The thread for space cadets!

[warpus]

Do your calculations account for the movement of the solar system during this time interval as well?

I think so! I just calculated how fast you have to go on average to traverse 19 lightyears in 70,000 years, which was the displacement in space-time in between us and the other star

 

[hobbsyoyo]

I think so! I just calculated how fast you have to go on average to traverse 19 lightyears in 70,000 years, which was the displacement in space-time in between us and the other star

If that's how you did it then sadly no it doesn't. It's a decent approximation absent data but your calculation assumes that the solar system has stood stationary while the other star moved away. In reality they are both moving with their own relative velocities so you can't just take 19 years/distance to get speed. But like I said it's a decent enough approximation that will get you in the right order of magnitude. Another decent approximation (that's probably slightly better) is to take your number and divide it in half - this assumes that the two star systems are both moving away from each other at roughly the same velocity rather than assuming one star is stationary.

 

[Lexicus]

If that's how you did it then sadly no it doesn't. It's a decent approximation absent data but your calculation assumes that the solar system has stood stationary while the other star moved away.

Doesn't that just mean it's an accurate calculation of that star's motion relative to the Sun?

 

[hobbsyoyo]

Apparent relative motion, yes. But the sun isn't stationary. Things get really sticky once you try and sort out the reference frames which is why I said it's a decent approximation but there are better ones.

 

[hobbsyoyo]

Canada is having a rough time right now in the space industry. They are trying to set up a launch site for a Ukrainian rocket (Cyclone 4) to do polar launches but the construction of that launch site has been delayed by a year due to the need to perform more environmental impact studies and other paperwork issues. Though in all honesty I'm sure Cyclone 4 itself will be the long pole in that schedule rather than the launch complex.

Also, a major ground station provider set up a station in Northern Canada like 2 years ago but can't actually use it because Canadian bureaucrats are seriously dragging their feet on awarding them a license. The company has filled out all of the requisite paperwork, the government is just sitting on it for unknown reasons. The company in question has stated they will pull their equipment and go elsewhere if no action is taken by the government in the next couple of months.

 

[Lexicus]

But the sun isn't stationary.

From its own frame of reference, it is, no?

Things get really sticky once you try and sort out the reference frames which is why I said it's a decent approximation but there are better ones.

LSR or galactic center?

 

[hobbsyoyo]

From its own frame of reference, it is, no?



LSR or galactic center?

Yeah but a frame of reference centered on the sun isn't super useful for these sorts of calculations precisely because the sun really isn't stationary. I mean nothing is, but assuming it is gives you velocity estimates for other objects that are not close enough to reality to be useful for anything but back of the napkin estimates.

It's kind of like being in aircraft 1 (speed 500 mph) and measuring aircraft 2's speed (also 500 mph) and concluding aircraft 2 is actually going well over Mach 1 because they are flying in opposite directions and you aren't accounting for aircraft 1's own velocity.

I truthfully don't know what a good reference frame would be for this situation, just that a sun-centered one that is assumed at rest is not a great one but will get you in the correct order of magnitude.

 

[warpus]

If that's how you did it then sadly no it doesn't. It's a decent approximation absent data but your calculation assumes that the solar system has stood stationary while the other star moved away. In reality they are both moving with their own relative velocities so you can't just take 19 years/distance to get speed. But like I said it's a decent enough approximation that will get you in the right order of magnitude. Another decent approximation (that's probably slightly better) is to take your number and divide it in half - this assumes that the two star systems are both moving away from each other at roughly the same velocity rather than assuming one star is stationary.

But I got the numbers from a piece that said "The star was 1 light year away from earth, and now it is 19 light years away" so it's only mentioning relative distance between the suns, which is enough to calculate the relative average velocity between them (right?)

 

[hobbsyoyo]

But I got the numbers from a piece that said "The star was 1 light year away from earth, and now it is 19 light years away" so it's only mentioning relative distance between the suns, which is enough to calculate the relative average velocity between them (right?)

You are measuring the relative velocity between them but then ascribing the entirety of that velocity to only one of the objects. It's like the airplane example I made.

It's kind of like being in aircraft 1 (speed 500 mph) and measuring aircraft 2's speed (also 500 mph) and concluding aircraft 2 is actually going well over Mach 1 because they are flying in opposite directions and you aren't accounting for aircraft 1's own velocity.

