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A comment under this question has me thinking; with it's huge main structure and giant solar panels the ISS presents a very large cross-section to Earth's rarified atmosphere at 400 km altitude. So much so that I have heard that every time the ISS goes into Earth's shadow the solar panels rotate to present a minimum drag surface, and some of them may do this even in the sunny parts of the orbit if full power is not needed to charge the batteries and run the systems currently drawing power.

Drag is bad because the ISS would soon reenter if not for regular shipments of rocket fuel from Earth to power engines that are burned to regularly re-raise the orbit, which sits at around 400 km these days.

Question: Roughly how hard does atmospheric drag push on the ISS? Is it more than one pound? The ISS is about 400,000 kg and so I'm guessing that the drag force must be quite substantial to make a dent in its velocity, even over months.


screenshot ScienceCasts: Space Coffee from ScienceAtNASA screenshot ScienceCasts: Space Coffee from ScienceAtNASA

From here click for larger.

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    $\begingroup$ Does this answer your question? What is the ISS drag? $\endgroup$
    – asdfex
    Mar 14 at 20:27
  • $\begingroup$ @asdfex When we close as duplicates we want to guide future readers to the best answer, not simply the oldest answer. So I've voted to close that as duplicate of this because I find this answer a lot more readable, informative and mathematically complete than the answer there. Interestingly the data there shows a 10x faster rate of descent as it's smack-dab in the middle of the late-2014 sunspot maximum associated with maximal atmospheric heating, so in this case perhaps merging the two will be the best way to serve future readers. I'll flag it and post something in meta about this. $\endgroup$
    – uhoh
    Mar 14 at 23:14
  • $\begingroup$ Are these two similar questions ripe for merging? $\endgroup$
    – uhoh
    Mar 14 at 23:26
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    $\begingroup$ My comment was mostly tongue-in-cheek! But yes, if you changed km to miles, kg to pounds (or stones?), and kept the pounds-force I would have found it less jarring. Of course, ideally as scientifically minded community, I'd stick to SI (and derived) units and thus simply ask for drag in Newton. No need to actually change the question though. I was mostly joking and should have added a ;-) to clarify! $\endgroup$ Mar 15 at 15:58
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    $\begingroup$ @Cristiano Oh! 20 to 50 mN Yes I see now! Reading sideways I saw it reversed somehow. Yes it does provide an answer to this question, thanks! $\endgroup$
    – uhoh
    Mar 22 at 14:47
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Here is a rough estimate. The ISS's height drops at the rate around 10 meters per day. The energy of a body of mass $m$ in a circular orbit of radius $r$ is $E=-\frac{\mu m}{2r}$, so $$ Fv=\frac{dE}{dt}= \frac{\mu m}{2r^2}\frac{dr}{dt}. $$ Since $v=\sqrt{\frac{\mu}{r}}$, $$ F=\frac{m v}{2r}\frac{dr}{dt}. $$ Substituting $m=4\cdot 10^5$ kg, $v=7800$ m/s, $r=6.8\cdot 10^6$ m, $\frac{dr}{dt} = 10/86400$ m/s, we get that $F$ is about $1/40$ of a Newton, or about $1/200$ of a pound-force.

Edit: 10 meters per day is the current rate, taken from the image enter image description here (source and credit: Heavens-Above). It may be unusually low, see the comments.

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    $\begingroup$ @uhoh To be honest, that the increase rate of the orbital speed is the same as the deceleration due to the braking force is somewhat of a coincidence. If the gravitational potential was proportional not to $r^{-1}$, but to some other power, they would be different. Though still, they would differ only by a constant factor. $\endgroup$
    – Litho
    Mar 14 at 12:20
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    $\begingroup$ @uhoh Just in case: it applies only when the orbit stays almost circular. $\endgroup$
    – Litho
    Mar 14 at 12:28
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    $\begingroup$ Litho, I used the same 10m/ day or 300m/ month. The solar cycle has a huge influence. But maybe a factor of 5 at most. $\endgroup$
    – asdfex
    Mar 14 at 15:09
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    $\begingroup$ Air density changes a lot with radiation. Even day/night side is clearly distinguishable $\endgroup$
    – asdfex
    Mar 14 at 20:04
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    $\begingroup$ So, I guess the reason they turn the solar panels at "night" is because shipping rocket fuel from the surface is so dang expensive, not because the drag is substantial. $\endgroup$
    – jpaugh
    Mar 14 at 23:08
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While Litho's answer addresses the first step in the question, of how much drag the ISS experiences, it doesn't directly address why that amount of drag might be worth worrying about, or spending money developing a technique to adjust the solar panel's angle to account for.

