# At what angle would a crashing rocket be most efficient in deflecting an asteroid? **URL:** \ **Category:** Factual Questions\ **Created:** [July 12, 2020, 7:49am UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099 "2020-07-12T07:49:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [July 12, 2020, 7:49am UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/1 "2020-07-12T07:49:15Z")
After hearing about NASA’s intent to test this maneuver, I was wondering whether there is a preferable angle under which the rocket should crash into the space rock. Here’s what the news said: _DART, NASA’s first planetary defense mission, will demonstrate asteroid deflection by crashing the spacecraft, built by the Johns Hopkins Applied Physics Laboratory (APL), into an asteroid at roughly 14,500 miles per hour._ At this speed, does the angle of the rocket’s trajectory really matter? ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [July 12, 2020, 11:48am UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/2 "2020-07-12T11:48:47Z")
It would matter in things like how much energy goes into a change in velocity and how much goes into rotation. I would wag that a straight shot at the center of mass would generally be the best at changing the velocity. ---
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [July 12, 2020, 1:17pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/3 "2020-07-12T13:17:37Z")
I am not good at this, but I thought this solution would be least favored because it would require the highest amount of energy to be successful and in the end it may simply slow down the asteroid instead of deflecting it. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 12, 2020, 1:40pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/4 "2020-07-12T13:40:17Z")
I’m not sure what you mean by “at what angle”, because I can interpret that two different ways. Do you mean relative to its motion, or relative to the body of the asteroid? Relative to its motion, the best angle would generally be very close to perpendicular. Relative to the body of the asteroid, aiming at the center or a glancing blow aimed at the edge would have the same effect on the trajectory. The latter would also make the asteroid spin, but that wouldn’t matter. Practically speaking, you’d probably aim for the center, for the same reason you aim for the center when shooting a gun: To make it more likely to get a hit at all. ---
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [July 12, 2020, 1:50pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/5 "2020-07-12T13:50:50Z")
> [@Chronos](#): > > Do you mean relative to its motion, or relative to the body of the asteroid? I was confused about that part, but thanks to your explanation I understand it a bit better. Isn’t it possible that a perpendicular shot taken at the center of the asteroid should merely slow down the space rock instead of deflecting it? Laser ablation sounds so much better, but I guess the technological effort would be too big. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [July 12, 2020, 2:21pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/6 "2020-07-12T14:21:38Z")
I guess the first step would be to find the asteroids center of mass. If it’s rotating they could calculate that. If not I guess they would have to assume the asteroid has a consistent density even if it doesn’t. The center of mass could then be calculated from a 3d representation of the asteroid. From there you’d want to represent the asteroids trajectory as a single vector line. Then direct your rocket at the center of mass and hit it at an angle perpendicular to that vector. ---
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [July 12, 2020, 2:49pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/7 "2020-07-12T14:49:39Z")
I’m not sure what method NASA is going to use. Here is what I found in a different article: _Dart is designed to intercept a small moon of the asteroid Didymos in late 2022 when the rock comes in 11 million kilometers of Earth. The spacecraft will crash into the moonlet, nicknamed Didymoon, in an attempt to redirect its course. “The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent […]”_ ---
**Author:** ![Gray\_Ghost](https://avatars.discourse-cdn.com/v4/letter/g/c0e974/32.png) [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)\ **Post date:** [July 12, 2020, 2:57pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/8 "2020-07-12T14:57:46Z")
> [@PlaceboTarget](#): > > Isn’t it possible that a perpendicular shot taken at the center of the asteroid should merely slow down the space rock instead of deflecting it? This is a body in orbit we’re talking about. Isn’t ‘slowing down’ the asteroid the same thing as deflecting it? Moreover, isn’t energy by the impactor that rotates the target, vs altering the momentum of its center of mass, wasted for our purposes? Wake me we when we try out @Stranger_On_A_Train 's idea he’d mentioned here, of having a large mass of some low-Z material like polyethylene, right next to a directed nuclear explosion, and use that now-rapidly moving gas cloud to move the threatening asteroid off its trajectory. ---
**Author:** ![Dr\_Paprika](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr_paprika/32/3042_2.png) [@Dr\_Paprika](https://boards.straightdope.com/u/Dr_Paprika)\ **Post date:** [July 12, 2020, 3:00pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/9 "2020-07-12T15:00:04Z")
It seems to me if you are shooting at something thousands of kilometres away, and hit it, you are already ahead. Much easier to miss. But I agree with Chronos about the angles. The relevant principle may be conservation of momentum, and to a lesser degree angular momentum and moments of inertia (which involve the centre of mass). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 12, 2020, 4:18pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/10 "2020-07-12T16:18:26Z")
It would definitely deflect it. The final speed could be greater or less than the original, depending on the exact masses and velocities. Given those parameters, the results would be exactly predictable, using very easy physical principles. In practice, of course, the reason you’d want to do this is that the rock is on a collision course with the Earth. At the end of the day, any collision with the Earth is disastrous, no matter whether the rock is aimed at the center or edge, and any miss, even a very close one, is fine. But it’s probably not initially aimed exactly at the center, which means that it’s going to be easier to make it miss in one direction than the other, which will determine how you want to aim your deflection mission. Of note, speeding up or slowing down the asteroid can also generate a miss, since the Earth is a moving target. It’s just that a sideways deflection will usually get you more bang for your buck. ---
