Any Astronomy Buffs Here?

[DevilHorse]

Speaking of Telescopes, the following article about the James Webb Telescope crossed my attention today:
https://www.airspacemag.com/airspace...EwMDM0NTQ2NwS2

Next great telescope, yadda yadda..

What caught my attention was that unlike the Hubble Telescope (which orbits the earth, although thanks to Newton's conservation of inertia, always faces the same direction in space), the James Webb would be placed at the L2 Lagrangian Point of the Earth.

Not sure how many of you are familiar with Lagrange, his many contributions to celestial mechanics, or specifically, the Lagrangian Points.
https://solarsystem.nasa.gov/resourc...agrange-point/
The James Webb will take a month to be 'placed' in the L2 Lagrangian point which is along the Sun/Earth access, but the one on the far side of the Earth from the Sun, and about 4 times the distance from the moon (1.5M km or 1M miles about). There are 5 Lagrangian Points between any 2 rotating masses; 3 unstable ones along the line between the masses, and 2 that are off to the sides of the rotating bodies (if you are familiar with the Trojan Asteroids that precede and follow Jupiter in a 60 degree triangle, they are in the 2 off to the side Lagrangian points of the Jupiter/Sun system).

The L2 Lagrangian point of the Earth/Sun system is where the Webb Telescope will find its new home and apparently it is where other satellites will be placed in the future because of desirable properties. Anyone with DirecTV or Dish network knows about GEOsynchronous satellites, which seem to sit in the sky at 22.2 KMile orbits above the equator. The L2 Lagrangian point is yet another stable point, relative to the sun, where the James Webb aperture and electronics can be protected (the Earth shades it from the sun). Some of these points are stable (i.e., the L4 and L5 where the Trojan asteroids are) and the others along the 2 body access are not (you can understand that with a little thought experiment that with 2 rotating bodies, a third body can be in perfect equilibrium between them, or outside of them, along the access as they spin, but the slightest perturbation, and the third body will fly off). This means that any satellite at the L2 Lagrange point will have to occasionally correct its position to stay at the L2 Lagrange point.

You may not have come across Lagrange Points unless you read Astronomy articles, took an intermediate mechanics physics class, or astronomy classes.

Larry
DevilHorse

[-jk]

Sounds like fun: https://www.nasa.gov/planetarydefense/dart

"DART is a planetary defense-driven test of technologies for preventing an impact of Earth by a hazardous asteroid. DART will be the first demonstration of the kinetic impactor technique to change the motion of an asteroid in space."

"The DART spacecraft will achieve the kinetic impact deflection by deliberately crashing itself into the moonlet at a speed of approximately 6.6 km/s, with the aid of an onboard camera (named DRACO) and sophisticated autonomous navigation software. The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, but this will change the orbital period of the moonlet by several minutes - enough to be observed and measured using telescopes on Earth."

The only problem with space stuff is that I have no patience!

-jk

[duke79]

Sounds like fun: https://www.nasa.gov/planetarydefense/dart

"DART is a planetary defense-driven test of technologies for preventing an impact of Earth by a hazardous asteroid. DART will be the first demonstration of the kinetic impactor technique to change the motion of an asteroid in space."

"The DART spacecraft will achieve the kinetic impact deflection by deliberately crashing itself into the moonlet at a speed of approximately 6.6 km/s, with the aid of an onboard camera (named DRACO) and sophisticated autonomous navigation software. The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, but this will change the orbital period of the moonlet by several minutes - enough to be observed and measured using telescopes on Earth."

The only problem with space stuff is that I have no patience!

-jk

THis is pretty cool but my concern is I hope they don't miscalculate and send the asteroid on a direct collision course with earth!

[CameronBornAndBred]

THis is pretty cool but my concern is I hope they don't miscalculate and send the asteroid on a direct collision course with earth!

That would be impossible. Those rocket guys are always on point with their calculations!

https://www.latimes.com/archives/la-...288-story.html

[DevilHorse]

THis is pretty cool but my concern is I hope they don't miscalculate and send the asteroid on a direct collision course with earth!

Here are my off-hand thoughts:

Just as a background, when a meteor/meteorite/bolide/bomb blows up on a trajectory to earth, the center of mass of all of the pieces land exactly where the object would have landed had it been intact. So if you put an explosive on an asteroid coming at earth, it blows the asteroid up, but the center of mass of the asteroid pieces still heads right through earth. Not that this applies in this case, but center of mass is important in a multi-body system.

