4 ms·

I'm not a scientist, etc., but isn't this potentially slightly bad news? The more stuff getting knocked free, the less momentum gets imparted to the asteroid i

I'm not a scientist, etc., but isn't this potentially slightly bad news? The more stuff getting knocked free, the less momentum gets imparted to the asteroid itself, so the less its course is changed.

Fatnino 4y ago
No, every bit of debris that flies off carries away some momentum.
- deleted 4y ago
  [deleted]
- BlueTemplar 4y ago
  I guess it's important because momentum is a vector, and the debris fly off in a direction on average opposite to where we anted to push it, so this also means that the momentum of the asteroid itself has been changed by (at least) that negative ? But why is this better than a perfectly inelastic impact without any debris ? Was a significant fraction of the asteroid blown away ?
  - Fatnino 4y ago
    You can apriximate the debris to rocket exhaust. There isn't an engine bell to direct it and make it all leave in the most efficient direction but it's still thrust.
    - BlueTemplar 4y ago
      I know, my above statement wouldn't make sense without this assumption.
- adrianmonk 4y ago
  That's my point, though. The goal is to knock the asteroid off course sideways. (Perpendicular to its direction of travel, so that it misses the Earth.) You slam something into it, and that changes the asteroid's momentum because total momentum is conserved in a collision. If the asteroid stays in one piece, then after impact, it will be going sideways at some particular velocity. If instead some pieces break off and speed off at a greater sideways velocity, then the parts that didn't break off have to be going at a smaller sideways velocity. In other words, if some debris breaks off, it carries away more than its "fair share" of momentum, and the remaining part gets less momentum.
  - Fatnino 4y ago
    We just need the target to change its orbit, in any direction, so that it misses the keyhole that will send it on a collision course with earth. If we only spot an incoming asteroid on its final approach we are screwed no matter what we do. DART is testing what we can do when we find a near earth asteroid that is going to pass near earth harmlessly but the "gravity assist", if you will, of this encounter with earth's gravity well sets it up to crash into us next time it comes around. There is a specific volume of space (keyhole) that should the asteroid pass through it at the most unfortunate time (for us) it will hit earth during the next encounter. We just need to make sure it doesn't pass through that bit of space at that exact time. Knocking the rock sideways or slowing it down or even speeding it up are all valid strategies. You may remember a while ago there was much concern about an asteroid named Apophis. When it was discovered, it's orbit wasn't so well understood and the error bars (more like error ellipse) included a keyhole that would set it on a collision course. Further study reduced the error somewhat but worryingly didn't rule out the keyhole, meaning that the odds of a hit started to appear more likely. Imagine something with 1/1000 chance of happening. Then you rule out 900 possibilities and end up with a 1/100 chance. This caused even more detailed study of its orbit to take place and the error was narrowed to the extent that the keyhole is ruled out entirely and now there is no concern Apophis will hit us in the foreseeable future.

Fatnino 4y ago

No, every bit of debris that flies off carries away some momentum.

deleted 4y ago
[deleted]
BlueTemplar 4y ago
I guess it's important because momentum is a vector, and the debris fly off in a direction on average opposite to where we anted to push it, so this also means that the momentum of the asteroid itself has been changed by (at least) that negative ? But why is this better than a perfectly inelastic impact without any debris ? Was a significant fraction of the asteroid blown away ?
- Fatnino 4y ago
  You can apriximate the debris to rocket exhaust. There isn't an engine bell to direct it and make it all leave in the most efficient direction but it's still thrust.
  - BlueTemplar 4y ago
    I know, my above statement wouldn't make sense without this assumption.
adrianmonk 4y ago
That's my point, though. The goal is to knock the asteroid off course sideways. (Perpendicular to its direction of travel, so that it misses the Earth.) You slam something into it, and that changes the asteroid's momentum because total momentum is conserved in a collision. If the asteroid stays in one piece, then after impact, it will be going sideways at some particular velocity. If instead some pieces break off and speed off at a greater sideways velocity, then the parts that didn't break off have to be going at a smaller sideways velocity. In other words, if some debris breaks off, it carries away more than its "fair share" of momentum, and the remaining part gets less momentum.
- Fatnino 4y ago
  We just need the target to change its orbit, in any direction, so that it misses the keyhole that will send it on a collision course with earth. If we only spot an incoming asteroid on its final approach we are screwed no matter what we do. DART is testing what we can do when we find a near earth asteroid that is going to pass near earth harmlessly but the "gravity assist", if you will, of this encounter with earth's gravity well sets it up to crash into us next time it comes around. There is a specific volume of space (keyhole) that should the asteroid pass through it at the most unfortunate time (for us) it will hit earth during the next encounter. We just need to make sure it doesn't pass through that bit of space at that exact time. Knocking the rock sideways or slowing it down or even speeding it up are all valid strategies. You may remember a while ago there was much concern about an asteroid named Apophis. When it was discovered, it's orbit wasn't so well understood and the error bars (more like error ellipse) included a keyhole that would set it on a collision course. Further study reduced the error somewhat but worryingly didn't rule out the keyhole, meaning that the odds of a hit started to appear more likely. Imagine something with 1/1000 chance of happening. Then you rule out 900 possibilities and end up with a 1/100 chance. This caused even more detailed study of its orbit to take place and the error was narrowed to the extent that the keyhole is ruled out entirely and now there is no concern Apophis will hit us in the foreseeable future.