If the laser was in the visible part of the spectrum, you MIGHT be able to see that with your eye, or certainly with a small telescope… but only if you were within the few kilometer diameter spot- the laser would have to be targeting you… the laser wavelength was in the short wave infrared, however, which is invisible to the human eye.

This is what makes lasercom so secure… unless you are in the small spot, you can’t detect it… it doesn’t matter how good you are at cracking encryption, unless you are actually in the beam, you have no hope of receiving it… this physical security makes it attractive for some applications.

The signal from the moon was pulsed rapidly, with the information content encoded in the timing of the pulses, and a data rate of 622Mbps was achieved using a signal format known as pulse position modulation.

A set of telescopes on the ground received nano-joules of signal per second, and those nanojoules were divided among 10′s of millions of pulses…

The receiver telescope collected the signal light and coupled it into optical fiber which carried these faint pulses to photon counting detectors… super-conducting nano-wire detectors that are kept at sub 1 kelvin temperatures. They are highly sensitive and able to detect a single photon.

The detectors detect the weak signal pulses and turn them into electrical pulses, and the modem processes the signal to get the data encoded in the timing of the pulses.

More later… its late and I have to get some sleep.

It was a message:

https://boingboing.net/2021/02/23/nasa-hid-an-easter-egg-in-the-parachute-for-perseverance.html

Geeks are cool.

Remember, mars is dusty, and all that dust would greatly compromise a laser transmitter on the surface… so forget the ‘almost certainly’ above… it would CERTAINLY be on an orbiter.

The width of the beam would depend on many factors… how collimated the beam is limited by the diffraction limit of the transmitting telescope. Larger transmitter telescope, the narrower the beam, the shorter the wavelength the narrower the beam… the beam divergence would likely be on the order of 10 micro-radians, or 2 arcsec… perhaps as small as 2-3 microrad, but let’s assume 10.

I think at mars’ closest approach to earth this gives you ~560 km spot on earth, but its late and I am doing this on my phone…

The laser transmitted in the Lunar lasercom demonstration had a footprint on the earth of ~ 6 kilometers in diameter. For comparison an RF signal would cover most of the continental US.

One of the most difficult problems in lasercom is how to aim something that precisely.. and you have to aim at where your target WILL be when your laser signal gets there… NOT WHERE YOU SEE IT.

The more collimated the beam is, the more power arrives at your receiver, BUT the more collimated it is, the more precise your aim has to be…

The data rate would vary widely based on distance, the closest Mars gets to us is ~56 million km, and the furthest is 400million km… a factor of 7.14 further, the data rate will vary with the received intensity, which goes as the distance squared. So the data rate will be ~ 50 times higher at its closest than at its furthest… but it gets worse than that, at it’s furthest, your receiver on Earth will be pointing close to the sun, this means MUCH more noise, you will have to pick the few signal photons you receive out of all the background sunlight…

The canceled mars lasercom demo was going to use Palomar telescope as a receiver…

More later…

Questions:

Would this communication be with a rover, or with a repeater on an orbiter?

What it the width of a LASER at this distance that still maintains enough strength to convey information?

I ask our expert because I was wondering if I had a LASER detector on earth, would I be able to get transmissions from mars? Could I watch a rover’s transmissions of data?

https://www.youtube.com/watch?reload=9&v=4czjS9h4Fpg

Press Conference:

https://www.youtube.com/watch?v=gYQwuYZbA6o

Just released. WOW