satellites being delivered per launch.

The Globalstar constellation started launching in 1998 and had about a dozen satellites go up in each launch. 

I was actually incorrect about the first GPS satellites; the original GPS launch was in 1978. Of course those particular satellites have long since been retired. The GPS sat launch campaign only sent up a single satellite per launch until the USA-66 mission in 1990, which launched two at the same time. 

3 hours ago, JoeSchmuckatelli said:

My intent was to suggest a single mission that could dump a bunch of satellites into lunar orbit for low cost and high return.

Yeah, you could do this easily enough. A simple relay like Quequiao needs a mass of no more than half a ton with enough propellant to brake itself into a lunar orbit, and so a Falcon 9 could definitely send four of them to TLI with enough margin for first-stage recovery.

[JoeSchmuckatelli]

52 minutes ago, sevenperforce said:

delivered between two and seven satellites to LEO per launch

I never knew that!  Thanks! 

[SunlitZelkova]

How hard was it to build a plutonium production reactor circa 1950?

The Hanford Site is located in a curious location- it was/is just within range of a one way Tu-4 strike. I am wondering if it would be feasible to knock out American nuclear weapons production in a single blow for a story I am writing. Or would recovery be easy?

Edited December 2, 2022 by SunlitZelkova

[kerbiloid]

3 hours ago, SunlitZelkova said:

How hard was it to build a plutonium production reactor circa 1950?

For comparison, in the USSR.
(But with money and on another continent it's easier.)

First they built the F-1 test reactor.

https://ru-m-wikipedia-org.translate.goog/wiki/Ф-1_(реактор)?_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp

https://nsrus-ru.translate.goog/o-reaktore-f1.html?_x_tr_sch=http&_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp

Then they built the A-1 industrial reactor. 

https://ru-m-wikipedia-org.translate.goog/wiki/А-1?_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp

 

3 hours ago, SunlitZelkova said:

The Hanford Site is located in a curious location- it was/is just within range of a one way Tu-4 strike.

Tu-4 aka B-29 isn't intercontinental, so it's far beyond its range (unless Alaska or Greenland were first captured).

B-36 was the first, and the first comparable (in sense of distance range) Soviet ones (Tu-95 and M-4) had appeared in 1952.

Edited December 2, 2022 by kerbiloid

[sevenperforce]

1 hour ago, SunlitZelkova said:

How hard was it to build a plutonium production reactor circa 1950?

The Hanford Site is located in a curious location- it was/is just within range of a one way Tu-4 strike. I am wondering if it would be feasible to knock out American nuclear weapons production in a single blow for a story I am writing. Or would recovery be easy?

The Hanford Site was selected because (among other, lesser important reasons) it was very close to a hydroelectric dam but reasonably far from population centers. They needed lots of space, lots of electricity, and not many people.

It would absolutely have been feasible to knock out the Hanford Site by a one-way strategic bomber mission (assuming the Soviets could have penetrated American air defenses and radar, which was quite formidable), but it would have only set back the nuclear weapons production by the amount of time that was invested in the Hanford Site. If the Soviets actually launched an attack on American soil, we would have put up plutonium production everywhere that had electrical power, population centers be damned.

[SunlitZelkova]

3 hours ago, kerbiloid said:

Tu-4 aka B-29 isn't intercontinental, so it's far beyond its range (unless Alaska or Greenland were first captured).

For a suicide mission with no return it would work. The crews would either be captured or ditch in the Pacific for a submarine.

These theories, put forward by American authors, assume Arctic bases were available though. I would assume airfields in places like Anadyr and Tiksi weren’t built until the mid 50s at least.

[kerbiloid]

1 hour ago, SunlitZelkova said:

For a suicide mission with no return it would work

Tu-4 official range = 5 100 km

Kaliningrad - New York = 6 600 km.
Chukotka - New York = 6 300 km

So, no way.
It could reach the North states from Far North, but not the Washington state.

Only if first capture and use the Alaska or Greenland, or use the Arctic drifting ice airdromes which haven't succeeded very much.
The Alaska or Greenland capture would require a lot of cheap light bombers like Il-28, which appeared in 1949, but were available in mass by 1952+.

So, for the sci-fi purpose maybe a secret ice base near the pole, secretly receiving several Tu-4 for that one dedicated target.

(Before the fighters could be alerted).

But by mid-1950s the A-bombs were rare and weak, so didn't mean much.

The first boosted device (operation Greenhouse, George and Item) were tested in 1951 (the Soviet one in 1953).

The first wannabe thermonuclear TX-16/EC-16/Mark 16 was available in 1952, six units, one of them exploded in 1952.
They were fueled with liquid deuterium, weighted 19 t, and could be delivered only by B-36 (piston-engined, 400 kmh speed, thus in a suicidal attack, as the liquid deuterium cistern unlikely could be chuted and stay intact for delayed explosion).
Were created only as "EC" (colloquial- Emergency Capability) to have something before a solid-fueled one gets ready (irl in 1954).

The real Soviet one came in 1955, but was not put in production.

