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Seems interesting, but can somebody smarter than me ELI5 this?

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They might do it again but bigger.

If there is a asteroid on a collision course with Earth, and it's big enough to cause some catastrophic damage, everyone on the planet will want some plan in action to make it go away. The simplest way to do that is to be able to do that is to alter the path of the asteroid ever so slightly that its course misses the Earth. Even though a satellite has very little mass, if you make it travel fast enough, you can impact enough velocity to give a big asteroid a nudge. Even if it's a tiny bit, that redirection propagating over millions of km will save the planet.

DART's mission target was actually a smaller space rock (roughly the size of the pyramid of Giza) that was orbiting a larger asteroid. Since they could easily detect the smaller rock's orbital period, they could calculate how much the impact would change its orbit. This would better inform scientists about what to expect from future redirect missions so that we could make a robust planetary defense system in case The Big One ever shows up.

Thanks! Makes much more sense to me.

Essentially, they are saying that the test was successful but it is still only one data point.

The final aim of this research is to be able to know, in case an actually dangerous asteroid were on a collision trajectory with Earth, what kind of impact would be sufficient to knock it onto a safe course.

So we know the mass, trajectory and velocity of DART and soon we'll know the exact effect the impact had on Dimorphos, but what we don't know is the effect of Dimorphos's characteristics. What if DART had struck another asteroid with, for example, the same mass but denser? What if the asteroid had been composed of a different material?

By carefully analysing the impact site and the asteroid itself with follow-up missions (like HERA) we can try to determine the effect of Dimorphos's characteristics so that in the future we may more easily predict the effect of an impact on any asteroid, provided we have some data about it.

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How much more effective would that same spacecraft be if it was 100 times faster?

So (correct me if I'm wrong) this is saying that we don't know yet if DART was actually successful? Obviously, it was successful in that they hit the target; however, they're not sure about whether it significantly changed the trajectory and are still trying to determine that?

Yes...we are waiting on the results.

When announced, you'll definitely hear about it ^(unless we've been taken out by an asteroid before then.)

10000 times more 'effective', if effectiveness correlates directly with energy.

DART was successful if we hit the rock and can make observations about how to improve a future similar maneuver.

Kinetic energy = 0.5 * mass * velocity^2

So a 100x increase in velocity equals a 10,000x increase in kinetic energy.

(although it isn't that simple because the asteroids aren't perfectly solid objects and are more like loose collections of gravel.)

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Shouldn't this be a simple matter of physics, though? Like, wouldn't the mathematicians already have the answer?

Edit: I greatly appreciate the educational responses!

Maybe, maybe not. I'm not a mathematician, physicist, or anyone in a position to know more than what I read online. My understanding is there are a LOT of variables that we don't necessarily entirely understand. The composition of the target is a big one, as well as density, uniformity of density (is one part of the target denser than the other, etc) and a bunch of other stuff. Not all space rocks are created equal, and it seems like the more we learn about these things the more we realize just how different they can be from one to the next.

We absolutely can calculate how much force one object of known mass and velocity can impart on another object of known mass and velocity, but when it comes to hitting space rocks there's a lot on the target side we simply don't know.

And even then, each situation will be different. If an object is a big enough threat to merit launching an intercept mission, it's likely that it will be too big to launch something at it capable of completely destroying it. Deflection will be the name of the game. Anyone who has played billiards will understand the critical role that impact angle plays when it comes to deflecting an object in the right direction.

Lots of assumptions as to the composition of the thing we're hitting.

The impact into a bowling ball vs an exercise ball is going to be very different.

Serious question: Why not just nuke it? A direct impact of a nuke on an asteroid the size of didymorphus/didymos would surely disintegrate or severely break it down and cause nothing more than a meteor shower. And the explosion would alter its path too.

A nuke in space would do less than you think. A nuke itself doesn't cause much impulse in a vacuum. It has inconsequential mass by itself, it just releases an immense amount of energy.

