Free planetarium

 
User avatar
DoctorOfSpace
World Builder
World Builder
Posts: 676
Joined: 22 Aug 2016
Location: SpaceX Mars Colony
Contact:

Asteroids, meteors, and meteorites

23 Jun 2017 14:43

That was quite a spectacle. 
CPU: Intel Core i7-5820K 4.2GHz 6-Core Processor - RAM: G.Skill Ripjaws V Series 32GB (4 x 8GB) DDR4-2400 - GPU: EVGA GeForce GTX 1080 Ti SC Black Edition
Quando omni flunkus, moritati
 
User avatar
N0B0DY
Astronaut
Astronaut
Posts: 69
Joined: 09 Dec 2016

Asteroids, meteors, and meteorites

03 Jul 2017 08:34

What do you guys think of this: Asteroid Impact and Deflection Assessment (AIDA) Mission    ? I hope they don't disturb its orbit much and inadvertently cause it to hit earth ...
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 746
Joined: 06 Sep 2016
Location: Bellingham, WA

Asteroids, meteors, and meteorites

03 Jul 2017 11:55

They couldn't change the orbit enough with this mission to cause it to hit Earth even if they wanted to. :)

If an asteroid would hit Earth many years in the future, it would take only a very small nudge now to make it miss then.  They would like to test this by actually nudging an asteroid in this way and measure the difference it makes.  But the problem is that the change in the orbit is so small that it is difficult to measure.  So the trick here is to impact the moon of a binary asteroid.  This changes the orbital period of the moon around the main asteroid very slightly, and eventually that grows into timing differences that we can measure.  But the orbit of the binary asteroid around the Sun is changed so little that for all practical purposes it might as well be zero, and it cannot turn it into an Earth-crossing orbit.
 
User avatar
N0B0DY
Astronaut
Astronaut
Posts: 69
Joined: 09 Dec 2016

Asteroids, meteors, and meteorites

03 Jul 2017 12:25

Thank you for your always enlightening info Watsisname! So let me see if I understand that correctly and sorry for any wrong assumptions I may write:
They plan to hit the small one and this will cause a small variation in its orbit around the large one which is marginally measurable. They will then extrapolate the result using computer modelling to simulate a more powerful impact on a larger body? Or the change in the orbit of the small one will also cause the binary orbit (the barycenter) to change slightly and hence affect its solar orbit very, very very slightly (almost zero) and use this to model a larger impact? I am confused..

I was always in favor of the idea to control-land a small craft on the surface of an asteroid and somehow plant it there using drilling and/or anchors/hooks. The point of landing must carefully be selected and if the asteroid is rotating it would be very challenging. Provided the landing and securing of the ship goes well and smooth it would then fire up its engine (it would have at least two engine nozzles pointing in opposite directions and extra fuel to spare) and change its orbit in a more controlled way instead of using impact technique. But this may be much more costly and complicated. But if it was for the safety of our planet I think cost shouldn't be a factor to cut corners..
 
User avatar
midtskogen
Explorer
Explorer
Topic Author
Posts: 247
Joined: 11 Dec 2016
Location: Oslo, Norway
Contact:

Asteroids, meteors, and meteorites

03 Jul 2017 22:13

N0B0DY wrote:
Source of the post I was always in favor of the idea to control-land a small craft on the surface of an asteroid and somehow plant it there using drilling and/or anchors/hooks.

This is pretty hard.  The Philae lander tried to anchor itself and utterly failed.  On a 150m asteroid the gravitational pull and escape velocity are very low.
NIL DIFFICILE VOLENTI
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 746
Joined: 06 Sep 2016
Location: Bellingham, WA

Asteroids, meteors, and meteorites

04 Jul 2017 02:52

Yeah, there's something to be said about the simplicity of a kinetic impactor to redirect an asteroid, vs. using a lander.  The lander has to slow down, land, stick the landing (poor Philae), and then you have to account for the asteroid's rotation and distribution of mass.  And this all assumes the lander remains operational and doesn't have a fault or failure.  It's a lot of challenges and risks.

Another method which is popular among the astrophysicists that look into this subject is the gravity tug.  Park a massive craft near the asteroid.  The asteroid pulls the ship, and the ship pulls on the asteroid.  Use thrusters to keep the position relative to the asteroid fixed, and basically you're just towing it with gravity.  The benefit of this method is it does not require any interfacing with the asteroid whatsoever.  No landing, no surface operations... its as close to the sci-fi tractor beam as you can get. :)

Then there's the impact method.  It is the most simple by far, but we've never done it with the purpose of seeing how well it works in mind.  It could also have application toward excavation and mining.