While it appears that the Didymos' primary object should be unaffected, because the moonlet is the target and will be perturbed, this is not necessarily true. No matter where DART hits on the Didymos binary, the center of mass will be affected and perturbed. Since the goal is that the moonlet's course around the Primary will only be perturbed, it isn't likely that the DART satellite will have much of an impulse anyway. Not sure if DART will be hitting the moonlet head-on or just a glancing blow to give the moonlet an oompf in a direction. The 6.6 km/s impact velocity cited in the article could be while the moonlet is going 50 km/s around the Primary, so the impulse could be relatively small; we don't have the information. It also might not be perpendicular to the velocity of the moonlet, which takes the edge off the impulse and the effect. Lots of variables here.

Based on the numbers provided, the Primary object can be estimated to be about 115 times the mass of the moonlet. If the moonlet (or Primary) is hit in the direction of the path of the orbit, there will be little change in the direction of the Didymos system, just the speed (and therefore the ellipse/orbit); if the hit is perpendicular to the present orbit of Ditimos, then there won't be a change in the speed, but there will be a change in the path/ellipse/orbit. I presume that the NASA scientists are attempting to cause as much change to the moonlet trajectory with as little to the Didymos combo orbital trajectory as possible. They seem to be looking at a very small observable affect. It has to be small enough to see, but not big enough to do (unforseen) harm. We have major trouble solving the 3 body problem in physics and this is a 4 body problem (2 bodies, 1 satellite, and the earth). The 2 body problem is completely solvable with math. The 3 body problem is successively solvable with computers.

Anyway, looks to be too small to make a difference in the Didymos system to hurl it toward earth.

Larry
DevilHorse

[duke79]

Here are my off-hand thoughts:

Just as a background, when a meteor/meteorite/bolide/bomb blows up on a trajectory to earth, the center of mass of all of the pieces land exactly where the object would have landed had it been intact. So if you put an explosive on an asteroid coming at earth, it blows the asteroid up, but the center of mass of the asteroid pieces still heads right through earth. Not that this applies in this case, but center of mass is important in a multi-body system.

While it appears that the Didymos' primary object should be unaffected, because the moonlet is the target and will be perturbed, this is not necessarily true. No matter where DART hits on the Didymos binary, the center of mass will be affected and perturbed. Since the goal is that the moonlet's course around the Primary will only be perturbed, it isn't likely that the DART satellite will have much of an impulse anyway. Not sure if DART will be hitting the moonlet head-on or just a glancing blow to give the moonlet an oompf in a direction. The 6.6 km/s impact velocity cited in the article could be while the moonlet is going 50 km/s around the Primary, so the impulse could be relatively small; we don't have the information. It also might not be perpendicular to the velocity of the moonlet, which takes the edge off the impulse and the effect. Lots of variables here.

Based on the numbers provided, the Primary object can be estimated to be about 115 times the mass of the moonlet. If the moonlet (or Primary) is hit in the direction of the path of the orbit, there will be little change in the direction of the Didymos system, just the speed (and therefore the ellipse/orbit); if the hit is perpendicular to the present orbit of Ditimos, then there won't be a change in the speed, but there will be a change in the path/ellipse/orbit. I presume that the NASA scientists are attempting to cause as much change to the moonlet trajectory with as little to the Didymos combo orbital trajectory as possible. They seem to be looking at a very small observable affect. It has to be small enough to see, but not big enough to do (unforseen) harm. We have major trouble solving the 3 body problem in physics and this is a 4 body problem (2 bodies, 1 satellite, and the earth). The 2 body problem is completely solvable with math. The 3 body problem is successively solvable with computers.

Anyway, looks to be too small to make a difference in the Didymos system to hurl it toward earth.

Larry
DevilHorse

Thanks so much for your explanation (even though I don't understand a single word of it!). Now I can sleep better, knowing we are safe.

[CameronBornAndBred]

Thanks so much for your explanation (even though I don't understand a single word of it!). Now I can sleep better, knowing we are safe.

I think I do, and it made me think of something I would never have pondered (and won't ever affect us in our species lifetime, most likely).

The Earth and Moon is a multi body system, also, and a pretty significant one. As we know, the Moon is slowly drifting away from Earth. Does the center of gravity of the two stay constant, though, and will there ever come a time that it has been adjusted enough that the Earth's orbit around the Sun is no longer what it is? And was Earth's orbit in the past different when the Moon was much closer? I'm guessing it had to have been before Theia knocked the stuffing out of us and thus created the Moon.

[DevilHorse]

I think I do, and it made me think of something I would never have pondered (and won't ever affect us in our species lifetime, most likely).

