# Sci-fi inspired tractor beams are real, and could solve a major space junk problem

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#### Unclear Engineer

Newtons = grams ??? (see https://en.wikipedia.org/wiki/Newton_(unit) )

So, is what you really mean that a 1 gigawatt photon source reflecting perpendicularly off a surface at will impart a force equal to the downward force of a 110 gram mass sitting on a scale on Earth's surface?

Considering the subject of the thread, it seems more enlightening to say that a 1 gigawatt laser would need to shine on a one metric ton (1000 Kg) satellite for 1000 x 20 = 20,000 seconds to achieve the 20 m/s velocity change assumed to be needed to deorbit the satellite. That is about 5.5 hours of continuous laser light shining on the satellite.

Still doesn't seem like such a bad idea until you ask how you would get that much energy into another satellite to make it capable of emitting that laser beam for that long. And ask what 5.5 gigawatt-hours of energy would do to the satellite if not all of it gets reflected and x% gets absorbed.

Caveat: I have not checked Bill's conversion from gigawatts of photon energy to momentum impulse at a reflecting surface.

#### billslugg

I was off in my calculation, A gigawatt would produce 3.3 Newtons against an absorber, 6.6 Newtons against a reflector. Force into an absorber equals watts divided by c, double for a reflector..
A Newton is that force required to accelerate a 1 kilogram mass to 1 m/s in one second. The force is about 110 grams and during that time the mass will move 1/2 of a meter.
A one gigawatt beam would produce a force of 1e9 divided by 3e8 or 3.3 Newtons.
If the object is a perfect reflector the force would be double that or 6.6 Newtons, about 730 grams of force.
A mirror can get up to about 99.9% reflectivity. That would cause about a megawatt to be absorbed. It would be required to spread this around a bit so the mirror would not melt.

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