N0B0DY wrote:
Source of the post So let me see if I understand that correctly and sorry for any wrong assumptions I may write:
They plan to hit the small one and this will cause a small variation in its orbit around the large one which is marginally measurable. They will then extrapolate the result using computer modelling to simulate a more powerful impact on a larger body? Or the change in the orbit of the small one will also cause the binary orbit (the barycenter) to change slightly and hence affect its solar orbit very, very very slightly (almost zero) and use this to model a larger impact? I am confused..

More the former.  The impact will have very little change on the orbit of the binary about the sun.  It's mostly a timing difference that it causes in the orbit of the moon about the main asteroid.  We can use that to see how effective the transfer of momentum was from the impactor to the asteroid, which then tells us the effectiveness of nudging an asteroid off of an Earth-crossing path.

Let's apply some physics and see how this works:

-The impactor will have a mass of 300kg, travelling 6.25km/s.
-The asteroid's moon is about 170m in diameter.
-Assume the density is about 2g/cm(typical of rocky asteroids).  Then its mass is about 5x109 kg.

Use the principle of conservation of momentum:
[math]

Choose a frame of reference where the asteroid's initial velocity is zero, and let the impactor's mass be negligible compared to the asteroid's mass. Then this will simplify to:

[math].  

Therefore the change in asteroid velocity is 

[math]

Plug in the numbers:  [math]

So this should change the velocity of the asteroid by about 0.4 millimeters per second!  This agrees with wikipedia's numbers.

This immediate change in speed by the impact is too small to be measurable.  But over time, it will change where the moon is in its orbit about the primary asteroid, and we can measure that.  The orbital period is 11.9 hours at a distance of 1.1km.  Technically it orbits the barycenter of the asteroid binary, but I'll assume it's a circular orbit with that radius to make it simple.  11.9 hours to make a 1.1km radius orbit is a speed of 161.3 mm/s.

Now the hard part.  The impact instantly changed the speed of the moon, but by the orbital mechanics, that ends up changing the orbit, which then changes the speed in a non-intuitive way.  If we hit the moon from behind, it speeds the moon up, which boosts it into a larger orbit, which is slower.  If we hit it from the front, we slow it down, which makes it drop to a lower orbit, which is faster.

Suppose we hit the asteroid from behind (how rude).  Then the new orbit will have the same periapsis, but a higher apoapsis, which is a slower orbit.  Let's use the Vis Viva equation to figure out the size (semimajor axis, "a") of the new orbit:

[math]

The mass M of the main asteroid is about 4.3x1011 kg (using the period of a circular orbit, v2 = GM/r).

Then by instantly boosting the velocity from 161.3 to 161.7mm/s, we change the semimajor axis from 1.1km to 1.105km, or a boost of only 5 meters.  Again we can't measure this.

But this new, bigger orbit is slower.  Using Kepler's Third Law,

[math]

the new orbital period is 11.98 hours, or about 290 seconds longer!  That's what leads to a visible effect.  After about a month, it will have fallen behind by half of its original orbit, so it would be on the opposite side of the main asteroid from where it would be if we had not hit it.

Similarly, a tiny nudge to an asteroid that would hit Earth can shift its orbit in such a way that it would end up missing Earth.  The orbit doesn't have to change a whole lot -- it can be more of a timing thing than a positional thing.  We can make it so that by the time the asteroid reaches the point in its orbit that originally would have hit Earth, now the Earth isn't in the way... if that makes sense. :)
 
User avatar
midtskogen
Explorer
Explorer
Topic Author
Posts: 247
Joined: 11 Dec 2016
Location: Oslo, Norway
Contact:

Asteroids, meteors, and meteorites

04 Jul 2017 03:52

So, if a 170m asteroid is headed for Earth, and we hit it three years in advance changing the speed by 0.4mm/s, then we can move it 3*365*24*3600*0.4mm/1000/1000=37km before impact.  If this is used to slow or speed up the asteroid's approach to Earth, it would at most offset the impact time by a few seconds and move the impact site by around 100 km assuming the lowest possible impact speed.  Not enough.  We would need a bigger impact and/or something more clever.

Watsisname wrote:
Source of the post We can make it so that by the time the asteroid reaches the point in its orbit that originally would have hit Earth, now the Earth isn't in the way... if that makes sense.