On Earth, we see a giant blast wave from a nuke. This is because it heats up matter(the atmosphere and ground around it), and that matter expands very quickly.

In a vacuum, you don't get that. It would heat up one side of the asteroid, which do to outgassing could alter it's course, and there is photon pressure. It's certainly been discussed. But I think at this point, especially with falling costs of mass to orbit, and kinetic impactor is a more viable option.

Why go all the way to nuking it if we can confirm crashing into at high speeds is an effective solution? A lot easier to plot a direct collision course than it is to time an explosion and/or make sure the device actually explodes on time.

I'm not arguing about the validity of the impactor. It's probably the best solution we have at the moment. I'm just curious why we would rather move it slightly off course than destroy it and never worry about it again.

Well for one thing the exercise ball would be slightly easier to hit size wise.

Next question.

And you can always do a series of kinetic impactors for increased power, like a train of StarLink satellites, slowly but systematically smashing it off course

I think the idea is that an extinction-level asteroid might be bigger than this one, so destroying may not be an option.

Also, nudging it out of the way of earth takes far less energy. And if you blow it up big chunks of it could still cause immense damage. They don’t have to be big. Enough pebble-sized meteorites in succession could heat up the atmosphere enough to kill off a lot of things.

There was a documentary that came out a while back that explained why nuking the surface of an asteroid wouldn’t do much to alter its course.

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With the advent of a super heavy launch vehicle that can be refilled in orbit like Starship, we could also just make a gravity tractor.

A fully expendable, fully refilled SS could get a beast of a gravity tractor out there.

When do we find out what, if any effect the impact had? I get the impact was small but we can measure orbital deviations by the feet this close in. You think we would know by now if it had any effect.

I mean in KSP I just put a bunch of grabbers on top of an enormous set of rocket engines and move em around that way... But smashing stuff is more fun!

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Just so happen that we have decided to test moving asteroids now...

Glad it wasn't just me that thought that. I'll go quitely now too...

We should just launch out supply of garbage at it instead

We should have just launched our supply of garbage at it instead

Given the high-visibility of this expensive mission, NASA is likely being VERY careful how they will message findings to the public.

In other words, stay tuned.

No need sir, you've been promoted to Head Mathematician. We're lucky to have access to a man of your caliber and scrutiny.

Good to know I'm not the only immature one on reddit.

I like where this is going...

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Lefty loosie tighty whitey hit the asteroid big and mighty.... oop, wait, sir I think we hit the wrong side of it.

As a bond villain, I totally wouldn't use this to redirect asteroids to impact certain parts of the Earth that need a cleansing. Totally wouldn't...

There's a treaty that says we can not use nukes in space.

To shreds you say?

No, because the impact is not an ideal collision. There is material ejected, which influences how much the momentum of the body changes. We have a rough idea of how much the velocity should change, but it’s still unknown how much, like on the order of tens of percent compared to theoretical.

If we underestimate how much of an effect an impactor has, we may expend far more resources or take more risks than are necessary to avoid a collision with Earth. Overestimate and we either get hit, or we have to send a second mission which may require an even larger deflection than the original mission (due to a shorter amount of time for the velocity variations to add up to a planetary miss).

So the ludicrous, yet slightly feasible, method of terraforming mars is off the table?

More like the impact into a loose pile of irregular boulders. The last couple of photos were stunning and unexpected. No smooth surface visible. Just jagged and loosely interlocked rocks.

Back to the chalkboard, folks! We need to explore the real possibility the probe emerged out the back of the asteroid without losing all of its momentum.

That is absolutely a great thought. People are responding to you saying that it's not the solution, but I had a discussion with someone working on DART who is a co-author of the paper this post is about. A nuke provides the greatest amount of energy per mass that can be delivered to an asteroid, so it absolutely is a 100% feasible option for planetary protection.

