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California-based Orbit Guardians is one of a small number of startups trying to tackle orbital debris. It's difficult to tell exactly what their plan is, but based on this animation, they seem to want to deorbit droplets of NaK, which I understand from other reading tend to be between 800-900km orbits. They seem to want to sort of push the droplets (retrograde?). This is nonsense, right? Even setting aside the danger of sending little droplets down through the most congested regions of LEO, it seems technically unfeasible. What kind of delta-v would you need for this anyway?

Screenshot from https://vimeo.com/536547567 showing an NaK droplet being mechanically deorbited by first capturing it then pushing it retrograde.

Screenshot from https://vimeo.com/536547567 showing an NaK droplet being mechanically deorbited by first capturing it then pushing it retrograde.

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    $\begingroup$ The back of my envelope (not to be trusted) says that each droplet needs a push of about 850 fps to get it to intercept the atmosphere. The pusher shown in the animation is moving quite a bit slower than that. $\endgroup$
    – Wayne Conrad
    Apr 19, 2021 at 22:39
  • $\begingroup$ Funny, they're going after the debris population that offers the least threat to satellites in orbit. Na-K droplets are very low density and are easily defeated by modest debris shields. $\endgroup$
    – Tristan
    Apr 20, 2021 at 2:37
  • $\begingroup$ What's the deal with the NaK? Are there droplets of NaK floating in orbit? $\endgroup$
    – Dohn Joe
    Apr 20, 2021 at 7:43
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    $\begingroup$ @DohnJoe Soviet BUK reactors, both as leaks and upon reactor core decommissioning. Total mass according to their paper is about 85kg, comprising maybe 4000 droplets of 1-5cm diameter. I'd be a bit worried about 4000 projectiles of 0.5g-1kg(most in the 1gram-5gram range) flying around at 7.4km/s, and destinied to eventually slowly drift down over the next couple centuries, passing through lower orbits. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Apr 21, 2021 at 7:03
  • $\begingroup$ I've just asked How much is really known about those liquid metal droplets orbiting the Earth? (e.g. sizes, composition, orbits…) Are any actually tracked? $\endgroup$
    – uhoh
    Apr 22, 2021 at 4:07

3 Answers 3

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I'm not sure what altitude they are aiming for at periapsis, but the $\Delta V$ to go from an 800x800km to 800x100km orbit is 194 m/s, and the $\Delta V$ to go from a 900x900km orbit to a 900x50 km orbit is around 234 m/s, so that's around the range that would be necessary. If the arm in their schematic travels 1 meter (which is probably very generous), then the acceleration to change the velocity by 200 m/s is $\frac{(200 m/s)^2}{(2)(1 m)}=20000\frac{m}{s^2}\approx2000g$. If the arm is shorter, the acceleration will be higher. Those levels seem...difficult to achieve.

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    $\begingroup$ 2000g is a pretty good kick, but not out of the range of achievable. It's on a par with hitting a baseball with a bat, and less than a tenth the acceleration of a handgun firing. $\endgroup$
    – Mark
    Apr 20, 2021 at 3:14
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    $\begingroup$ No info on their page or that I could find otherwise. But, their animation does show the solar panels being stowed before kick, so it is conceivable that they plan to use a really strong action. The droplets are very light and uniform almost-perfect spheres, thus not too difficult to handle. If the pusher is light enough, it might be able to achieve the needed speeds for deorbit from one kick. How hard is is to slam a round metal ball massing maybe 50g, to 800km/h? (if the droplet size were predictable, I'd just use a compressed gas charge, like a paintball gun) $\endgroup$
    – CuteKItty_pleaseStopBArking
    Apr 21, 2021 at 7:10
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Sounds perfectly plausible.

As the other answers have shown, you need roughly 200 m/s delta V to reliably and quickly deorbit such a droplet. For every gram of mass in a droplet, you need to deliver an energy of 20 Joules. An ordinary consumer-class electric jackhammer easily delivers 50-70 Joules in a single impact, so you could readily deorbit hundreds of 2-gram droplets in a single second with common off the shelf hardware (assuming you manage to hit them). Scaling this up to a single blow of several hundreds of joules once in a while won't be too hard. (Say, build a small electric motor to gradually compress a spring or some gas in a cylinder and then release it to deliver a kick.)

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Orbit Guardians - bs, right?

Scary? Yes! But no, not necessarily 100% bs.

Answers to How hard do you have to throw something off the ISS to make it deorbit? are in the "ballpark" of 90 m/s (fastest thrown Cricket ball is 45 m/s).

However, at 800 km you'd need to throw it harder. Reusing my stuff from there:

$$v^2 = GM \left(\frac{2}{r} - \frac{1}{a}\right)$$

Periapsis altitude (km) Apoapsis altitude (km) Semi-major axis (km) Apoapsis velocity (m/s)
Initial 800 800 7178 7451
Final 80 800 6818 7252
Difference 199
World's record 45

So you'd have to match the orbit of the droplet almost exactly, gently capture it and pull it in, then eject it with about 200 m/s of retrograde delta-v in order to deorbit it immediately.

You would not want to do it more gently and reach only say 300 or 200 km. Even though that would eventually deorbit in say months or a few years (depending on details of periapsis and solar activity) the uncertainty in the orbit (due to the mechanics of throwing and interaction with the atmosphere) means it could hit something valuable or even crewed!

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  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – ChrisR
    Apr 20, 2021 at 17:27
  • $\begingroup$ What does the freezing robot question have to do with this (honest question) $\endgroup$
    – Organic Marble
    Apr 21, 2021 at 11:17
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    $\begingroup$ @OrganicMarble I know that you are often pressed for time and loath to read long posts, but the answers there explore several issues related to robotic technology implemented in space. The technology described in the question and its links is a robotic gun, which approaches a "droplet of liquid" in space, folds back its solar panels (see the video) and somehow captures the droplet, draws it into the spacecraft, then suddenly propels it forward acting like a space gun. This propulsion should be accurate and repeatable/reliable enough to deorbit and not to hit anything. Check those answers. $\endgroup$
    – uhoh
    Apr 21, 2021 at 11:43
  • $\begingroup$ @OrganicMarble there is also some relevant/useful information in the comments as well. It goes to the OP"s "... it seems technically unfeasible." $\endgroup$
    – uhoh
    Apr 21, 2021 at 11:51
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    $\begingroup$ Thanks, I read the question, it seemed totally irrelevant so didn't proceed to the answers. $\endgroup$
    – Organic Marble
    Apr 21, 2021 at 12:15

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