Any Astronomy Buffs Here?

[CameronBornAndBred]

Artemis is on its way!

Remember when we were tracking the progress of the JWST?

Here is a tracker for Artemis as it makes its trip to and from the moon:
https://www.nasa.gov/specials/trackartemis/

It's interactive and has a few nice things. You can track numerical progress with the gauges; you can also grab the screen (click and keep depressed) and change the perspective on the view.

Larry
DevilHorse

Artemis will be halfway there very soon. (currently within 200 miles of that point)

[CameronBornAndBred]

Artemis will be halfway there very soon. (currently within 200 miles of that point)

And there it is.

ArtemisHalfway.jpg

[DevilHorse]

And there it is.

ArtemisHalfway.jpg

I'm not sure if anyone is interested in random musings and 'back of the envelope' calculations about the numbers relating to the approach that Artemis is making towards (not to) the moon, but I thought I'd share some thoughts/observations.

I noticed the obvious intuitive reduction in speed (velocity includes direction, so not appropriate for this thought) and I thought OK, the earth's gravity is slowing down Artemis as it approaches the moon. At some point the affects of gravity of the moon will be more than that of the earth on Artemis, and Artemis will start to accelerate (increase its speed). Where is that? Rather than doing the basic calculation to determine that point, I realized that this is just the L1 lagrange point (that spot between the earth and the moon where gravity pull is equal - and unstable). The L1 Lagrange point for the earth moon system (rounded off to the nearest 1000) is about 38,000 miles from the moon and 201,000 miles from the earth, along the straight line between the earth and moon. That checks out since the distance from the centers of the Earth and Moon is approximately 239,000 miles.

But if you look at the Artemis miles to moon/from Earth tickers, you notice that the sum of those distances is way bigger than 239K miles. It is more like 319K miles. Of course, Artemis approaches where the moon WILL BE in (approximately) an elliptical path (ellipse being in 2 dimensions, but close to an ellipse for about the first half of the journey only). This suggests to me that the locus of points around (3 dimensions now) the moon, where the gravity and the earth are equal (the set of points where there is no acceleration/deceleration due to gravity) is approximately an ellipse around the moon, in the plane of the earth/moon system, with the L1 and L3 Lagrange Points intersection this theoretical shape; the 3 dimensional shape is actually called the equigravisphere (funny, the DBR spell checker doesn't like this word). The equigravisphere is actually a sphere, but the moon is not at the center. So the distance of Artemis from the moon that the speed will start to increase is approximately 51,000 miles from the moon and not 38,000 miles; that should occur in about 48 hours from now (that makes it 11/20/2022 at ~2pm, but I'm just interpolating this number). Note that a sphere/circle is a special subset of an ellipsoid/ellipse, so the guess was right.

Of course, Artemis is not landing on the moon and will first be in an orbit 249 miles above the lunar surface (about 0.1% of the earth/moon distance).

Perhaps more than you wanted to know, but I found it interesting.

Disclaimer: I have never taken a course in Astronautics, but this approximation is probably close. Assuming that the Artemis will be doing mid-course corrections (added F=ma) the results might vary a little. But good enough for a 'back of the envelope' calculation.

Larry
DevilHorse

[DevilHorse]

Nasa wipes the Demon Deacons off the map.



https://www.msn.com/en-us/news/techn...185eb406fca4bf

Hopefully Duke can finish the job today!

Here's an interesting article from yesterday.

Protecting the Earth from space rocks doesn't just require DART-like rockets; it requires detecting mid sized astronomical projectiles long enough for the DART-like defenses 'save earth.'
https://www.msn.com/en-us/news/techn...e10d78ba404b07
Although the lead time on this asteroid was very little, so was the asteroid. The interval between identifying threatening space rocks/junk and being able to do something about it is an ever decreasing one. This embryonic space technology is very encouraging and apparently evolving rapidly (probably because it is garnering publicity and interest). Time to get more funding.

Jupiter and Mars are beautiful at night mid-evening.

Larry
DevilHorse

[CameronBornAndBred]

This sounds like a looming nightmare.

Intended to be the start of an orbiting communications network that can be accessed by standard smartphones, the recently launched prototype BlueWalker 3 satellite is now one of the brightest objects in the night sky.
...
BlueWalker 3 is certainly an impressive bit of hardware. Its 693-square-foot (64-square-meter) antenna array is the largest commercial array in low Earth orbit, capable of reflecting much more light than the SpaceX Starlink satellites, for example.
The intention of parent company AST SpaceMobile is to get more than 100 satellites up in the sky by the end of 2024, many potentially even bigger than BlueWalker 3. That's a significant worry for scientists.