The Earth and Moon is a multi body system, also, and a pretty significant one. As we know, the Moon is slowly drifting away from Earth. Does the center of gravity of the two stay constant, though, and will there ever come a time that it has been adjusted enough that the Earth's orbit around the Sun is no longer what it is? And was Earth's orbit in the past different when the Moon was much closer? I'm guessing it had to have been before Theia knocked the stuffing out of us and thus created the Moon.

No, the center of mass does not stay constant. As the moon moves away (about 1.4" per year from the center of the earth) the center of mass moves fractionally. The moon is 1.2% of the earth's mass; the center of mass is a function of the sum of the [sum of the mass times the distance from the center] of each of the objects involved../divided by the total mass. So the center of mass is essentially (approximately) moving at 0.007" per year away from the center of the earth. Right now, the center of mass of the earth moon system is INSIDE THE EARTH. The earth's radius is about 4000miles and the Center of Mass is about 1620miles. So we have a little wobble that happens 12 times a year as we rotate around the sun.

To answer your question: The earth's orbit is not changed by what is happening in the Earth/Moon system. The Center of Mass is what is circulating around the sun as a system and is the package that is affected by gravity (in a Newtonian view of things). Many of our physics problems or more easily solved in the center of mass frame.

There are other wobbles in our trip around the sun. Our earth's axis, that points toward the North Star (Polaris). This is a natural affect, the way a (spinning) top twists at the end of its wobble when you were a kid; it actually nutates and precesses (2 kinds of turnings). The access of the top changing its orientation is the nutation; the long spinning out spiral is called precession.. but I digress. Anyway the nutation of the earth's access is 26,000 years. In 13000 years, the North Star will be VEGA!!

Now, we have a problem. In 50 billion years, the moon will stop moving away from the earth and will start to move toward the earth, eventually crashing into us (unless of course we have a satellite like DART to deflect it). HOWEVER, in 5 billion years, the sun will run out of Hydrogen and Helium and will become a Red Giant and grow large enough that it will envelope the earth. No more air conditioning bills, but, well.. you know. Buy marshmellow futures.

I hope it is ok to put some things in inches and some in miles. Easier to understand in this context I think.

Larry
DevilHorse

[sagegrouse]

Here are my off-hand thoughts:

Just as a background, when a meteor/meteorite/bolide/bomb blows up on a trajectory to earth, the center of mass of all of the pieces land exactly where the object would have landed had it been intact. So if you put an explosive on an asteroid coming at earth, it blows the asteroid up, but the center of mass of the asteroid pieces still heads right through earth. Not that this applies in this case, but center of mass is important in a multi-body system.

While it appears that the Didymos' primary object should be unaffected, because the moonlet is the target and will be perturbed, this is not necessarily true. No matter where DART hits on the Didymos binary, the center of mass will be affected and perturbed. Since the goal is that the moonlet's course around the Primary will only be perturbed, it isn't likely that the DART satellite will have much of an impulse anyway. Not sure if DART will be hitting the moonlet head-on or just a glancing blow to give the moonlet an oompf in a direction. The 6.6 km/s impact velocity cited in the article could be while the moonlet is going 50 km/s around the Primary, so the impulse could be relatively small; we don't have the information. It also might not be perpendicular to the velocity of the moonlet, which takes the edge off the impulse and the effect. Lots of variables here.

Based on the numbers provided, the Primary object can be estimated to be about 115 times the mass of the moonlet. If the moonlet (or Primary) is hit in the direction of the path of the orbit, there will be little change in the direction of the Didymos system, just the speed (and therefore the ellipse/orbit); if the hit is perpendicular to the present orbit of Ditimos, then there won't be a change in the speed, but there will be a change in the path/ellipse/orbit. I presume that the NASA scientists are attempting to cause as much change to the moonlet trajectory with as little to the Didymos combo orbital trajectory as possible. They seem to be looking at a very small observable affect. It has to be small enough to see, but not big enough to do (unforseen) harm. We have major trouble solving the 3 body problem in physics and this is a 4 body problem (2 bodies, 1 satellite, and the earth). The 2 body problem is completely solvable with math. The 3 body problem is successively solvable with computers.

Anyway, looks to be too small to make a difference in the Didymos system to hurl it toward earth.

Larry
DevilHorse

Actually, I don't buy this. If you put a rocket on the side of a moonlet, and set it off, it would deflect the path of the moonlet in the direction opposite the rocket thrust. If instead of a rocket, you have an explosive, the same thing would happen -- the moonlet's path would be diverted. It would be much as what would happen if you placed an explosive charge on the side of an object sitting on the ground -- the object would be moved away from where the explosion occurred.

[PackMan97]

THis is pretty cool but my concern is I hope they don't miscalculate and send the asteroid on a direct collision course with earth!