In any case I hope I'm not coming off as nit-picky. I'm just trying to show how tricky these sorts of calculations are. Space is hard.

 

[Lexicus]

I truthfully don't know what a good reference frame would be for this situation, just that a sun-centered one that is assumed at rest is not a great one but will get you in the correct order of magnitude.

From my admittedly quite limited knowledge it seems like the "local standard of rest" is one of the common frames of reference astronomers use to solve these kinds of problems. Apparently you can also figure out motion relative to the Milky Way by getting a star's proper and radial motion and distance from the Sun and then using something called "coordinate transformation".

 

[hobbsyoyo]

Coordinate transformations are simple in concept and devilishly difficult to actually calculate. You have reference frame X, Y, Z for object 1 and A, B, C for object 2. You can correlate between the two frames in a manner similar to A = X+2, B = Y-3 and C = Z + 0.5. Except it's of course way more complicated because you also have to account for velocities, accelerations, angular velocities and angular accelerations for objects in each frame (meaning each axis of each frame) and then correct for the differences. So you wind up with these ugly matrices that mathematically correct for all of the differences to place object 2 in object 1's reference frame.

And then it gets even more complicated when you step back and realize that not only are objects 1 and 2 moving in their own reference frames but the reference frames themselves may be moving and have to be corrected for.

I'm not familiar with local standard of rest reference frames and I suspect that while they would be good for astronomical observations they would not be so great for planning spacecraft trajectories which I am more familiar with. If you only care where a star is in the sky and how it is going to move relative to the Earth then I'm sure LSR is fine. But if you actually really care about how everything is moving relative to anything else so that you can move between objects in time and space (wherein time is measured in months and years and space measured in millions of kilometers) then all of a sudden small inaccuracies that a simplified reference frame bring up mean you miss your target by many thousands of kilometers.

 

[red_elk]

You are measuring the relative velocity between them but then ascribing the entirety of that velocity to only one of the objects. It's like the airplane example I made.

Wouldn't the relative speed calculated by Warpus useful for some purposes, like extrapolating the distance between the Sun and this star for next several thousands years?
I think everybody understand that both stars are moving with their own speeds around the center of galaxy, but for a short time period a linear approximation of their motion should be good enough.

 

[warpus]

No no my number is only the relative speed between the stars. How anything else moves in the solar system should be irrelevant.

Say Mike is 1km away from you and 60 minutes later he's 19 km away from you. Whether you were moving or standing still doesn't matter, these numbers are relative to your position. So what I did was basically calculate that Mike must have been moving about 18 km/h relative to you in order to cover that distance and end up that far away from you. If you were also moving at the time (like the Earth is), Mike's velocity wrt to any other point (galactic centre, etc.) would have been different. But if I'm calculating the average velocity between you and Mike I can ignore everything else that's moving, and can only focus on the relative movements of those 2 bodies

Let me know if that makes sense. After all I'm just a guy who plays video games and you're an actual rocket scientist

You are measuring the relative velocity between them but then ascribing the entirety of that velocity to only one of the objects. It's like the airplane example I made.

Yeah that's it! That's all I was calculating, but not saying that. I'm just saying that is the other star's velocity relative to us. I could have re-worded that and said that is our velocity relative to the star

I have no idea if that number is useful for anything, but it's useful in my mind to see a number that tells me how fast it is moving relative to my shoes. That number is far easier for my brain to wrap itself around than the initial quote: "It was 1 light year away, now 75,000 years later it's 19 light years away". My brain just goes "Huh okay I can't conceptualize that at all"

 

[Berzerker]

is 1 LY close enough to affect Oort Cloud objects? The theory that the inner solar system is occasionally bombarded with comets from passing stars with subsequent mass extinctions comes to mind.

 

[uppi]

In reality they are both moving with their own relative velocities

Sorry, but that statement is so bad that it is not even wrong. In space, there is no such thing as an absolute speed. When you have two objects, you can only ever measure the relative speed of those. Asking how that relative velocity is distributed among the two objects - whether one is stationary or whether both are moving - doesn't even make any sense, unless you include a third object. There are reasons to include a third object, one of them if the reference frames of the objects significantly accelerated, but I don't think any of those apply here.

It's kind of like being in aircraft 1 (speed 500 mph) and measuring aircraft 2's speed (also 500 mph) and concluding aircraft 2 is actually going well over Mach 1 because they are flying in opposite directions and you aren't accounting for aircraft 1's own velocity.