So, a really nuke war could begin by 1956+ (like the Dropshot'49 plan had presumed, in 1957).

Until then, a conventional but assisted with nukes could take place.
In such mostly-conventional war the Soviets didn't have enough nukes and planes to bomb US, while the Americans had two hundred tactical nukes to bomb mostly nothing, because the USSR was still living in a semi-war mode, with distributed industry, and population living in almost war conditions, building nothing but factories, shelters, and residential wooden barracks (because why build good if it anyway will be bombed in several years).
In 1956 (XX congress of the CPSU) it was officially decided to start a peaceful international competition instead of preparing to immediate global war, the army was decreased, the prisoners from labour camps were mass released, the residential construction turned to more comfortable multi-storey houses (aka "khrushchevka", a cheap 5-storey building), the urbanization ran faster (causing economical problems by higher QoL standards), the propaganda got much softer ("Khrushchev's Thaw"), the first international festival took place in Moscow, and several years later, after the Cuban Crysis, then-modern ICBM were put on duty.

Exactly at that time first long-range ballistic missiles (IRBM, ICBM, SLBM) came on service, and the sides decided to make better defence on the Earth and to start competing in space.

1957..58 - ICBM delivered first satellites (coincidentally matching the then-modern nuclear warhead mockup mass and dimensions).
1958 - US moved from projects Horizon and Lunex to less ambitious but necessary Apollo superproject.
1960s - space race, finished with cost-to-efficiency parity by mid-1970s, and limiting the space usage only with civil (i.e. spy and perspective military R&D) needs, later expanded with global communications.

Edited December 2, 2022 by kerbiloid

[kerbiloid]

_(multipost)

Edited December 2, 2022 by kerbiloid

[DDE]

3 hours ago, SunlitZelkova said:

Anadyr

Anadyr's Ugolnyi airfield was built by the late 1950s, yes, but it was more of a planned development, complete with a nuclear weapons dump carved into sheer rock, and a battery of Pioneer MRBMs for SEAD. In a crash program, you could probably get an infrastructure going a lot earlier, using the competencies built up by ALSIB (who too used Anadyr).

2 hours ago, kerbiloid said:

So, for the sci-fi purpose maybe a secret ice base near the pole

These were explored by Tu-16s and Tu-95s. It was marginal. Could probably work, but marginal.

Edited December 2, 2022 by DDE

[kerbiloid]

2 hours ago, DDE said:

These were explored by Tu-16s and Tu-95s.

By Tu-4 and Il-28, it was in 1940s.

[Rutabaga22]

How can an orbit be slightly retrograde? I thought an orbit was either prograde or retrograde.

[Jacke]

12 hours ago, Rutabaga22 said:

How can an orbit be slightly retrograde? I thought an orbit was either prograde or retrograde.

I think what's meant is an orbit with just over 90° inclination.  Commonly Sun-synchronous orbits.

https://en.wikipedia.org/wiki/Sun-synchronous_orbit

Edited December 3, 2022 by Jacke

[18Watt]

4 minutes ago, Rutabaga22 said:

How can an orbit be slightly retrograde? I thought an orbit was either prograde or retrograde.

I would call a polar orbit which slightly is inclined retrograde to be, in fact, slightly retrograde.  But yes, you are technically correct.  An orbit is either prograde, polar, or retrograde.  

If we're talking about equatorial orbits, then yes, it's either prograde or retrograde, no in-betweens.  But for orbits above 70 degrees inclination, well, those I could see myself calling slightly pro or retro- grade.

[magnemoe]

On 12/1/2022 at 4:38 PM, sevenperforce said:

Constellations have been around for quite a while now. The GPS constellation was launched beginning in 1993 and the Iridium communications constellation was launched beginning in 1997.

We haven't had much in the way of a need for a lunar relay constellation. All crewed landings took place on the near side of the moon, as well as all robotic landers up until Chang'e 4 (which had a dedicated communications link to Earth via a separately-launched relay, Queqiao, at L2). The orbiters we have (like the LRO) are out of communications with Earth half the time, but that's fine since they can just uplink whenever they come back around.

You wouldn't need six smallsats; you can get away with three. That's because you're on the "Earth" side of the moon half the time anyway, so as long as one of the satellites has line-of-sight to a satellite that has line-of-sight to Earth, you're fine. One problem with constellations around the moon, at least in low lunar orbit, is the distribution of mascons that make it hard to maintain orbit. There are a handful of frozen orbits, though, or you could put the constellation in a more distant orbit.

For Artemis II, the crew in Orion will be isolated from Earth briefly during the flyby, but not for very long. It's a free-return after all. Artemis III will place Orion in NRHO which has constant communications with Earth, and the landing will be on the south lunar pole with line-of-sight to Earth. In subsequent missions, Gateway will be in NRHO, and it will have relay capability if NASA wants to do landings on the far side of the moon.

I guess the medium orbit as in 1 moon diameter is more affected by earth and the sun than the gravity irregularities as their effect fade fast with increasing distance.  You want above 1/2 diameter anyway  so you always see one satellite who can view earth. If you want to observe the moon close up you need an low orbit.