“We’re smashing into an asteroid,” NASA’s Launch Services Program senior launch director Omar Baez said during a press conference. “I can’t believe we’re doing that”
For success, almost all of the mass of the target needs to acquire the correct momentum (all that's available from the Dart craft) to get into the right orbit. Serious egg on face (or worse) if some of the asteroid ended up coming our way.berkeman said:What am I missing?
This is not simple. The technique (as I understand) mostly involves changing orbital speed. So if you slow the asteroid, for instance, it might not intersect. But what if you can split the body and make part go faster (and not hit earth) and part go slower (and not hit earth). This might be much more effctive. That is sort of what a collision with a lot of ejecta might look like. Or perhaps the ejecta (a small fraction of the original mass) will hit Earth (bad) but the major body will not (good). Anyhow that is how I understand the complexities.sophiecentaur said:For success, almost all of the mass of the target needs to acquire the correct momentum (all that's available from the Dart craft) to get into the right orbit.HIS are many
That seems to be a common way of manoeuvering craft in orbit. I imagine that's because time, position and direction of the craft (in this case asteroid) are known accurately and errors in direction of engine impulse have least effect when applied forward or reverse.hutchphd said:This is not simple. The technique (as I understand) mostly involves changing orbital speed.
The problem is, that as long as the pieces are around they will orbit each other. So either you completely tear apart the pieces (awful lot of energy to dig them up from their gravitational well) of you just added one more parameter (their orbit phases) to the calculations (what if they are happen to be lining up with Earth when they are close?)...hutchphd said:But what if you can split the body and make part go faster (and not hit earth) and part go slower (and not hit earth).
Sorry, if conservation of momentum works in inelastic collisions, how does ejected material change anything? The CoM of the system of the asteroid and the interceptor stays the same, right? Or is the objective to eject lots of low-mass stuff one way, and thus push the asteroid the other way?mfb said:The big question is how much material it ejects at what speed. As energy scales with velocity squared the ejected material can deliver a much larger momentum change than the primary impact because it has so much more mass.
The target is very large. Unless DART hits very close to the edge it cannot fly through independent of the composition of the object. Even water with the same volume would stop DART. That part of the momentum change is guaranteed if DART doesn't miss.sophiecentaur said:How sure can we be that a target asteroid can be treated as one solid lump an that the impact will not result in a large part of it just continuing nearly on its original course?
This is not a risk. It's nowhere close to an Earth-crossing orbit. I mean... some ejected pebbles might, obviously, but that's not a concern.sophiecentaur said:Serious egg on face (or worse) if some of the asteroid ended up coming our way.
Splitting an asteroid deliberately, and doing it with such a precision, is pure science fiction.hutchphd said:But what if you can split the body and make part go faster (and not hit earth) and part go slower (and not hit earth).
No, they have the largest effect, just like the propulsion itself. That's why it is done whenever possible - maximal effect for a given amount of fuel.sophiecentaur said:That [changing orbital speed] seems to be a common way of manoeuvering craft in orbit. I imagine that's because time, position and direction of the craft (in this case asteroid) are known accurately and errors in direction of engine impulse have least effect when applied forward or reverse.
Spacecraft don't do that, it would be a waste of fuel. It's more efficient to have any intersecting trajectory and then slow down (relative to the planet) when the spacecraft is at its closest approach.sophiecentaur said:Rather like a course to settle a craft into orbit with a planet with minimal fuel use, the courses of planet and craft are arranged to coincide and be more or less parallel and similar speeds.
If you want to change the perigee, yes. If you want to change the apogee you should do it at perigee. Usually you don't want to change either of them in particular, you want to change the orbital period which can be done at any time. Finding a suitable trajectory from Earth is more important. That might take years for some objects, but in this case it's under a single year to the September 2022 impact.sophiecentaur said:Corrections of the asteroid orbit at the apogee are easiest, I understand.
The material is ejected from the impact site, so it is directional.berkeman said:Sorry, if conservation of momentum works in inelastic collisions, how does ejected material change anything? The CoM of the system of the asteroid and the interceptor stays the same, right? Or is the objective to eject lots of low-mass stuff one way, and thus push the asteroid the other way?
That makes sense. Changing gravitational potential with a rocket is not good value (efficiency at lift-off is an example) changing KE is better.mfb said:maximal effect for a given amount of fuel.
I was rather thinking that asteroids 'visiting' from further out (high apogee) could be taken to a higher perigee and permanently out of harm's way?mfb said:If you want to change the perigee, yes. If you want to change the apogee you should do it at perigee. Usually you don't want to change either of them in particular, you want to change the orbital period which can be done at any time
Do you really mean that arrival from a radial direction is better than a near tangential direction? That seems to disagree with point about changing KE rather than GPE. The paths that I have seen to get into orbit round Mars seem to have been as I described.mfb said:Spacecraft don't do that, it would be a waste of fuel. It's more efficient to have any intersecting trajectory and then slow down (relative to the planet) when the spacecraft is at its closest approach.
Interesting paragraph. You make the point that, in the light of the masses involved, 'splitting the asteroid in two halves' is not a scenario - just a varying small amount of ejecta.mfb said:Toy scenario with rounded numbers for simplicity:
An important point is that the ejected/split matter needs to leave the asteroid to have any effect (speed above escape velocity). It's easy to do that with just some ejected matter, but certainly will be far more difficult with half the asteroid.sophiecentaur said:just a varying small amount of ejecta
That would need a deflection of the order of a kilometer per second (and I don't see a plausible scenario where this would leave the asteroid intact), while asteroid defense scenarios look at millimeters to centimeters per second.sophiecentaur said:I was rather thinking that asteroids 'visiting' from further out (high apogee) could be taken to a higher perigee and permanently out of harm's way?