Those speeds are not the "real" speed of the aircraft either. 500 mph refers to the speed compared to the air, the aircraft are moving in. Which is very relevant, because that determines how the aircraft handles, how much fuel is used and so on. But the air is moving at the equator with roughly 1000 mph compared to the center of gravity of the earth. So if one aircraft was moving east and the other west at the equator, one would have a speed of 500 mph and the other one of 1500 mph when compared to that. And even that is not the end of it, because the center of gravity of the earth moves around the sun and then the sun moves around the center of the Milky Way, which moves relative to other galaxy. So for the aircraft, there is no such thing as a correct speed and what you want to know depends on the question you are asking. If you want to know whether the aircraft is supersonic or not, you need to measure the speed relative to the air around. If you want to know how long it takes until two planes 1 km apart collide, you might as well assume that one aircraft is stationary and the other one is moving at 1000 mph. That is as good as calculating their relative speed to Andromeda (assuming we can neglect time dilation), but much easier to calculate.

In space, there is not enough air to be consequential, so that frame of reference won't be useful at all.

Yeah but a frame of reference centered on the sun isn't super useful for these sorts of calculations precisely because the sun really isn't stationary. I mean nothing is, but assuming it is gives you velocity estimates for other objects that are not close enough to reality to be useful for anything but back of the napkin estimates.
[...]
I truthfully don't know what a good reference frame would be for this situation, just that a sun-centered one that is assumed at rest is not a great one but will get you in the correct order of magnitude.

The sun is one of the best reference frames you can use for this, because what else are you going to take. The center for he Milky Way comes to mind, because the sun is in an accelerated motion around it, but for this problem it is a rather bad one. Apart from that we don't know exactly where it is, the effect is pretty small over 70000 years. If you assume that the sun moves in a straight line relative to the center instead of the real, curved orbit, you make an error of roughly 5E-5 light years (about 3 AU). Since the other star feels roughly the same acceleration towards the center, the relative effect will be even much smaller. I don't think that we know the distance right now to a precision of 10^-5, much less the distance 70000 years ago. So it doesn't even make sense to include the effects of the motion relative to the center of gravity into account when you want to know the relative motion.

In fact, I would assume that since the distance between the sun and that star was supposed to be quite small in the past, the effect of assuming constant motion instead of accelerated motion caused by the attraction to each other is much bigger than anything else. Even if you needed a really accurate number, you would probably mostly look at effects that the gravitation caused the nearest stars has. So you would be looking at a multi-body problem, in which none of the bodies is really dominant. So choosing the sun as the reference point is really the best option here, because there is no reason to choose anything else over it and at least you know quite accurately where it is, relative to you. Any small acceleration of the sun can then be handled by a small pseudoforce acting on all other objects.

Long story short: Assuming that the sun is stationary isn't just good for a calculation on the back of an envelope for this problem, but it is really what you should do!

 

[Kaitzilla]

Big news, the EM Drive is a bust!
https://news.nationalgeographic.com...ics-independent-tests-magnetic-space-science/

The group, led by Martin Tajmar of the Technische Universität Dresden, tested the drive in a vacuum chamber with a variety of sensors and automated gizmos attached. Researchers could control for vibrations, thermal fluctuations, resonances, and other potential sources of thrust, but they weren’t quite able to shield the device against the effects of Earth’s own magnetic field.

When they turned on the system but dampened the power going to the actual drive so essentially no microwaves were bouncing around, the EmDrive still managed to produce thrust—something it should not have done if it works the way the NASA team claims.

The researchers have tentatively concluded that the effect they measured is the result of Earth’s magnetic field interacting with power cables in the chamber, a result that other experts agree with.

They need to run one more test in the vacuum chamber after shielding for the Earth's magnetic field to be certain, but I think it is safe to stick a fork in it.

 

[Kaitzilla]

is 1 LY close enough to affect Oort Cloud objects? The theory that the inner solar system is occasionally bombarded with comets from passing stars with subsequent mass extinctions comes to mind.

