What impact will the deorbiting of thousands of satellites have on the atmosphere?

Question

With the creation of mega satellite constellations like Starlink, there are several thousand satellites being launched each year. This means that as these satellites go out of order in a few years, there will be thousands of satellites being deorbited and burning up in the atmosphere each year.

Will this large number of deorbitings have an impact on the atmosphere (e.g., particles from the burn staying suspended in the atmosphere)? Has the impact of all these deorbited satellites on the atmosphere been determined?

Answer 1

Not much research has been done on this question in recent years, but some researchers are worried enough to research into wooden satellites. The question on the environmental impact of deorbiting satellites burning up in the upper atmosphere was partly addressed in a 1994 report (warning: not peer reviewed) by the Environmental Management of the Space & Missile Systems Command in the United States. Their focus was to consider the impact of deorbiting space debris on ozone at the time, and their conclusion was that deorbiting space debris has very little impact on stratospheric ozone. They considered two types of impact using a combination of lab and model measurements:

• Heterogeneous mechanisms, or small particles (in their case Al₂O₃) on which ozone depletion can occur (such as in polar stratospheric clouds). The report cites experiments by Marino Molina (MIT) from which they conclude it takes 10⁴ – 10⁵ years to destroy one percent of stratospheric ozone. Although small, it would seem that a major increase of re-entry mass should motivate revisiting this study; if their figures are accurate and numbers are linear, a factor 1000 increase would translate to 10–100 years to destroy 1% of stratospheric ozone, close enough to warrant at least some worry, although most likely still in the safe range (there is always a natural amount of ozone destruction and regeneration).

• Homogeneous mechanisms: spacecraft paint and the Zeldovich mechanism produce nitric oxide, but the study estimates that the paint causes the destruction of one ozone molecule per billion days and the Zeldovich mechanism even less, so if their conclusions are correct, this mechanism is negligible.

There may be other impacts than ozone depletion, but ozone is probably the most sensitive substance that may suffer from the impact of orbital debris reentry for us to worry about in practice.

The impact is probably still minor, but to actually answer this question, you need to consider:

1. Where does the satellite break up?
2. What pollutants are released in the process, and how much of each?
3. What is the lifetime of those pollutants?
4. What is the ultimate fate of these pollutants after their lifetime?

A complete answer would take an in-depth study. We've made great progress in the recovery of the ozone hole and let's not be taken by surprise again; few if any people expected fridges to cause skin cancer, after all. To address the questions a bit:

1. Comparing with the total mass of the atmosphere is not useful. The total atmospheric mass is not relevant because satellites break up in the upper atmosphere, which is very thin compared to the rest. The stratosphere does not exchange much mass with the lower atmosphere, so substances can stay in the stratosphere very long (unless destroyed in chemical reactions or heavy enough to fall down due to gravity).
   You don't need much mass to have a high impact. CFCs have concentrations in the parts per billion range, but with their lifetime of decades can and do break down large amounts of ozone. So we can't simply dismiss the problem based on atmospheric mass considerations alone.

2. The materials from which satellites are built are different from propellants involved in launch. Therefore, you can't simply dismiss satellites as far less massive than rocket propellants and therefore declare the impact of re-entry negligible compared to the the impact of launch. Much of a satellite material is metal, which will deposit rather quickly (see next question), but other materials could in theory have an impact (John et al. considered paint and concluded its impact was negligible). Some satellites contain unusual materials: for example, Kosmos 1402 was a Soviet spy satellite containing a nuclear reactor and thus fuel. Leifer et al (1987) have shown that more than a year after Cosmos-1402 deorbited a 53±20% excess of ²³⁵U was measured at an elevation of 36 km. I don't know what happened since then. Fortunately Starlink will not contain nuclear reactors.

3. Lifetime, often defined using the half-life or the time it takes for the concentration to halve, is crucial to determine impact. A piece of metal that falls down has no impact on the atmosphere, but exclusively anthropogenic CFCs lingering around for decades may, even in relatively small concentrations. I don't know if anyone has estimated the lifetime of the ²³⁵U from Kosmos, but Murphy et al. (2018) may have detected evidence of it in the upper troposphere (they found one particle and could not tell the source). Due to its low concentration this is rather of academic interest than something to really worry about.

4. There are only two ways a substance can leave the upper atmosphere: by physically leaving (to the troposphere) or by destruction (chemical reaction). If it reaches the troposphere it will have a very low concentration compared to pollutants originating from the surface, and if it reacts we go back to question 2. Larger particles may fall down quickly, but molecules can stay around for a while. The John et al. found that mostly the small particles were enhancing ozone depletion.

