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Comment by Boothroid | Hacker News Reader

I would think many countries have disused mineshafts that might be good candidates for a far longer drop

It isn't sufficient to just have a deep hole in the ground. You need a vacuum too!

I had to poke around to find this: "Before the experiment, 18 high-performance pumps make sure that the drop tube is almost free of air containing only one ten thousandth of the normal air pressure. Due to the vacuum, the air resistance is so low that the Bremen Drop Tower can provide one of the best quality of microgravity - in some aspects even better than on the International Space Station (ISS)"

> in some aspects even better than on the International Space Station (ISS)

Woa, does anyone know in what way this tower provides less gravity than the space station?

I think they're implying that if you want to test to effects of microgravity, you don't want something like 14 psi of air pressure happening at the same time, because interactions with the air flows might confound whatever you're trying to test.

The station is likely better in terms of gravity, but in terms of test conditions, you're somewhat limited by the astronauts being there.

High-quality vacuum is fairly readily available at the ISS.

The centre of mass of the ISS follows a fixed orbit round the earth (to first order approximation that is, occasional thrust is required to maintain this trajectory). Moving away from the centre of mass, by definition, puts you into a slightly different orbit. This perturbed orbit has a different trajectory and orbital speed. This will manifest itself as very small reactionary forces relative to the ISS structure, in other words what you and me call gravity. In plain English, except for at the exact centre of mass position within the ISS (Or any point exactly on the trajectory traced by it) objects will appear to drift.

In the microgravity tower, translations within the experimental container will not result in divergent trajectories, so no drift will occur.

I think you've just answered for me why the ISS is effectively flat and two-dimensional rather than three. Also this strongly suggests that the station has a spin moment corresponding to its orbital period, about 90 minutes.

Perhaps the ISS has more vibration from all those life support systems?

Correct, that's the main problem there. Demanding experiments are even carried out when the astronauts are sleeping, they also tend to vibrate ;-)

Wouldn't it be easier to add some rope pulling the capsule to compensate for the air resistance? Or would it add too much "noise" because of the imperfections of such system?

I was thinking a electromagnetic rail controlled by precise sensors combined with very powerful gyroscope would also work. Probably still need to evacuate some of the air.

They're building something similar in Hannover: https://www.hitec.uni-hannover.de/einsteinelevator.html

Sure, the rope could pull an evacuated chamber, inside of which another chamber could fall without much noise. (assuming you could handle the velocity and stretch in that much rope. thrusters are probably easier)

The longest vertical shaft you're likely to find is around 3km, which gives you a little over 20 seconds of freefall and a couple seconds of violent deceleration.

But the problem is, you're dealing with acceleration squared. The Bremen tower already does almost half that time (9.3s with the slingshot), so it's probably not a big enough difference to warrant the effort and travel.

Now you just need to retrofit a zero-gravity environment into the disused mineshaft..

So then something akin to a vacuum chamber in a freefall miner's lift then? Surely cheaper than evacuating the whole chamber.

If that vacuum chamber is falling through air, then it's not in freefall.

So then you accelerate the lift to the correct speed

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