Yup!
https://en.wikipedia.org/wiki/Scholz's_Star

Solar System flyby
Estimates indicate that the WISE 0720−0846 system passed about 52,000 astronomical units (0.25 parsecs; 0.82 light-years) from the Sun about 70,000 years ago.[2][5] 98% of mathematical simulations of the star system's trajectory indicated it passed through the Solar System's Oort cloud, or within 120,000 AU (0.58 pc; 1.9 ly) of the Sun.[2] Comets perturbed from the Oort cloud would require roughly 2 million years to get to the inner Solar System.[2] At closest approach the system would have had an apparent magnitude of about 11.4, and would have been best viewed from high latitudes in the northern hemisphere, in the autumn mostly.[4] A star is expected to pass through the Oort Cloud every 100,000 years or so.[4] An approach as close or closer than 52,000 AU is expected to occur about every 9 million years.[2] In about 1.4 million years, Gliese 710 will pass somewhere between 8,800 and 13,700 AU from the Sun.

Here's a video of what it probably looked like.

How long until Earth's inevitable destruction?
The wiki said 2 million years for the comets to get here.

1 light year is 9.461 trillion km.
Let's be generous and say it passed through 0.6 light years from Earth 70,000 years ago.
Comets go about 2km/s right?
Speeds up to 20km/s as it gets close to Sun?

2km/s for 70,000 years is 31557600*70000*2=2.209 trillion km
0.6 light years * 9.461 trillion km/ light year = 5.6766 trillion km

I guess if the comets speed up as they fall towards the sun, 2 million years seems about right?

Anyway, Gliese 710 flybys in 1.4 million years and gets the coveted Earth kill first!
https://www.sciencealert.com/rogue-...ystem-encounter-earlier-than-thought-1-29-mya

Berski and Dybczyński found that Gliese 710 would enter the Oort cloud to pass by the Sun a distance of about 13,365 astronomical units (each astronomical unit is the distance between Earth and the Sun).

According to the new research, it will graze us at a distance of 4,303 AU. That's not actually very close - it's over 100 times the distance to Pluto, which orbits the Sun at an average of 39.5 AU. But it still has the potential to disrupt the Solar System.

If humans are still around on Earth at that point, we don't have to worry about any disruptions to our orbit. Gliese 710, which is only about 60 percent of the Sun's mass, may only have that effect on the outer Solar System, if at all, and shouldn't affect anything within 40 AU.

What it could do, however, is behave like the proverbial fox among the Oort cloud's chickens. The Oort cloud is a theoretical sphere of icy planetesimals that is thought to surround the solar system at distances between 200,000 and 50,000 AU.

If Gliese 710 enters the Oort cloud, it could send these planetesimals careening through the solar system - resulting in showers of comets.

1 light year is 63240 AU, so Gliese 710 could blow through 9 times closer than Scholz's Star did.

Gliese 710's doom comets would arrive 300,000 years later 1.7 million years from now, followed by Scholz's doom comets 300,000 years after that.

 

[Berzerker]

isn't that pretty good evidence the Oort Cloud doesn't exist?

If a star passes thru it every 100k we should see craters and extinctions every 100k or so, right?

Course that doesn't mean the Earth gets pummeled each time, but still...

 

[Kaitzilla]

isn't that pretty good evidence the Oort Cloud doesn't exist?

If a star passes thru it every 100k we should see craters and extinctions every 100k or so, right?

Course that doesn't mean the Earth gets pummeled each time, but still...

Jupiter saves us from being pummeled.
https://imgur.com/gallery/ggj28F9

Jupiter took an Earth-ending hit back in 1994.
This one was moving at 60 kilometers per second.
https://en.wikipedia.org/wiki/Comet_Shoemaker–Levy_9

28,000,000 Megatons of boom if you add up all the impacts!
Not as much as the 8 mile wide asteroid that killed the dinosaurs off with 100,000,000 Megatons, but close.

In fact, Jupiter gets constantly pummeled due to its size and gravity.
http://www.slate.com/blogs/bad_astr...r_hit_by_asteroid_or_comet_in_march_2016.html

In 2009 something relatively big hit the planet (and Hubble caught the aftermath). It was hit again in June 2010 (with a cool color photo this time), and then again in August 2010. A repeat performance was held in September 2012.

Interestingly, meteor and comet impacts have a limited range of impact speed with Earth.
They will hit between 11km/s and 71km/s
Anything faster simply leaves the solar system instead of menacing Earth.
http://mathscinotes.com/2011/07/speed-of-a-meteor/

 

[Thorgalaeg]

Something moving at 1000km/s could also hit Earth while pass through the solar system as a bullet. But It would be extremely improbable, like a random bullet hitting a fly in a cathedral. Maybe in some eons when Andromeda and the Milky Way collide.