They won't miscalculate. It will be done in purpose.

[DevilHorse]

Actually, I don't buy this. If you put a rocket on the side of a moonlet, and set it off, it would deflect the path of the moonlet in the direction opposite the rocket thrust. If instead of a rocket, you have an explosive, the same thing would happen -- the moonlet's path would be diverted. It would be much as what would happen if you placed an explosive charge on the side of an object sitting on the ground -- the object would be moved away from where the explosion occurred.

You are talking about 2 different things I think. Although I didn't disagree with what you diagnosed the intent of DART. It is indeed an external force being introduced.

A rocket that is introducing thrust from outside of a system (setting it off I presume) is also introducing force into the system (F = M A), so you are changing the Center of Mass and the trajectory of the system. In the case of DART, (hitting the system from the outside) this is very small and the intent is to perturb the moonlet so as to make its orbit different, but not the center of mass change, as I understand it. Changing the Center of Mass or the orbit a lot would be dangerous; changing the moonlet orbit around the Primary asteroid is safer. Doing it without changing the Center of Mass of the system is a neat trick. I imagine they've done calculations about what this will do to the orbit. If the DART strike is perpendicular to the direction of the Asteroid system, the center of mass speed may not be changed at all; F(direction of orbit) is unchanged; the kick may only be perpendicular to the orbit to exaggerate the moonlit orbit; that is what they said they are trying to do.

However, if you put an explosive on a meteor, (and it explodes), when the pieces fall (all over the place) the center of mass hits exactly where it would hit as if the object didn't fall apart; the center of mass doesn't change. There is no outside force in an explosion. That would be different if the explosion was next to the meteor (a nuance), which would create an impulse to blow a meteor to the side; but that would require significant precision in space where I have no idea what kind of an impulse gets created by an explosion in a vacuum.
There is discussion of this same subject of explosions on an object here:
https://physics.stackexchange.com/qu...center-of-mass

Larry
DevilHorse

[DevilHorse]

Any northern dukies notice extra aurora activity this evening?
https://www.swpc.noaa.gov/news/g2-mo...1-october-2021

Larry
DevilHorse

[OldPhiKap]

Four went up . . .

B12B0A66-6720-4E97-BAFB-E04395A97FDE.jpg

. . . Only one came back.

[CameronBornAndBred]

Four went up . . .

B12B0A66-6720-4E97-BAFB-E04395A97FDE.jpg

. . . Only one came back.

Instead of posting pics about space tonight, you should be enjoying Space (after Drums) at Cellaris. Tsk tsk.

[OldPhiKap]

Instead of posting pics about space tonight, you should be enjoying Space (after Drums) at Cellaris. Tsk tsk.

True. And you are right to call me out on it.

The bus came by, but I did not get on.

[DevilHorse]

It seems to me, based on this map:

https://www.usgs.gov/media/images/fr...king-around-us

If a recruit can't make up his mind between Kentucky and Duke, just mention that Kentucky is twice as likely to get a big earthquake.
That should clinch any decision. No?

Larry
DevilHorse

[DevilHorse]

Lucy mission to Trojan Asteroids:
https://twitter.com/i/status/1446226496541048837

https://www.nasa.gov/mission_pages/lucy/overview/index

Launching Saturday.

Larry
DevilHorse

[DevilHorse]

Lucy mission to Trojan Asteroids:
https://twitter.com/i/status/1446226496541048837

https://www.nasa.gov/mission_pages/lucy/overview/index

Launching Saturday.

Larry
DevilHorse

Lucy took off this morning and has deployed its solar arrays:
https://twitter.com/i/status/1449332038092402691

Larry
DevilHorse

[duke79]

https://dnyuz.com/2021/10/18/how-a-n...lthy-asteroid/

This is cool....but also quite scary!

[DevilHorse]

https://dnyuz.com/2021/10/18/how-a-n...lthy-asteroid/

This is cool...but also quite scary!

Has this not been discussed in bits and pieces (pun intended) up-thread?

1) Hit it far enough away to deflect it if a) you can detect it b) if you have the technology - rocket/bomb ready to go

2) If not try a rocket that will blow it into many smaller pieces so our atmosphere will absorb the smaller pieces (risky because you can't guarantee that you'd be left with Only smaller pieces because that will depend on i) great aim ii) knowing what the asteroid is made of which is REALLY hard to determine at distance

3) Try to blow it up enough so that most of it blows beyond the target of the earth and goes around us, while the center of mass of the asteroid still goes through us. The few bits of Asteroid that still hit our atmosphere will hopefully be very small.

Any of the above would be acceptable, but none are guaranteed.

Larry
DevilHorse