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Several projects related to interstellar travel have been proposed, like interstellar travel with miniature light sail starships , or the Breakthrough Starshot project

Question. Could a laser array and miniature light sails be used to deflect asteroids that are on a collision path with Earth?

Since the miniature light sails could reach high speeds (and collide with the asteroid) , could a large number of these miniature light sails be used to deflect dangerous asteroids, even large ones ?

How efficient would such a deflection system be, in comparison with other methods?

Edit. These notes have been added later.

Due to the answer, comments and feedback from PcMan, Cornelis and uhoh it seems to me that with current technology the only feasible way to deflect asteroids is to build a laser array around Earth (probably powered by the Sun ) that would carry Davy Crockett devices with light sails to far away dangerous asteroids on a possible collision path with Earth. The nuclear blasts in the vicinity of the asteroid (similar to a laser burn ) should efficiently deflect it. If the temporal consciousness of humankind is limited to decades then this is irrelevant and a waste of money.  If it spans centuries or millennia then there is nothing more important than protecting life on Earth against complete obliteration (the real odds for that time span are at least 50/50).  With current technology that's the only way. Funny thing, no space agency on Earth  (whether American, Russian,  Chinese,  Indian or European) currently has sufficient (or any ) funds allocated to this kind of research.  Real defender "Starlink' project, anybody?

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Yes it would work.

But no, it would not work very well at all.

If you are going to have the supremely powerful laser array available, you would be better off beaming the asteroid directly. The momentum transferred by boiling off a thin layer of the surface of the asteroid is many magnitudes more effective than using the same laser to accelerate lightsails.
Why? Because when you push a lightsail, you only transfer 2*the photon's momentum to the lightsail. The photon bounces off, still carrying virtually all of its energy away with it. But by heating the surface material causing it to boil off, you generate a lot of thrust by absorbing the totality of the photons' energy, converting it to heat, and using this energy to generate thrust.

Example: a 303 Mw (photon energy) laser will impart 1 Newton of thrust via light reflection off your lightsail. The same 303Mw applied to 86 kilogram of surface material will flash heat it to 5000K, per second
This 86kg will promptly depart the asteroid at about 1.3km/s, imparting a force of 72.8 million newtons.

So, directly burning the asteroid is millions of times more effective than directing lightsails against it.

Of course, if you have lots of warning time, you need not bother building the laser array. Just paint the asteroid white (or black), and use the incredibly gentle pressure of sunlight to shove it out of the danger path, by altering its natural https://en.wikipedia.org/wiki/Yarkovsky_effect

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  • $\begingroup$ In that case, nuclear blasts in the vicinity of the asteroid would have similar effects as laser burning but probably even more efficient. The issue with your assessment is the laser beam divergence, since we can assume that the laser array is close to Earth and the asteroid is far away. Thank you for your answer @PcMan $\endgroup$
    – Cristian Dumitrescu
    Jul 3, 2021 at 16:27
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    $\begingroup$ @CristianDumitrescu Quite Correct! The only real disadvantage of the nuclear option is get the nukes there, as the asteroid is typically several AU distance away. And then the more trivial problem of timing your detonations exactly, as the closing velocity of the nuke to the asteroid will be rather large, but you want a detonation as close as possible without actually fracturing the asteroid. A laser array, on the other hand, is kinda point-n-shoot even at interplanetary distances (if you have good enough focus) $\endgroup$
    – CuteKItty_pleaseStopBArking
    Jul 3, 2021 at 16:31
  • $\begingroup$ Maybe you could steer a Davy Crockett bomb to its destination with light sails, but that would not be a miniature light sail anymore. A quantitative analysis would be interesting @PcMan $\endgroup$
    – Cristian Dumitrescu
    Jul 3, 2021 at 16:49
  • $\begingroup$ "Lots of warning time", important distinction. But with a laser array in place, light sails and Davy Crockett devices ready to be deployed, you only need sufficient warning time,, important distinction. $\endgroup$
    – Cristian Dumitrescu
    Jul 4, 2021 at 4:55
  • $\begingroup$ I would encourage rthe reader (few will get this far ) to understand Descartes' ontological argument related to the existence of God. Also translate that argument into this current context. What's more important than the long term preservation of life on Earth (emphasis on long term)? What resources could be justifiably spared when meeting this goal? The correct answer is "none". There is no higher purpose than this. And yet here we are, absolutely nothing is being done. Funny. $\endgroup$
    – Cristian Dumitrescu
    Jul 4, 2021 at 8:02
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Yes this solution can be used, but a large amount of time would be required (how much would depend upon the size of the object, and the amount of energy being delivered).

Most deflection solutions share this same requirement - as much time is possible, and this is the primary driving force behind the development of the early asteroid impact detection solutions currently in use (the earlier we detect an risk, the more time we have to deal with it).

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