The largest issue with the nuke is the timing. DART impacted going roughly 6 kilometers per second. This means that a nuke would need to be timed within a few milliseconds of impact so it would be able to transfer energy to the asteroid without wasting it, but needs to be detonated before impact as the speed is so extreme if the spacecraft impacted the asteroid first the nuke would be destroyed without detonating.

Detonating a nuke in space is actually rather trivial though, the largest issue is navigating and guiding a spacecraft to the asteroid. DART was able to locate Didymos over a month out, but only was able to see Dimorphos a few hours before impact. Didymos is nearly 1,000 meters in diameter, Dimorphos is about 170 meters while the asteroids we are actually worried about are closer to 50 meters. Since they are smaller than we can detect from Earth, we would likely only discover it possibly weeks or months before impact, whereas something much larger would be discovered years earlier.

I have heard from people who worked on DART, that a potential DART-2 mission is a dry-run of nuking an asteroid without the actual nuke onboard, so that the design would already be finished if we discovered an asteroid with limited time before impacting Earth. That way they could re-build from existing designs, add the nuke and launch considerably faster in the case of an emergency.

Breaking it into multiple pieces/re-directing it both end up going together, at some point of pushing it you're going to break it up anyway because of the force required. One concern of breaking it up is that if it still impacted Earth, while it would be less likely to kill us all, it would have a high likelyhood of damaging many of our satellites in orbit. In 2014 a comet nearly hit Mars, and all the satellites (since there are not many around Mars) navigated to be on the opposite side of the planet to where it was likely to hit. Earth satellites wouldn't realistically have the same ability.

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They’re always moving though.

But what if it comes back?

Is that the film NASA uses as a training tool, for people to spot how many things it gets wrong?

Because if those high speeds aren't enough, what's your next plan?

It doesn't matter how good the math looks, you always test.

That's how we ended up with relativity. Classical physics works great at a human level, but it's wrong at the cosmic and quantum levels, and testing proved that.

We’ll just launch another one then

But we can't just make garbage, everything gets recycled.

Not true, simulations already show it would be very effective

https://www.nytimes.com/2021/10/18/science/asteroid-nuclear-bomb.html

My favorite factoid about DART is that the impact was significantly more energetic than if they had literally made the entire impactor out of TNT, landed it on the asteroid and then blown it up.

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Depends. Can you fill in the following unknowns for the mathematicians?

1. How much energy went into changing the spin of the asteroid rather than its orbit
2. How much energy was redirected sideways by the potential initial collision with a non-perpendicular surface?
3. How much energy went into blasting loose material in all directions like a cue ball hitting a racked set of pool balls, multiplied by how many thousands or millions or billions for the unknown number of pool balls in the equation?

Mathematicians need to have data to work with.

I was thinking what if this mission of theirs went so wrong that is changed the trajectory of that asteroid to actually hit earth instead, oh the irony. But I'm sure they thought of that and there was no risk of that actually happening haha

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I never said it wouldn't be effective. I am saying there is no shockwave from a nuke in space. That it doesn't behave like people might think.

I literally said in my post the ways it would affect an asteroid's motion.

What do you mean? It looks extremely similar to all of the other asteroids that we've landed on/ photographed recently...

The one they impacted wasn't a near-earth asteroid, for that very reason.

I would imagine that the composition would be less of a concern than knowing the exact mass of the asteroid.
A collision in space is as good of a scenario you could get in terms of applying conservation of momentum. Then, knowing that the worst case scenario is a plastic collision (because the probe going through the asteroid completely is very much impossible), you could know with very good certainty the final momentum (assuming we have fairly precise and accurate information on the velocity of both objects). I'm purely speculating as I haven't looked into the mission, but I also doubt the scientist expected a best case scenario of a bounce from the asteroid.

Wouldn’t landing rockets on an impactor and over time send any additional fuel toward it be a more effective method? Turning the object into a movable, steerable body would ease it safely out of its orbit rather than blow it to pieces and send fragments that could still pose a threat to the planet.