We've seen the complaints about the Starlink satellites, and the valid reasons for them in the numerous streaks showing up in photos. These will dwarf those. The article goes on to explain that not only will astronomy be affected visually, but given the nature of the satellite system that they very well may affect radio array satellites as well.
Ugh.
https://www.msn.com/en-us/news/techn...57cd232f00d44d

[DevilHorse]

Two new minerals were discovered in a meteorite that fell in Somalia in 2020:
https://www.the-sun.com/tech/6793570...wo-substances/

Wonder what, if any, properties these minerals have; can they be recreated in the lab?

There are more than 4000 minerals existing (naturally) on Earth.

Larry
DevilHorse

[DevilHorse]

Hopefully Duke can finish the job today!

Here's an interesting article from yesterday.

Protecting the Earth from space rocks doesn't just require DART-like rockets; it requires detecting mid sized astronomical projectiles long enough for the DART-like defenses 'save earth.'
https://www.msn.com/en-us/news/techn...e10d78ba404b07
Although the lead time on this asteroid was very little, so was the asteroid. The interval between identifying threatening space rocks/junk and being able to do something about it is an ever decreasing one. This embryonic space technology is very encouraging and apparently evolving rapidly (probably because it is garnering publicity and interest). Time to get more funding.

Jupiter and Mars are beautiful at night mid-evening.

Larry
DevilHorse

More space rocks headed here today. The activity in finding these objects seems to have picked up; and it is a good thing too.
https://www.msn.com/en-us/news/techn...ystemprelaunch

This rock is 140 feet wide while Dimorphous (hit by DART) is 530 feet. No doubt a craft like DART could have had an impact on its' orbit/trajectory.

For perspective:
Estimate for the size of the asteroid that would obliterate life on earth is one with a diameter of 60 miles (316k feet). This new asteroid is about 1/2000 the diameter of the killer asteroid, and therefor 1/8_billion the mass (for the same density). I expect we would see the big one coming, but how far in advance? Bellus anyone?
It is also estimated that space rocks smaller than 82 feet would burn up in the atmosphere.

Larry
DevilHorse

[WillJ]

Two satellites collided in 2009, but I'm confused about some of the physics...can someone tell me where my reasoning goes off the rails?

1. The two satellites were at the same altitude but on different orbits - sort of like one going around equator and one going around the poles.
2. Their collision must have slowed their orbital velocity as they glanced/collided off of one another. The reduction in velocity would be greater as the collision became more "head-on."
3. Given that their altitude didn't change and their velocity fell, they should fall to earth.

[DevilHorse]

Two satellites collided in 2009, but I'm confused about some of the physics...can someone tell me where my reasoning goes off the rails?

1. The two satellites were at the same altitude but on different orbits - sort of like one going around equator and one going around the poles.
2. Their collision must have slowed their orbital velocity as they glanced/collided off of one another. The reduction in velocity would be greater as the collision became more "head-on."
3. Given that their altitude didn't change and their velocity fell, they should fall to earth.

Allow me to take a shot.

Before answering, let's be on the same page as to what an orbit around the earth is geometrically. It is an ellipse with the earth at one of focii.

1. The two satellites were at the same altitude but on different orbits - sort of like one going around equator and one going around the poles.

You could have 2 orbits in the same plane (around the equator), but one ellipse has a bigger eccentricity (math translation: eccentricity of a circle is 0, eccentricity of an elongate ellipse that is so far out it is almost a parabola.. For eccentricity = 1 it IS a parabola where the orbiting object doesn't come back. Higher eccentricities are hyperbolas). 2 orbits in the same plane, with different eccentricities would intersect at 2 points. In Newtonian mechanics, the trace of the orbits intersect at the same place, relative to the earth, always. I'll spare you any mention of General Relativity (not relevant here).

Another definition: The eccentricity of an ellipse. It is the ratio of the distances from the centre of the ellipse to one of the foci and to one of the vertices of the ellipse, i.e., e = c/a where a is the length of semi-major axis and c is the distance from centre to the foci. With a circlular orbit, c is 0 because the focus is the center of the circle.