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [July 12, 2020, 4:40pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/11 "2020-07-12T16:40:24Z")
Of course. After going through all these answers it makes sense. 🤦‍♂️ While I was reading my own quote initially, I wondered if hitting an asteroid’s moon will really help scientists understand better how to deflect any space rock heading for the earth. Now I can see this plan is also quite doable except it will be very easy to miss the target. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 12, 2020, 5:11pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/12 "2020-07-12T17:11:22Z")
> [@Chronos](#): > > Of note, speeding up or slowing down the asteroid can also generate a miss, since the Earth is a moving target. It’s just that a sideways deflection will usually get you more bang for your buck. FOr curiosity I did some quick simple calcs about Earth orbit size, speed, etc. Baring screw-ups it shows that the Earth passes through its own diameter about every 18 seconds. So even for a meteor destined for a perfectly centered hit on the disc of the Earth as seen from its angle of approach, all we need to do is alter the arrival time by +/- 9 seconds and the Earth will be somewhere else when the meteor crosses our orbit. If the strike wasn’t going to be centered then we’d need even less change in arrival time, but we’d need to get the +/- part correct. ---
**Author:** ![Gray\_Ghost](https://avatars.discourse-cdn.com/v4/letter/g/c0e974/32.png) [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)\ **Post date:** [July 12, 2020, 6:01pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/13 "2020-07-12T18:01:36Z")
> [@"LSLGuy](#): > > FOr curiosity I did some quick simple calcs about Earth orbit size, speed, etc. Baring screw-ups it shows that the Earth passes through its own diameter about every 18 seconds… 7 minutes, per the [wiki](https://en.m.wikipedia.org/wiki/Earth%27s_orbit), not 18 seconds. But right, otherwise. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 12, 2020, 8:35pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/14 "2020-07-12T20:35:58Z")
Hmm. I wonder where I fouled up? My error is a factor of 23.3, so it’s not something obvious. I didn’t save my quicky spreadsheet. Oh well. On the basis of my small number I had thought to question **Chronos** ’ assertion that changing the meteor’s along-orbit speed & hence arrival time was generally a less effective strategy than changing orbital track. Glad I didn’t. That 23x makes the tradeoff a lot less favorable. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [July 12, 2020, 8:58pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/15 "2020-07-12T20:58:52Z")
The 7 minutes is right. Circumfrence of Earth orbit is around 600,000,000 miles. Circumfrence of Earth is around 8,000 miles. So orbit is around 75,000 Earth circumfrences. Around 525,600 minutes per year divided by 75,000 is around 7. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 12, 2020, 9:36pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/16 "2020-07-12T21:36:18Z")
> [@Darren\_Garrison](#): > > Circumfrence of Earth is around 8,000 miles. Nitpick: diameter of Earth is around 8000 miles. But diameter is what you wanted in this calculation, so it’s all good. ---
**Author:** ![Dr\_Paprika](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr_paprika/32/3042_2.png) [@Dr\_Paprika](https://boards.straightdope.com/u/Dr_Paprika)\ **Post date:** [July 13, 2020, 3:17am UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/17 "2020-07-13T03:17:25Z")
I don’t know much about ballistics, but if the target is small relative to missile strength, it is possible the goal is not to deflect the asteroid but break it into smaller pieces which would presumably reduce harm and change orbits. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 13, 2020, 12:54pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/18 "2020-07-13T12:54:47Z")
Smaller pieces merely leaves a shotgun aimed at Earth instead of a rifle. That actually _increases_ the odds we take a hit from at least some of it. Oops. As to the others above … I figured out my error. The number of minutes in a year is 365_24_60, not just 365\*60. Dumb-ass mistake. :smack: as we used to say. And the 1.5% difference between 23.33 and 24 was just from me using rounded numbers for earth size & orbit radius. Which is plenty close enough. Should’ve just wiki-ed instead of calculating. ---
**Author:** ![Dr\_Paprika](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr_paprika/32/3042_2.png) [@Dr\_Paprika](https://boards.straightdope.com/u/Dr_Paprika)\ **Post date:** [July 13, 2020, 1:32pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/19 "2020-07-13T13:32:42Z")
Maybe, but that might depend on how far out the asteroid is. The combination would probably deflect them, and gravity acting on a smaller mass might change their path too. If something is already in orbit around the Earth, that’s a different situation. The goal is not to miss. One’s ability to optimize angles might be limited. And if going for the centre of mass, the thing might fracture anyway. But as some comedian said, there might be more scientists working on the Big Bang Theory than helping save the Earth from rare events. The Economist has a good article on why very rare events like this matter, and why it is governments job to foresee them. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [July 13, 2020, 2:02pm UTC](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099/20 "2020-07-13T14:02:36Z")
> [@Dr\_Paprika](#): > > The combination would probably deflect them, and gravity acting on a smaller mass might change their path too. If something is already in orbit around the Earth, that’s a different situation. “Might” won’t work. You might also have the same mass hitting Earth. Deflecting the path may be canculated. [Next page](https://boards.straightdope.com/t/at-what-angle-would-a-crashing-rocket-be-most-efficient-in-deflecting-an-asteroid/915099.md?page=2)