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TLDR: "[Dimorphos] ... seems to be a loose pile of rubble barely held together by gravity — whose surface would probably shatter spectacularly when DART hit it

TLDR: "[Dimorphos] ... seems to be a loose pile of rubble barely held together by gravity — whose surface would probably shatter spectacularly when DART hit it. DART [impacted] Dimorphos at more than 6 kilometres per second. The impact caused the asteroid’s orbit around another [larger] space rock [Didymos] to shrink — Dimorphos now completes an orbit 33 minutes faster than before the impact. The impact ejected at least one million kilograms of rock from Dimorphos’s 4.3-billion-kilogram mass. The debris formed a tail that stretched for tens of thousands of kilometres behind the asteroid. Various telescopes watched over weeks as the tail shifted and evolved under the pressure of the Sun’s rays; the Hubble Space Telescope even detected a second tail, which had disappeared by 18 days after the impact. One factor [in successfully changing Dimorphos's orbit] is that the spacecraft hit a spot around 25 metres from the asteroid’s centre, maximizing the force of its impact. Another is that large amounts of the asteroid’s rubble flew outwards from the impact. The recoil from this force pushed the asteroid further off its previous trajectory. Researchers estimate that this spray of rubble meant Dimorphos’ added momentum was almost four times that imparted by DART4." It's good that this technique worked on a "loose pile of rubble", but I'm not sure we can assume all asteroids are composed in the same way. A denser rock of iron or similar might be more difficult to divert.

ericbarrett 4y ago
No, this was a successful test! Very small changes in the energy of an object can cause large changes in its trajectory. An impact at this same scale could be the difference between a direct collision with Earth, and not even getting inside the moon's orbit. We don't need any mass to be knocked off. In fact, it's probably even more effective if the target stays intact. What we do need is time. If the asteroid is weeks away, we've got a great chance of deflecting it. If it's hours, we'll certainly be struck.
- Tepix 4y ago
  We need years, not weeks
  - jessaustin 4y ago
    We'll need decades to install a system that could solve this problem when given weeks of notice. Perhaps those decades of effort will start soon? Alternatively, if we knew a particular asteroid would threaten us in one decade, that one decade might be enough to do something about it. Many asteroids won't give us that sort of notice, since orbits can have century-long periods.
nine_k 4y ago
Shaking off 1/4300 of the mass, roughly, is likely not the most important part. Changing the trajectory even a slighted bit may mean much more after the asteroid travels a few million km. To avert a direct impact at Earth, we only need a correction of about 7000 km of the orbit intersection point, at the very worst case. I'm still intrigued at a possibility of detonating a megaton-classl nuclear device at a very close proximity to a tough metal / rock asteroid. Consider the blast happening close to the surface but on the "side", as much off-center as possible. Won't it evaporate enough material for a jet of it to significantly push the orbit sideways, thus shifting it enough away from a collision course?
- ilyt 4y ago
  Wouldn't just doing the boom on surface made more sense ? Less energy wasted and possibly more directional blow if bomb landed in a crater.
  - aaronblohowiak 4y ago
    That can be harder to get to in a hurry than next to it
  - nine_k 4y ago
    The idea is to irradiate and evaporate a slightly larger area "below" the blast, so that more of the ejected matter had the impulse with the direction across the course, obstrucred by the asteroid on one side and thus not symmertical. (Much if the energy will anyway be wasted on beams / jets that are symmetrical and thus compensating each other.)
    - ilyt 4y ago
      That's why I mentioned dropping it in crater, to redirect part of it into desired direction
- Teever 4y ago
  But the question is how much did the change of mass result in the change of trajectory?
ordu 4y ago
> A denser rock of iron or similar might be more difficult to divert. May be, or may be not. But I think it doesn't make it impossible to divert them, even if it is more difficult. The idea is to struck it in a carefully chosen moment, when small differences in delta-v can translate into huge differences in a probability of the impact with Earth. A pile-of-rubble kind of asteroids seemed the most troublesome, because when you shoot it bullets can just pass through. Or asteroid can break into a lot of pieces amplifying a probability of the impact with Earth: one of pieces can strike Earth and it is can be enough for a catastrophic consequences. But the outcome of an experiment assures us that it is not a case. A lot of small boulders fly on a lot of different trajectories, but they are small, they will burn in atmosphere if they get here. ps. I feel it is important to state that I'm not an astrophysicist or someone like this, so all written above is my own opinion that can be completely wrong.

ericbarrett 4y ago

No, this was a successful test! Very small changes in the energy of an object can cause large changes in its trajectory. An impact at this same scale could be the difference between a direct collision with Earth, and not even getting inside the moon's orbit. We don't need any mass to be knocked off. In fact, it's probably even more effective if the target stays intact. What we do need is time. If the asteroid is weeks away, we've got a great chance of deflecting it. If it's hours, we'll certainly be struck.

Tepix 4y ago
We need years, not weeks
- jessaustin 4y ago
  We'll need decades to install a system that could solve this problem when given weeks of notice. Perhaps those decades of effort will start soon? Alternatively, if we knew a particular asteroid would threaten us in one decade, that one decade might be enough to do something about it. Many asteroids won't give us that sort of notice, since orbits can have century-long periods.

nine_k 4y ago

Shaking off 1/4300 of the mass, roughly, is likely not the most important part. Changing the trajectory even a slighted bit may mean much more after the asteroid travels a few million km. To avert a direct impact at Earth, we only need a correction of about 7000 km of the orbit intersection point, at the very worst case. I'm still intrigued at a possibility of detonating a megaton-classl nuclear device at a very close proximity to a tough metal / rock asteroid. Consider the blast happening close to the surface but on the "side", as much off-center as possible. Won't it evaporate enough material for a jet of it to significantly push the orbit sideways, thus shifting it enough away from a collision course?

ilyt 4y ago
Wouldn't just doing the boom on surface made more sense ? Less energy wasted and possibly more directional blow if bomb landed in a crater.
- aaronblohowiak 4y ago
  That can be harder to get to in a hurry than next to it
- nine_k 4y ago
  The idea is to irradiate and evaporate a slightly larger area "below" the blast, so that more of the ejected matter had the impulse with the direction across the course, obstrucred by the asteroid on one side and thus not symmertical. (Much if the energy will anyway be wasted on beams / jets that are symmetrical and thus compensating each other.)
  - ilyt 4y ago
    That's why I mentioned dropping it in crater, to redirect part of it into desired direction
Teever 4y ago
But the question is how much did the change of mass result in the change of trajectory?

ordu 4y ago

> A denser rock of iron or similar might be more difficult to divert. May be, or may be not. But I think it doesn't make it impossible to divert them, even if it is more difficult. The idea is to struck it in a carefully chosen moment, when small differences in delta-v can translate into huge differences in a probability of the impact with Earth. A pile-of-rubble kind of asteroids seemed the most troublesome, because when you shoot it bullets can just pass through. Or asteroid can break into a lot of pieces amplifying a probability of the impact with Earth: one of pieces can strike Earth and it is can be enough for a catastrophic consequences. But the outcome of an experiment assures us that it is not a case. A lot of small boulders fly on a lot of different trajectories, but they are small, they will burn in atmosphere if they get here. ps. I feel it is important to state that I'm not an astrophysicist or someone like this, so all written above is my own opinion that can be completely wrong.