Bonus fact: The eccentricity of earth's orbit around the sun is 0.0167. The longer axis of the ellipse occurs during our winter. The moon's eccentricity around the earth is 0.0549. These differences in eccentricity is why sometimes we have annular eclipses or longer eclipses (one factor anyway). The most eccentric (yes, that is what astronmers call it) of the planets is Pluto at 0.2444 which is why it sometimes is closer to the sun than Neptune and sometimes not.

But I digress.

2. Their collision must have slowed their orbital velocity as they glanced/collided off of one another. The reduction in velocity would be greater as the collision became more "head-on."

Collisions that preserve energy are called elastic collisions. These are ideal descriptions. A collision between 2 satellites is definitely not elastic. Elastic collisions preserve the energy in the system. With satellites "crunching" you have lost energy to deformation of the satellites themselves, and no doubt turned pure kinetic energy into some rotational energy of the satellites; so the system will lose a great deal of energy which will contribute to a change of orbit.

There are a lot of factors that would go into where the energy went during this crash. But you will definitely lose energy in total and most likely both satellites will suffer.

3. Given that their altitude didn't change and their velocity fell, they should fall to earth.

Let's use a recent example mentioned in this forum about what would (actually) happen.
Remember DART and it's crash into the asteroid moonlet Didymos? That was a head-on collision (almost) with DART slowing down the energy of Didymos. How did they tell if DART had an impact (literally and figuratively) on Didymos. They used the same technique that they used to discover that Didymos was orbiting Dimorphous; they looked at the light coming from the asteroids and noticed the change in light from it, indicating the orbital period of Didymos. Within a couple of days they found that the DART/Didymos collision had altered the orbital period from 11 hours 55 minutes to 11 hours 23 minutes; a faster period by 32 minutes!!! What this tells us is that with less energy, starting from the same orbital position, Didymos's orbit changed for the faster. That is because with less 'speed' the moonlet started to fall toward Dimorphous and had a speed up to result in a faster orbit around. No doubt the orbit became more eccentric too. The new closest point of the orbit was now closer to Dimorphous, which explains the additional speed and shorter orbital period.

My point here is that we don't know the angles. Did your 2 satellites hit head-on or did they have a glancing blow because they were going in the same direction and one just caught up to the other. These points matter a great deal to determine how much energy was lost by each satellite. One thing is for sure, the paths of both satellites after the crash are ellipses. It is still very probable, if it wasn't a head on collision, that some kind of orbit would continue, not unlike Didymos. It is also possible for the energy of a satellite was greatly decreased, and the angle after the crash was so oblique to the original orbit, that an orbit that continued on was not possible. Consider that a new orbital ellipse could have part of its path go inside the earth; that would mean a crash would definitely happen.

This is an idealized scenario. With satellites and the earth there is friction from our atmosphere that will degrade any orbit. I ignored that to make the answer simpler. Any orbital (elliptic) path that touches our atmosphere will soon put the satellite into a death spiral.

I did not look up the Iridium-Kosmos Problem in answering this.

Feel free to ask questions.

Larry
DevilHorse

[WillJ]

Thanks for the information, DH....very interesting. I hadn't thought of the possibility of a collision between coplanar objects.
I was motivated to ask the question because of academic economists' concern about a "kessler spiral," which I guess occurs when orbiting satellites crash, creating trash, which creates more crashes, which creates more trash, until we have a trash field that would imped the launching of new satellites. Even after your information, I'm puzzled why the crashes - which are, as you note, not completely elastic - don't result in stuff falling to earth. My understanding (perhaps incorrect) is that any orbit is a very careful balance of altitude and velocity, and that any change in velocity at fixed altitude will result in either a fall to earth (if velocity is lost) or a leap off into outer space (if velocity is gained). So I'm still a bit confused, tbh.

[DevilHorse]

Thanks for the information, DH...very interesting. I hadn't thought of the possibility of a collision between coplanar objects.
I was motivated to ask the question because of academic economists' concern about a "kessler spiral," which I guess occurs when orbiting satellites crash, creating trash, which creates more crashes, which creates more trash, until we have a trash field that would imped the launching of new satellites. Even after your information, I'm puzzled why the crashes - which are, as you note, not completely elastic - don't result in stuff falling to earth. My understanding (perhaps incorrect) is that any orbit is a very careful balance of altitude and velocity, and that any change in velocity at fixed altitude will result in either a fall to earth (if velocity is lost) or a leap off into outer space (if velocity is gained). So I'm still a bit confused, tbh.

