Knowledge

If I shine a flashlight at Mars, does a small amount of the light actually reach it?

Let’s say you had a very nice flashlight with a 5 degree beamspread and a 1 watt visible beam.

At 60 million km to Mars at close approach, the beam spreads out over 2.5××101313 sq km or 2.5××101919 sq meters. That means about 1 photon per second per square meter arrives at Mars.

Mars does not move far enough in the time it takes light to get there to need a lead ahead angle with a flashlight. If you were using a narrowbeam laser, you would need to calculate and precisely apply a lead ahead angle.

So yes, a very small amount of the light gets there. Not enough to detect with even advanced sensors, but some gets there.

I assumed a clear line of sight to Mars. No clouds, no volcanic dust in the air. You can subtract about 10% for atmospheric extinction in the Earth’s atmosphere. You can subtract a lot more if Mars is very close to the horizon as seen from your position. I also did not take the light through the Martian atmosphere to the Martian surface.

You can lose upwards of 90% of the light getting through the Martian atmosphere. None of that changes the basic answer. Yes, some light gets there, but it is below a detectable level.

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