In conclusion: the impact is probably small, but the potential for ozone depletion if re-entry flux is increased by several orders of magnitude is probably sufficient to warrant a dedicated research project to quantify this again. And any detectable impact is an impact, which is worth monitoring even if the impact is well within safe limits. Someone go write a research grant proposal ;)

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Relevant papers I found:

• Leifer et al., Detection of Uranium from Cosmos-1402 in the Stratosphere, Science, 23 Oct 2017, doi: 10.1126/science.238.4826.512
• Murphy et al., An aerosol particle containing enriched uranium encountered in the remote upper troposphere, Journal of Environmental Radioactivity, Volume 184-185, April 2018, Pages 95-100.
• John et al., The Impact of Deorbiting Space Debris on Stratospheric Ozone, 1994 (see online PDF).

Answer 2

The mass of Earth's atmosphere is 5E+18 kg and the Troposphere alone has 3/4's of that. With an average height of 13 km that makes its volume $4 \pi r^2 h$ or about 6.6E+18 m^3.

If we break up one thousand 100 kg satellites into semi-porous PM2.5 particles that works out to be 1.5E-08 micrograms per cubic meter, and we generally worry about tens of micrograms per cubic meter when it comes to particulates.

Of course we will definitely be breathing them!; there'd end up being a few satellite particles in every cubic meter of air until they settled out or were incorporated in rain and washed out, but that's nothing compared to the crap we breathe in every minute. Anthropogenic particulates constantly deposit radioactive, volatile and carcinogenic and heavy metal-laden material so deep into our lungs that the cilia can't bring it back out. A burned up constellation is nothing in comparison.

According to this answer to Is Earth getting heavier or lighter? every year 4E+07 kg of meteoric dust enters our atmosphere, and people who collect dust from rooftops and separate it with magnets can show some of it to you!

One thousand 1000 kg satellites is is only 0.25 % of the annual meteoric infall. We need to worry far more about particulates we produce from automobiles and industry than we would about deorbiting a satellite constellation!

Also; as @Uwe points out for ever kilogram we put into orbit, several tens of kilograms of material is added to Earth's atmosphere. Some of that will be carbon particulates from kerosene and LOX, but any time a launch uses SRBs (solid rocket boosters) there are a lot more particulates introduced, which is why SRB exhaust glows so much more brightly than Keralox, and why that glows so much more brightly than Methalox. See the following for side-by-side comparisons:

• What makes exhaust from aluminum-based SRB propellant so bright?
• How do rocket propellant combinations rank in terms of "brightness"?
• What is the cause of the blue light from LH2/LOX rocket engines?
• Is it possible to create different colors in rocket exhaust?

below: Screenshot from the NASA video Smoke and Fire! NASA's Space Launch System Rocket Booster Test as an example of soot particulates produced by a (in this case quite large) solid rocket booster.

Answer 3

An article has been published last month on this topic.

Today, half a ton of satellites is falling into the Earth's atmosphere each day. It is expected that a constellation such as Starlink, once it is fully deployed, will cause 2 tons of satellites to enter the atmosphere a day. This number should be multiplied by the number of starlink-like constellations.

Two tons a day may be smaller than the 54 tons of meteors a day that go through the Earth's atmosphere. But contrary to what is stated in uhoh's answer, the amount matter that comes in from satellites is not directly comparable to the amount of meteorites, since the composition of the two is very different. The fact that the total mass is much smaller doesn't matter much if the composition is different (compare adding a ton of nitrogen into the atmosphere to adding a ton of CFCs: the effect on the atmosphere would be very different. This might be an extreme example but you get the idea). Satellites for instance contain a lot of aluminum, whereas meteoritic infall contains oxygen, magnesium and sillicates and only 1% aluminum. This means that the infall of iron would be at least multiplied by four.

each mega-constellation will produce fine particulates that could greatly exceed natural forms of high-altitude atmospheric aluminum deposition

This aluminum oxide from a large constellation of satellites could potentially disrupt the ozone layer. And aluminum deposits into the atmosphere has been suggested as a way of increasing the Earth's albedo.

Answer 4

To add another perspective to the discussion:

The mass fraction of the satellites consisting of materials that naturally occur in the solar system, i.e. iron, will probably not have a notable impact on anything because the total mass of natural objects hitting Earth is far larger than that of satellites that will reenter in the foreseeable future:

NASA states an estimate of 44,000 kg of meteoritic material hitting Earth each day (source pointed out in a comment to a related question). Most of this material is vaporized during entry.

The point is that reentering satellites contribute to this on a much smaller scale. Therefore this effect is negligible.

This answer does not address particularly toxic substances contained in satellites or in general any artificial materials but just the ones that are constantly added to the atmosphere naturally and en masse.