Did you know that if an astronaut, on a space walk, throws a wrench at the earth that wrench would not hit the earth but would circle around the craft/person where it was thrown from? Realizing that the speed of satellites in low earth orbits are on the order of 4-5 miles per second (that's about 15000 MPH) in their orbits, relatively small changes in velocity would keep anything (wrench or satellite) in some kind of orbit. Compare a baseball pitcher, throwing at 100MPH vs 15000 MPH; not much difference to the wrench/baseball relative to its orbit. You are talking about a huge change in velocity of some part of the 2 satellite system due to a crash. If you imagine that few collisions would be at that relative speed. It is much more likely to be a slower crash, hurting the satellites (creating debris) but keeping an orbit.

Stuff does fall to earth. Some of it burns up before getting here, most hit the ocean, a few aim at cows in Montana.
https://theconversation.com/space-de...roperty-188062
Only one person is known to have been hit by space debris.

Most satellites rotate around the earth with the same direction as the earth rotates around the sun; it is a convention that most use. But other satellites, that have different purposes, can have different orbits (i.e., polar orbits or E/W vs W/E orbits), but care is taken to try and have a different height so as not to create crashes. This would suggest that co-planar 'bumps' between satellites might be more likely than head on collisions. Of course, that doesn't consider encounters with the space junk from the crash you mentioned.

There is more thinking and planning going into the removal of satellites from orbit. Most of the debris that is up 'there' will slowly fall to earth (some disintegrating), but I haven't seen the relative probabilities of a piece of junk falling clear vs. hitting a satellite. I think they are trying to identify (large) objects that might be close to debris fields. Not unlike DART, better identification and planning is probably in the early stages and will improve in time; hopefully before another big event happens.

Larry
DevilHorse

[WillJ]

Great stuff, DH. Thanks for the education.

[DevilHorse]

How to create a wormhole:
https://www.quantamagazine.org/physi...uter-20221130/

Larry
DevilHorse

[Skydog]

How to create a wormhole:
https://www.quantamagazine.org/physi...uter-20221130/

Larry
DevilHorse

I think I went down a wormhole reading that. Fascinating stuff. Or fascinating non-stuff. Not sure of anything anymore.

[-jk]

Twice timely XKCD:



-jk

[DevilHorse]

Twice timely XKCD:



-jk

I could have posted this in the UFO thread (however buried that is). The Venn Diagram intersection between Astronomy and the UFOs threads are things like wormholes.

Xkcd is the best.

Larry
DevilHorse

[DevilHorse]

This sounds like a looming nightmare.



We've seen the complaints about the Starlink satellites, and the valid reasons for them in the numerous streaks showing up in photos. These will dwarf those. The article goes on to explain that not only will astronomy be affected visually, but given the nature of the satellite system that they very well may affect radio array satellites as well.
Ugh.
https://www.msn.com/en-us/news/techn...57cd232f00d44d

Found this slightly positive article:
https://www.military.com/daily-news/...ial-risks.html

Anyone watching Mars dance with the moon this week?
On Wednesday the 7th there was an occultation of mars.
https://twitter.com/MadeInCanada_eh/...497601/photo/1
https://twitter.com/i/status/1600703524907556864

Just a bit of cosmic posterizing.

Here's a back of the envelope calculation of the apparent size of Mars vs. the Moon:
https://twitter.com/AndrewRBate/stat...734786/photo/1

Larry
DevilHorse

Larry
DevilHorse

[DevilHorse]

Artemis is scheduled to come down at 12:40pm ET.

Here is a link:
https://www.nasa.gov/content/live-co...on-to-the-moon

Larry
DevilHorse

[Skydog]

Found this slightly positive article:
https://www.military.com/daily-news/...ial-risks.html

Anyone watching Mars dance with the moon this week?
On Wednesday the 7th there was an occultation of mars.
https://twitter.com/MadeInCanada_eh/...497601/photo/1
https://twitter.com/i/status/1600703524907556864

Just a bit of cosmic posterizing.

Here's a back of the envelope calculation of the apparent size of Mars vs. the Moon:
https://twitter.com/AndrewRBate/stat...734786/photo/1

Larry
DevilHorse

Larry
DevilHorse

The 100 to 1 ratio in apparent sizes IS surprising. But there is that whole “distance” thing. For example, my girlfriend’s head looks to be 20 times the size of the moon.

[DevilHorse]

https://twitter.com/latestinspace/st...589318/photo/1

Larry
DevilHorse