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Military grade lasers shooting down incoming missiles have been in the news since Ronald Reagan was the US president. The problems, as I recall are mostly about diffusion in atmosphere.

The Kessler syndrome suggests at some point the amount of debris in LEO is going to be so great it creates significant issues.

When LEO is so full of debris that you can no longer find a safe window to launch through, could you use laser mounted on your rocket to clear a launch path?

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  • $\begingroup$ I think there may be a problem that lasers go in straight lines and rockets don't; and the earth rotates as the rocket travels etc.... $\endgroup$ – Brian Tompsett - 汤莱恩 Dec 31 '16 at 11:00
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    $\begingroup$ The laser must be so powerful and precisely aligned that debris would be totally vaporized or transformed into very fine dust. Dividing larger parts of debris into smaller ones would not help, it would increase the risk of a collision. $\endgroup$ – Uwe Dec 31 '16 at 13:09
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    $\begingroup$ Do you mean "People have been shooting down the idea of military grade lasers shooting down incoming missiles since Ronald Reagan was the US president"? $\endgroup$ – uhoh Dec 31 '16 at 15:13
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    $\begingroup$ Even the craziest of the Strategic Defense Initiative crazies weren't crazy enough to want to use lasers to shoot down incoming missiles. The idea was to shoot down outgoing missiles (i.e., during the boost phase). Burn a tiny hole into the rocket's fuel and boom! the rocket takes itself out. At least that was the idea. $\endgroup$ – David Hammen Dec 31 '16 at 21:19
  • $\begingroup$ I know I've seen this. Oh, wait, here it is. Randall has done at least three relevant xkcd what-ifs: in no particular order, Laser Umbrella and Snow Removal and Enforced by Radar. $\endgroup$ – user Jan 3 '17 at 12:43
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Well, for a laser to help in this situation, you'd need to vaporize the debris. That means heating the entire piece to its evaporation temperature. This needs either a very powerful laser, or enough dwell time at a lower energy level.

Both are difficult to achieve. For high laser power, you're talking about the sort of power that is generated by the first stage engines, plus tons of laser equipment.

At lower power, you need to account for the fact that the debris you're worried about has a speed on the order of kilometers per second relative to the launch vehicle. This makes it tricky to target the debris accurately. If you need 5 seconds dwell time for an object 10 cm across, you need to hit it when it's 5-50 km away and keep hitting it continuously. Objects that small are very difficult to hit.

You'd be better off putting the laser on a satellite, where it can shoot at targets at leisure, and not care how long it takes. That way you can also do selective ablation on larger pieces, and produce retrograde thrust to deorbit them. Power's still a problem though.

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