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The ISS recently jettison some garbage and expects it to take 2-4 years to de-orbit and burn up.

From Gizmodo's ISS Ditches 2.9-Ton Pallet of Batteries, Creating Its Most Massive Piece of Space Trash:

The pallet is packed with nickel-hydrogen batteries, and it will stay in low Earth orbit for the next two to four years “before burning up harmlessly in the atmosphere,” according to a NASA statement. SpaceFlightNow reports that the pallet is the “most massive object ever jettisoned from the orbiting outpost.”

How come they don't give it a little extra push towards earth and have it de-orbit sooner?

Or perhaps they did shove it as hard as they could and this is just how long it takes.

It just seems logical that they would want to get it out of the way ASAP.

uhoh
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  • Welcome to Space SE. Could you put a reference in for that event? – Puffin Mar 16 '21 at 18:01
  • The Canadarm2 can only move at 0.6m/s, and that is unloaded. velocity quickly drops with load. You can't simply "push harder" with an apparatus like that. Yes, the SSRMS can handle masses of up to 100 tons, but not whip them around with ease. I'm not an expert on the load limits of that system though, so I can't really tell you the maximum operational limits for a 2.9 ton payload like the garbage pallet. But 0.6m/s is the upper bound anyways. – Polygnome Mar 16 '21 at 18:01
  • @Polygnome where'd you get the 0.6 number? That's different from what I remember. 0.6 is close to the number for the shuttle arm, but the SSRMS is about half that IIRC – Organic Marble Mar 16 '21 at 20:39
  • Because the extra push would take extra energy they'd like to reserve for more important things, maybe... They probably could hurry the garbage down if it was important, but since it's hurting no one to have some lithium-battery satellites orbiting earth for a few years, then why waste the fuel? That's my guess. Fuel is very expensive to ship up there. –  Mar 17 '21 at 03:23
  • @OrganicMarble Its possible I mixed up the Shuttle and ISS arm when looking up the numbers. Doesn't really change the sentiment of my comment, though, since its the same magnitude. You simply won't get much acceleration out of the arm. – Polygnome Mar 17 '21 at 10:46
  • @Polygnome it makes your argument better since the SSRMS is even slower. – Organic Marble Mar 17 '21 at 11:38
  • Related: This video explains what tools you'd likely want to use to help you eject the garbage pallet faster. https://www.youtube.com/watch?v=cxNJoaBLLNM&ab_channel=ScottManley

    With that said, one might consider pushing it off as hard as possible as way to boost the station's orbit.

     – Justin Braun Mar 17 '21 at 15:37

3 Answers3

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I assume you refer to the jettison of the garbage pallet containing the old batteries, which came in at about 2.9 tons. Those were jettisoned using the Canadarm2.

The Canadarm2 has the following operational limits:

Speed of Operations

  • Unloaded: 37 centimeters / second (1.21 feet / second)
  • Loaded:
    • Station Assembly - 2 centimeters / second (.79 inches / second)
    • EVA Support - 15 centimeters / second (5.9 inches / second)
    • Orbiter - 1.2 centimeters / second (.47 inches / second)

(Numbers from Canadarm2 and the Mobile Servicing Subsystem)

I am not exactly sure where on this range the pallet falls - but presumably somewhere between 2cm/s and 15cm/s. The upper limits is 37cm/s.

Orbital velocity of the ISS is ~7660m/s. That means the pallet can only get about 0.005% of change of velocity from the arm.

Combine this with the fact that the pallet is very dense and has a small-ish cross-sectional area, the ballistic coefficient is rather high, so it doesn't experience much drag (compared to a large, light object).

Going by this Q&A, jettisoning stuff by hand yields velocities of about 0.6m/s, but only for stuff that is significantly lighter than 2.9 tons.

@Uwe already gave a good comparison with the Shuttle, which needed 90m/s for the re-entry burn. The jettison was done with 0.37m/s.

You can not simply "push harder" because of the operational limits of the Canadarm2 and the human body, depending on who does the jettison.

And with such a small change in velocity, it takes a while to de-orbit. Which doesn't really matter. Making it de-orbit faster would incurs high costs for no apparent benefit.