Whether that's more effective will depend on the asteroid's speed.  If we can hit it far out in the solar system while it's moving slowly, yeah, to change the timing should be much more effective.  If it's already in the inner solar system, it seems less obvious which way to nudge it.
NIL DIFFICILE VOLENTI
 
User avatar
Watsisname
Science Officer
Science Officer
Posts: 746
Joined: 06 Sep 2016
Location: Bellingham, WA

Asteroids, meteors, and meteorites

04 Jul 2017 18:09

Yeah, for redirecting a similar size asteroid from Earth, we would use a larger impact mass (or higher velocity, or both).  For this mission, 300kg at 6km/s is sufficient to make a measurable effect -- that's the benefit of hitting the moon of a binary asteroid.  Hitting it with more mass moving more quickly would increase the mission cost, without providing much more in the way of useful data.

For a real Earth impact avoidance, the cost/benefit analysis would be very different.  Procuring funds is not so much of an issue if it means avoiding a hundred megaton impact over a populated area.  We also know we can get more momentum -- we've already achieved more than 300kg at 6km/s.  New Horizons was 470kg at the Jupiter flyby, which had been accelerated directly from Earth at 16km/s.  Most space missions are designed to get a payload somewhere in a reasonable time at minimum cost, so they minimize mass and use time to grab whatever free delta-v they can get from flyby's.  In this case, like with New Horizons, we want to get a big mass moving fast, and don't care about slowing down when we get there.

Three years lead time is also fairly short for an asteroid of that size.  It is more probable to have a lead time of decades.  (The probability continues to grow with time, but eventually uncertainties will dominate and you can afford to wait to get better constraints on the orbit, or prepare a better solution for dealing with it.)  For hundred meter class asteroids with decade or so lead time, what this shows is that it takes about two orders of magnitude more momentum to deflect the asteroid sufficiently.  It's doable, and might be done with several small nudges instead of one big one.

midtskogen wrote:
Source of the post Whether that's more effective will depend on the asteroid's speed.  If we can hit it far out in the solar system while it's moving slowly, yeah, to change the timing should be much more effective.  If it's already in the inner solar system, it seems less obvious which way to nudge it.

With more than one orbit of lead time, changing the orbital period is usually the best way to go, since the effect accumulates. The most efficient way to change the orbital period is to hit it prograde or retrograde.  Inclination shifts are more expensive and don't change the orbital period.  Hitting radially inward or outward also doesn't change the period.  That being said, the manner in which you can get a craft from Earth to intersect the asteroid's orbit may provide a better trajectory.

With less lead time, the best method will depend a lot more on the particular circumstances, and then it definitely isn't obvious what the most efficient trajectory would be.
 
User avatar
midtskogen
Explorer
Explorer
Topic Author
Posts: 247
Joined: 11 Dec 2016
Location: Oslo, Norway
Contact:

Asteroids, meteors, and meteorites

05 Jul 2017 00:39

We can probably get more than three years for objects whose origin is in the asteroid belt.  Oort cloud objects, however, will likely give us less time.  In any case, the yearly likelihood for any of these objects hitting Earth is very low, so in my opinion this is not an activity that we should spend insane amounts of money on.  So simple impact strategies sound reasonable.  It will not rescue us from a 1 km object coming straight for us from the outer solar system, but the chances for that happening during our current technological level are extremely slim.
NIL DIFFICILE VOLENTI
 
User avatar
midtskogen
Explorer
Explorer
Topic Author
Posts: 247
Joined: 11 Dec 2016
Location: Oslo, Norway
Contact:

Asteroids, meteors, and meteorites

03 Aug 2017 15:21

The Perseids are coming.  We don't see that many of them in Norway because the night is still short and not yet perfectly dark, but I recorded one quick Perseid last night.  Recorded by four different cameras.  Three of them recorded at 10 fps, the fourth with a camera capable of 30 fps, which actually makes a huge difference.  Our stations have until now been based on a setup of four cameras each delivering [email protected] to cover the entire sky.  New stations will likely be based on this new camera, so we get four cameras each delivering [email protected] The resolution is slightly lower, which probably doesn't matter because the lens wasn't good enough for 5 megapixels anyway, but the new camera has much better sensitivity and dynamic range.  Even at 30 fps I've been able to detect stars down to magnitude 4.3.
NIL DIFFICILE VOLENTI

Who is online

Users browsing this forum: No registered users and 1 guest