Also an object which ejects mass in space is known as a rocket engine. That ejecta plume was a rocket engine. It's a nonlinear equation that you have to run in real time to get a result. At the very instant you collide the two masses together the mass of the larger object changes.

Yes, Newton's laws of motion still hold, although the three body problem is hard to solve. Dimorphos, Didymos, and Dart make for an interesting computation.
The thing is, an asteroid is not a rigid body. How much of DART's inertia affected the speed of Dimorphos, and how much of it just knocked pieces of Dimorphos off of the main asteroid, and how all of this affected the speeds of both Dimorphos and Didymos is something that is complicated enough that it needs to have that calculation tested.

I did some rough calculations of how much energy the DART craft had and it was about 1/1000 of the atomic bomb over Hiroshima. 1/1000 doesn’t sound like much but this was just an initial try. And that’s still equivalent to 15 tons of TNT. Not bad. The problem with a nuke is you can’t just slam it in, you first have to slow down, match speeds , then land and tunnel into the middle of it . Maybe all doable but dang .

Right you would have to tunnel into the middle of it

Yes that is probably how we will do this next. But fat ion engine and some solar panels or maybe nuclear power station

Depends on your frame of reference. Asteroidlings may consider us to be in motion relative to their stasis.

I think everyone here is ignoring how material gets ejected by the impact. They know the momentum of both the spacecraft and asteroid very well. There's nothing new the impact will show in regards to that very basic momentum conservation problem.

What they are actually trying to characterize is the way debris is ejected by the impact. This ejecta carries off its own momentum and can impart a significant additional momentum to the asteroid.

There was a huge amount of ejected mass from the collision - that is likely the majority of the momentum change, and what we are measuring.

Plastic vs elastic collisions fail when explodey stuff happens.

Could one make a shaped charge or cone shaped nuke? I feel like that would deflect a pile of barely held together rocks better than our own Little Rock with solar panels on it

We have to wait to see how the orbit is modified after many cycles to get real numbers.

Prediction was like a 2% change after however many cycles.

So they don't even have the all the data yet to analyze.

Unless the ejected mass went somehow forward, I don't see how that would steal momentum from the asteroid. Maybe if the ejected mass went perpendicular in a manner was not even in all directions? Kinda stretching it.

Anyway, explodey stuff would only help, not hurt.

You would absolutely sink a nuke to the right depth so that it does have maximum impulse.

But that's kinda my point. Anything ejected would only "push off" the asteroid and help nudge it off course, given that they mostly get ejected either in the opposite direction to the spacecraft, or perpendicularly in a roughly even manner (this last one is speculation on my part since I imagine it depends heavily on the angle of impact and possibly more things I don't know, but I don't think it's a wild assumption)

Crashing into a body requires affecting it's path further in advance because we only have such big rockets that can only add so much dV.

Energy density of a nuke is way higher than rocket fuel of equivalent mass.

Precisely timing a device is trivial in the grand scheme of thrust vector control, orbital navigation, and all of the other control systems operating the device. Like sure it's hard, but an extra PCB, sensor system, and software isn't going to exactly break the bank.

An excellent point. When dealing with rockets, you have to account for the changing mass of the vehicle as propellant is exhausted. In order to integrate it into my list and remove the overlap with item 3 (being the part involving the energy in the ejecta), I would phrase it as: 4) How much mass has departed the asteroid?

They better tell us. I want my last-night-on-earth orgy.

No you wouldn't. The most realistic option for a nuclear avoidance strategy is to use a surface standoff detonation some tens or hundreds of meters above the surface.

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Only for asteroids we can predict collisions with well in advance.