Polygnome
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  • I was with you until the 'human body' bit. Relevance? The SSRMS controls don't require strength to operate. – Organic Marble Mar 17 '21 at 12:31
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    @OrganicMarble No, but not all garbage is jettisoned using the arm. In this case it was, but at other times it was done by literally pushing the garbage away with body strength, e.g. here: https://twitter.com/Space_Station/status/1354461242610642945 – Polygnome Mar 17 '21 at 12:34
  • Oh, ok. I thought you meant who was flying the arm to jettison the p/l. I could pick some robo nits with this but I like it well enough for a +1 – Organic Marble Mar 17 '21 at 12:35
  • @OrganicMarble Feel free to nitpick, if that leads to an improvement in the answer that just helps with spreading knowledge. As I said, I'm not really an expert when it comes to the arm. – Polygnome Mar 17 '21 at 13:14
  • Well - the last 3 of the bullets you give are "loaded" rates, not just the Station Assembly one. "Loaded" just means the arm is grappled to a payload. Maybe you just copied it from the source wrong. The "Orbiter" one was a for a hypothetical case of the SSRMS moving the shuttle, it never happened. Part of configuring the SSRMS for ops is loading in files to its computers that define the maximum allowable rates for a particular payload. It's not simply a matter of available arm power vs mass of the payload (although that is obvs. a major consideration). – Organic Marble Mar 17 '21 at 13:35
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    @OrganicMarble I've changed the list markup a bit, its now much clearer. – Polygnome Mar 17 '21 at 13:58
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    There's also the tiny issue of recoil; you may not want to push any harder in order to minimize the momentum change on the ISS itself. I'm sure the mass differential is big enough that it wouldn't be worth worrying about, but there is a point where you could push hard enough to significantly affect the ISS' orbit. – John Bode Mar 17 '21 at 17:28
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There are two main reasons it takes so long:

  1. lots of mass and very little delta-v.
  2. a large ballistic coefficient.

The first is due to the limitation of the deployment mechanism. The second means that the drag takes longer than say a flimsy structure.

Erik
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  • The garbage has a large ballistic coefficient? Do you have a source? This would be surprising, to say the least. That box-like container is not at all aerodynamic. Nor would it make sense for it to be, if the purpose is to let drag bring down to earth and break it apart on the way down if possible. Why do you say it has a large ballistic coefficient? –  Mar 17 '21 at 03:29
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    @user39728 because it obviously does. Dense bodies tend to have high ballistic coefficients. Suggest you read the wikipedia article. – Organic Marble Mar 17 '21 at 03:40
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    In the region we are talking about, we are dealing with rarefied gas dynamics. For quick looks, this is simply about density. Batteries are dense. – Erik Mar 17 '21 at 04:38
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    OK, you know better than I do, but to say "because it obviously does" is hardly a good way to start an answer. Educate me so I can ask less stupid questions next time ;-) –  Mar 17 '21 at 04:39
  • Thank you, Erik. –  Mar 17 '21 at 04:41
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    @user39728 what can you throw farther : a beach ball, a small plastic ball, or a golf ball? – eps Mar 17 '21 at 06:34
  • @eps: The density issue has been explained, so you're not adding much at this point. Also, the balls are of wildly different sizes, so you would have two variables at play, and it would not be immediately obvious which of them accounts for the difference in travel between the balls. –  Mar 17 '21 at 18:44
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    @user39728 One way to look at it is to use a related concept, the coefficient of drag $c_d$. Given a fluid (liquid or gas) with density $\rho$ and an object moving through the fluid with a velocity $v$ and a cross section $A$ to the fluid, the force on the object is $F=\frac12 \rho v^2 c_d A$. Shape is important for aircraft, watercraft, and cars. It is not important at all for things in the very low density upper atmosphere. A standard value for $c_d$ in the upper atmosphere is 2.2. In comparison, a very good parachute (which are designed to be rather non-aerodynamic) has a $c_d$ of 1.5. – David Hammen Mar 18 '21 at 02:38
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    Almost everything is essentially a parachute in space. All that matters is cross section. Given two objects with the same cross section, one very light and the other very massive, the two objects experience more or less thee same drag force. Dividing by mass, the very light object will experience a much greater drag acceleration than will the very massive object. The relation between the ballistic coefficient $BC$ mentioned in the answer and the coefficient of drag $c_d$ I just wrote about is simple: $BC = M/(c_d A)$. – David Hammen Mar 18 '21 at 02:50
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    This results in a nice expression for drag acceleration: $a_d = \frac12 \rho v^2/BC$. The inverse relation between ballistic coefficient and drag acceleration means the higher the ballistic coefficient the lower the drag acceleration (and vice versa). – David Hammen Mar 18 '21 at 02:52
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The Space shuttle did a deorbit burn for three minutes to decrease the orbital velocity by only 1 %. This delta v was only 90 m/s. But 90 m/s are 324 km/h, much more than a little extra push. A slow walker needs about 10 minutes for one km, 1.67 m/s or 6 km/h.

If the gentle push is similar to the slow walkers speed, it is only 1.85 % of the neccessary delta v of 90 m/s. The deorbit burn speed is comparable to a ICE-3 high-speed electric multiple unit train licensed for 330 km/h.

Try to imagine an ICE-3 passing by at full speed in a distance of some 20 m. Is this like a gentle push?

Uwe
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