In fact, even if the amount of drag saved by rotating the solar panels were insignificant on a daily basis, designing that feature could still have made sense while developing the IIS, from a project-management perspective, because the ISS is a massive, long-term endeavor.

For one thing, shipping fuel to space is very expensive! Circa 2010, it cost about $210 million per year to keep the ISS in orbit. That's a huge incentive to invest in "minor efficiencies." It follows that NASA would be willing to spend a little less than \$1 million per year to save even half a percent of that fuel.1 And, considering the longevity of the ISS, they would have been willing to spend significantly more up-front.

1 They might be willing to spend more than the dollar amount of fuel saved; the "launch weight" budget of any mission is far more constrained than the "launch cost" budget.

I'm not sure how long NASA originally planned to keep this thing in space; but this NASA-RSA agreement from 1998 uses phrases like permanent human presence; mature operations; and, provide [...] maintenance plans five years in advance.

Additionally, because the ISS already needed a system to rotate the solar panels to track the sun, adding additional features to that big, unavoidable requirement would have been comparatively cheap.

Up to a point, adding more sophisticated logic to the controller would not have increased development costs substantially, since controller had to be built and coded, anyway. Perhaps it required upgrading the hardware to get better range-of-motion that was strictly necessary to track the sun.

That might affect the launch weight, which is normally a no-no; but if it saves weight on future launches, it might still be worth it. "One extra feature" would have added a small additional cost to the overall build budget.

Verification that the feature works and that it doesn't conflict with other design requirements could easily been more expensive than the feature itself; you have to make sure everything else still works when you go tweaking the power availability. Yet, the total cost from a man-hours perspective would have been amortized across hundreds of other features that were already intrinsic to the the design, and already had to be cross-checked against each other for mutual compatibility.

Furthermore, the entire point of the ISS was to be a long-running, upgradable system. They could have released a more sophisticated (but less critical) update to the panel control system well after the 1998 launch date --- at a time when schedules, budgets and oversight constraints had all relaxed somewhat. I'm sure they do over-the-air updates for some software-only upgrades today, although I wouldn't guess about how it worked in 1998.

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  • $\begingroup$ Does not even attempt to answer the question that was asked "Roughly how hard does atmospheric drag push on the ISS?" $\endgroup$ Mar 15 at 0:26
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    $\begingroup$ @OrganicMarble There's already a great answer covering the most explicit part of the question; however, I was speaking more to the motivation of the question than the phrasing. $\endgroup$
    – jpaugh
    Mar 15 at 0:27
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    $\begingroup$ @OrganicMarble Then, please read the whole question: "Roughly how hard does atmospheric drag push on the ISS? Is it more than one pound? The ISS is about 400,000 kg and so I'm guessing that the drag force must be quite substantial to make a dent in its velocity, even over months." From Litho's other answer, that drag force turns out not to be very substantial. What it doesn't address, is why bother adding this feature when the impact on drag is so small? $\endgroup$
    – jpaugh
    Mar 15 at 0:35
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    $\begingroup$ @jpaugh "Too long to be a comment:" is a standard phrase used by Stack Exchange answer authors to help readers accept a post that might not necessarily directly answer the question in some people's opinion but yet provides helpful and insightful additional information for the benefit of future readers. It's a standard thing. $\endgroup$
    – uhoh
    Mar 15 at 0:41
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    $\begingroup$ @uhoh That is true. I do believe I addressed the "a long time" bit better than the first answer; although my answer does not rely as much on math as it does the some-what softer area of project management. It's important to remember that space is not just a physics equation. It's a place that people go. Everything comes into play. $\endgroup$
    – jpaugh
    Mar 15 at 0:45

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