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Comment by patall | Hacker News Reader

That sounds very scary to me. If they don't care whether a single satellite is working or not, they are basically asking Kessler syndrom to start within years. Multiple start-ups (etc.) and the article covers not even have a tiny bit about what happens at the end of life of the satellites, what could go wrong ...

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The other comments that are replying to you have it fairly accurate. It's not widely known but it isn't a secret either: the point of the doves is to be deployed en masse and with more capability with every subsequent build. A quick refresh in the constellation means there is fresh hardware in space every so often.

This means the intention is indeed for re-entry into the atmosphere after the operational lifespan of the dove has lapsed. (They physically stay in space only a few years depending on altitude, orbit, exposed atmospheric drag and a couple of other things)

The last two flocks alone have put close to 130 sats in space so the debris issue is something that is taken quite seriously at the company.

Source: I work at planet. I generally lurk on HN but I created an account just now to reply to this :)

EDIT: Relevant blog post from a couple years ago - https://www.planet.com/pulse/keeping-space-clean-responsible...

Unrelated and probably a stupid question -- but I am curious. What would these satellites see if they looked the other way - i.e. Outwards towards space. Would they see just darkness? Some stars? Lots of stars? Would you know?

I don't work for Planet but I know the doves have a space-looking camera that is used to help understand the satellite's orientation based on the location of stars in the image.

I'd hazard a guess at probably not a lot. The onboard telescopes and cameras are likely calibrated to look at and focus on specific sized patches (say 15km by 15km) of the earth between certain altitudes (say LEO distance + -450m to 9000m).

Turning them to look outwards might just capture something if it happened to be inside the size of patch and whatever the telescope/camera combo can focus on (which would be the LEO distance plus or minute a bit). But space is big, even at 500km above the earth, so the likelihood of finding something in your field of view such as another satellite is probably quite low. Stars and other astronomical objects might be too dim/out of focus.

[Disclaimer: not a rocket scientist]

It seems Planet currently only relies on passive de-orbiting, how long are sats expected to stay in orbit? And are active de-orbiting options being considered?

Between 6 months and 4 years ideally. 25 years max. If you can't prove reentry within 25 years in simulations you do not fly on the rocket at all in the first place.

It sounds like you're relying on the satellites de-orbiting and burning up in the atmosphere.

Are there any steps taken to ensure the satellites de-orbit in a timely manner? Are there any estimates on how many of the satellites will de-orbit and when, absent such an intervention?

Anything in LEO will de-orbit unless you actively keep it there, due to atmospheric drag. This is like watching someone drop a ball and asking if they are taking any steps to ensure it will fall.

Obviously, but how long does that take, and is that long enough that collisions are negligibly probable?

It's possible that the problem will take care of itself, but the information provided is not adequate to model the problem and prove that the problem will take care of itself.

It's a Bloomberg article, not an intro to astrodynamics. If you know the orbit you can figure out when it will decay relatively accurately.

I'm not asking the Bloomberg article, I'm asking the person who works for the company who responded upthread.

This is simply ignorant of the plans. Firstly, there are levels of functionality, being able to provide service being the highest one. Out of service satellites are still most likely to be able to execute a controlled re-entry. Secondly, these satellites will be in low Earth orbit. If they become completely inoperable they have an orbital lifetime of only a few years before they re-enter. Only with routine station keeping maneuvers can they stay in orbit for even their sub-decade planned lifetimes. So there's very little risk of a Kessler syndrome type situation developing. At any given time the number of derelict satellites in orbit would be quite small if not zero.

> Out of service satellites are still most likely to be able to execute a controlled re-entry

How? By a burn? That's a lot of fuel reserve to use up.

It takes minimal energy to deorbit from LEO. By changing the shape of the orbit you spend some time in denser atmosphere which increases drag which quickly deorbits you. Controlled just means they start the processes at a specific time.

How much reserve fuel does an out of service satellite need to use after it burns up in Earth's atmosphere?

How much fuel do you imagine it takes a satellite to re-enter if it will happen automatically in a matter of a few years?

The answer to both is, of course, not much, not much at all.

You don't have to worry too much about the low Earth orbit (LEO) sateillites as they will deorbit soon enough. My understanding is that there is also ongoing research into methods to deorbit them at end of life.

In the case of a Kessler syndrom event it remains a concern. Collisions will push some debris higher while causing other pieces to enter early. The result would be a debris cloud that persists for decades even in LEO.

From first principles, I'm not seeing how a LEO collision pushes a significant amount of debris notably higher, but I'm certainly missing something:

Post collision, all debris orbits will still be passing through the point of collision. Any deflection with a vertical component (up or down towards the earth) will have a part of their orbit go through thicker atmosphere, which will make them deorbit faster. That leaves deflections which are in the plane spanned by the two orbits ("sideways" and "forwards/backwards"). If those deflections in any way slow down the piece of debris, that will also go through lower atmosphere and deorbit.

Disregarding debris under those effects, the remaining debris will have two more things going for it: They'll be out of the LEO orbit for a large part of their (now elliptic) orbits, and they'll be smaller so they'll slow down more from friction (due to the square-cube law).

Of course, cascading effects could still affect all satellites in LEO (and humanity's access to orbit for years), but it doesn't seem to me like it'd be a "permanent" issue in LEO? What am I not seeing?

When collisions are more frequent than once per orbit you get effects where more than one collision cause a bit of debris to have itself lifted. MOST debris is deorbited, but a small fraction is lifted into a higher orbit overall. As these collide with other objects the total mass of the system might go down (from reentry), but the number of objects and the frequency of collision goes up, continuing the process of some objects getting into higher energy orbits.

This is not too dissimilar to the process of evaporative cooling in a liquid, or gas escape from an atmosphere.

> more frequent than once per orbit

My gut feeling says that statistically, even just going from one impact to two impacts being likely would require an immense density of satellites, let alone having more collisions than that.

Then there's also the fact that every impact would have a loss of kinetic energy (because it gets converted to heat as the objects deform), which would also make a reduction in orbit likely.

If the debris keeps fragmenting, which maybe could increase odds of impact, the remaining kinetic energy would be divided over each object. The smaller the debris gets, the more drag it should feel too, because of the square-cube law[0]. So that too would only make it more likely to deorbit.

[0] https://en.wikipedia.org/wiki/Square%E2%80%93cube_law

https://en.wikipedia.org/wiki/Kessler_syndrome

Not how orbits work. A collision can't cause, for example, an object with a circular orbit at 400km (passive reentry regime) to become fragments with a circular orbit at, say, 2000km (non-passive reentry regime.) Like snaily said, all fragments originating from a collision will still pass through the point of collision, which, if it is still in the upper atmosphere, will lead to reentry. Orbital debris is actually very dissimilar to gas escape.

Multiple collisions. Multiple.

The proportion of fragments that would have their orbits boosted, through multiple collisions, to an orbit higher than the upper atmosphere, is trivial. Nearly every angle of collision between two objects in orbit lowers their periapses. The risk of Kessler Syndrome doesn't come from objects in upper-atmosphere orbits somehow getting boosted out through collision chains, it comes from collisions between objects already in higher orbits not strongly affected by atmospheric drag (>600km).

It can me infinite collisions. It does not matter. Conservation of momentum still applies. The system of collisions only have a finite amount of energy. And it's chaotic rather than engineered. So it's not cumulative, much more likely to happen at numerous different angles cancelling previous collision trajectories out.

Maybe this is a good time to bring up the European Space Agency film about space debris [1].

[1]. http://www.esa.int/spaceinvideos/content/view/embedjw/484820

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