No. The most common approach for direct flights is a Hohmann transfer, which arrives at the target at significant relative speed along the orbital direction. That relative velocity is reduced around the time of closest approach to enter an orbit, or it's shed in the atmosphere for a direct landing.sophiecentaur said:Do you really mean that arrival from a radial direction is better than a near tangential direction?
That's what I was thinking of.mfb said:a Hohmann transfer, which arrives at the target at significant relative speed along the orbital direction.
Useful information, thanks. Why would the required velocity change be as high as 1km/s? Would it be because of the need to increase the total orbital energy so much? Sort of makes sense if the original orbit had high eccentricity.mfb said:while asteroid defense scenarios look at millimeters to centimeters per second.
Not a control, but a tool to detect small velocity changes. Measuring the changed orbital period can be done far more precisely than measuring the velocity change itself. There is saying that comes from optics, but it's true almost everywhere: Reduce every measurement to a frequency measurement if you can.sophiecentaur said:The idea of using two similar ('twin') asteroids for the experiment, with one acting as a control, is also very smart.
That's what I meant; to see the difference that the 'treatment' has achieved. The with and the without cases.mfb said:Not a control, but a tool to detect small velocity changes.
But more outside Earth's orbit than inside? If perihelion were taken to >1AU the problem would be solved. But you have pointed out that the energy involved to do that would be too much.mfb said:Both perihelion and aphelion are somewhere random,
. . . . going right back to Kepler's Laws, in fact. We can see the 'when' with a telescope much easier than the where or how fast.mfb said:Measuring the changed orbital period can be done far more precisely than measuring the velocity change itself.
Both are somewhat random but only the set of asteroids with perihelion <=1 AU and aphelion >=1 AU can be potentially dangerous obviously (the actual range is slightly larger because of Earth's eccentricity). That's a post-selection.sophiecentaur said:But more outside Earth's orbit than inside? If perihelion were taken to >1AU the problem would be solved. But you have pointed out that the energy involved to do that would be too much.
I also have a question about DART? If the spacecraft does not hit the target exactly head on (which is likely), some of the collision energy will go into rotational energy of the target moon. A 'glancing blow' would cause most of this energy to become angular momentum of the moon. We will not know the 'angle of attack'(?) for a long time, (if ever) and I don't see how they will be able to determine how much rotational energy the moon has when the second spacecraft arrives.mfb said:The big question is how much material it ejects at what speed. As energy scales with velocity squared the ejected material can deliver a much larger momentum change than the primary impact because it has so much more mass.
Another question is how accurately we can hit the asteroid when approaching at 6.6 km/s, and what that means for smaller targets. No mission ever had a target that small with that approach velocity.JWST had a mishap in the integration with Ariane 5. Four days delay to check that vibrations didn't exceed specifications, now the launch is planned for December 22.
You are very much on the mark, the issue with DART is momentum transfer, from the impacter to the body of the target.Galexy said:I also have a question about DART? If the spacecraft does not hit the target exactly head on (which is likely), some of the collision energy will go into rotational energy of the target moon. A 'glancing blow' would cause most of this energy to become angular momentum of the moon. We will not know the 'angle of attack'(?) for a long time, (if ever) and I don't see how they will be able to determine how much rotational energy the moon has when the second spacecraft arrives.
Also, I suppose the dynamicists are assuming that the moon is tidally locked to Didymos to begin with, but that is not necessarily the case. There is no way of determining if the moon has any residual rotational energy left over from its formation (or from a recent meteoroid hit).
The way I see it, the unknowns associated with the rotational energy of the moon before and after the collision will seriously complicate the experiment. Am I wrong?
100% unless it's a really bad hit. The unknown part is the momentum of the ejected debris.etudiant said:Ideally, the impacter transfers close to 100% of its momentum to the asteroid.
Doesn't conservation of momentum apply to net linear and angular momentum? A glancing blow would involve a lot of spin and less linear momentum change. Likewise, if the moon were spinning and a glancing blow reduced the spin then effectively there would be change in linear momentum.mfb said:The rotation of the moon is irrelevant for changing its center of mass motion.
Yes, and both are independent.sophiecentaur said:Doesn't conservation of momentum apply to net linear and angular momentum?
That is non-intuitive as a general principle. It implies on the face of it, that if a moving ball were to hit a bar with a force, normal to the axis and at the end, that the bar would only rotate and with no change in its linear momentum; i.e. the bar would stay where it is. Would that actually be the case? Will the share of linear momentum between bar and ball, after the collision not depend on where the ball strikes.mfb said:Yes, and both are independent.
DART will hit it with 6 km/s. A surface velocity of 1 m/s in any direction is completely irrelevant even if the mission fails and delivers a glancing hit only.
No, it doesn't imply something that's obviously wrong...sophiecentaur said:It implies on the face of it, that if a moving ball were to hit a bar with a force, normal to the axis and at the end, that the bar would only rotate and with no change in its linear momentum; i.e. the bar would stay where it is.
After the inelastic collision the far heavier bar has essentially 100% of the initial momentum of the ball. Its rotation is irrelevant, the tiny bit of kinetic energy the buried ball ends up with is irrelevant as well. We only care about the center of mass motion.sophiecentaur said:Will the share of linear momentum between bar and ball, after the collision not depend on where the ball strikes.
So what? It's irrelevant.sophiecentaur said:but the initial angular momentum depends on the path of the ball and the final angular momentum of the bar will involve the linear velocity of its CM relative to the ball.
