Peter Rodes Robinson
3 min readFeb 27, 2021

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How to Protect a Space Elevator

A comment in Reddit:

>>The problem with this whole thing isn't *if* the tether falls, it's *when.* You yourself state that by necessity the tether *must* be on the Equator. Because of that, it's in the path of *literally every satellite between the surface and geostationary*. It's *going* to get hit, and it's not going to survive. In my mind, that's the single biggest reason that space elevators will never happen. Because it's physically impossible to prevent them from getting hit by satellites.<<

I believe this is a real problem, but not an unsolvable one.

First the geostationary satellites are not a problem. They don't move relative to the surface or relative to the tether. And there would not be any independent geostationary satellites at all in the vicinity of the tether since they could simply be attached to the tether.

So the problem is the other satellites. The tether does not have a large cross-section: say 10 meters wide. Compared to the length of orbits, it is a tiny target.

The orbit of the International Space Station (420 km above the earth) is approximately 21345 km long. This is also the length of the circle swept by the tether at the same height. So the odds of a given satellite at that height impacting the tether in one orbit is 10 / 21345000 or 1/2134500.

(Please check my math.)

In effect any given satellite would crash into the tether roughly one time out of every 2 million orbits. Of course there are about 6000 satellites. So now we calculate a probability of a collision at 1/355 per 90 minutes (roughly). (Most satellites are higher and would come around less often.) That probability approaches certainty after 23 days. Not good.

But here we get to the heart of my argument: satellites must be capable of performing evasive action. The ISS does this routinely: in 2013 there were 67 notifications of "potential conjunctions".

https://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/

It takes a lot more fuel to move a 400 ton space station than to move (translate) a typical satellite. And they only need to move over a few hundred meters . Perhaps by using ion engines powered by the solar arrays the amount of fuel needed would be minimal.

The location of the tether would be precisely known. In fact it would probably be lined with radar beacons saying "Here I am". A satellite could triangulate with those beacons to calculate its path relative to the tether within millimeters.

The ISS is protected within a perimeter of 2 to 25 kilometers. The perimeter of the tether could be smaller because its location is so precisely known. Let’s assume we want to avoid anything passing within one kilometer. So the assumed width is 2000 meters instead of 10 meters.

2000 / 21345000 * 6000 ~= .5

Roughly a 50% chance that one of the 6000 satellites will make an evasive manoeuvre every 90 minutes or one every three hours. For one satellite that means an evasion every 18,000 hours: about every two years. If we assume that satellites are also maintaining their altitude, the fuel for evasive manoeuvres is likely a fraction of the fuel needed for station keeping.

This assumes that every satellite in orbit is upgraded with detection and manoeuvering capability before construction of the tether is begun. That is a small problem compared to building the tether.

We still need to consider space junk.

https://en.m.wikipedia.org/wiki/Space_debris

We can assume that the tether will be considerably tougher than the ISS. Flakes of paint and micrometeoroids impacting the tether (all that stuff that is not even tracked) should not be a problem (though presumably there would need to be routine maintenance to smooth out the dents).

What about the rest of the space debris? Stuff big enough to damage the tether? I think we have to get rid of it. Not a simple task to say the least, but still small in comparison to construction of the tether.

I should add the possibility of defensive lasers on the tether. These could be used to push away a rogue satellite which is no longer manoeuvring properly or the occasional piece of space junk. Probably wise to have as a backup plan.

IMHO we should build the first space elevator on the moon. A much easier problem gravity-wise and where the consequences of catastrophic failure are minimal. And for goodness sake: keep the lunar space free of junk.

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Peter Rodes Robinson

Thinker, dreamer, retired programmer, living in the Caribbean. UBI advocate. I offer inexpensive English editing (first hour free). RodesScholar@gmail.com