Every answer telling you some supposed reason about why this is infeasible or ineffective is wrong. NASA and related, credible parties have conducted studies and generally conclude that a nuclear device is one of the most effective strategies we have of asteroid avoidance. The catch is that it would not be used in the manner you describe to "disintegrate" an asteroid - rather, it would most likely be detonated in a surface standoff detonation and use the vaporized surface as ejecta to propel the asteroid in the opposite direction.

https://web.archive.org/web/20160303220543/http://www.nasa.gov/pdf/171331main_NEO_report_march07.pdf

In the case of a large and/or imminent threat, this would likely be one of our only options. No other feasible strategy, whether kinetic impactor or gravity tractor, is capable of transferring so much energy to an asteroid in such a short amount of time. DART and the gravity tractor approach are both similarly mature technologies, but they require either for us to see the threat years in advance or for the threat to be very small - possibly both. A nuclear device is capable of redirecting a much larger asteroid with a much shorter notice.

There are technical challenges to address, yes, but relatively minor ones especially given the level of spaceflight technology we've already had for decades.

The obstacles right now are geopolitical, not technical. The Outer Space Treaty bans the use of nuclear weapons in space. That means we can't even test it. The DART II mission that /u/ItsMyImPulse mentioned would certainly be interesting, though - a dry run to confirm our technical ability to pull off a mission like this would do a lot for our preparedness in case we ever do need to actually do it, without violating the treaty.

That treaty would go out if the window if it was determined that nuking an asteroid would stop it from impacting a populated area.

It's all about how much energy we can transfer to the asteroid, and a spacecraft moving through space at 6.1km/s is carrying far more kinetic energy in its own velocity than it could ever carry in fuel. Crashing the spacecraft into the asteroid and transferring all of its own kinetic energy into it is way more efficient and economical than trying to land it, then use a thruster to steer it away.

Keep in mind we don't really need to "steer" it, we only need to nudge it in a general direction that we know will affect its orbit enough to miss Earth.

Additionally, any thruster we do use for such an application would likely have to be an ion engine of some sort, because otherwise the specific impulse of normal chemical rockets would be far too low. We'd have to send a ton of fuel to make any noticeable change to its orbit. However, ion engines provide barely any thrust and would take years, if not decades, to push the asteroid by any noticeable amount - which also means we'd need to notice years or decades in advance. Transferring all that energy in a single instant with a kinetic impact event is much better in that regard.

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Because a nuke would blast in all directions, losing most of the energy to space. Smacking into it at 14,000 mph directs almost all the energy directly into the asteroid. Much more effective.

The energy impacting the asteriod doesn't directly translate to change in velocity. The problem is accounting for heat, ejected particles, ECT. Not a mathematician btw

Europeans are launching a follow up mission in 2023

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At least a kilogram, but I could be wrong.

About 800 feet should do it

Not really. The main reason they hit this asteroid was because it was a binary pair. This makes is way easier to measure the tiny change in orbit.

Measuring stuff in the real world has error bars and reducing those error bars takes time.

"The DART β determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos’s near-surface."

Encountering this sentence has hobbled my self-confidence.

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Yes, and they picked a binary pair that didn't cross Earth's orbit.

I want to add to this, we currently today can’t even reliably shoot a missile to blow up another missile in the way you’re describing, by detonating it next to the attacking missile. It’s much, much easier to calculate the incoming attack missile trajectory and fire a defense missile to slam into it head on to destroy it. If the US can’t do this with things going much slower, that are much smaller, with a controlled trajectory, and like a million times closer, there’s no way we can do it in space yet.

> Shouldn't this be a simple matter of physics, though? Like, wouldn't the mathematicians already have the answer?

If it was a perfectly inelastic collision of two rigid bodies, it'd be child's play. In reality, it's neither.

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The probability that an extinction-sized asteroid will hit in the next 100 years are infinitesimally small. We already mapped out all the large asteroids in the solar system. NASA is actually worried about asteroids big enough to vaporize one city, because they're so numerous and hard to see.

The Asteroid Redirect mission that was supposed to happen a few years ago was cancelled due to lack of funding. DART is the replacement mission that was able to get funded.

Why? You have the internet. Just go look up what the words mean.