SpaceX plan to launch Starships towards Mars; many of them.

This will involve burning hundreds to thousands of tonnes of propellant in low(ish) orbit, since each may require several refueling steps.

When the Mars bound ships are being boosted first to a higher Earth orbit, would it be possible to use the exhaust to slow down a specific, targeted piece of space debris, removing just enough velocity that it will deorbit significantly sooner than it would otherwise.

I acknowledge that this would require precision and orbits lining up extremely precisely, and that there are several alternative ways to deorbit dead satellites, but as a thought experiment in orbital mechanics, exhaust dynamics and aerodynamics, would this at least be possible?

  • 2
    $\begingroup$ I agree that this seems highly implausible, but it’s an interesting question! I can think of several reasons why it might not work, and I’d like to know where the deal breaker really is. Like what is the likelihood that the exhaust would break up the object rather than deorbit it gracefully? Or does the breaking up kit pose any real risk if you know it would deorbit? $\endgroup$
    – Eric G
    May 24 at 17:42
  • $\begingroup$ How do you define practical? If you are asking whether it's possible to alter the orbit of a satellite using exhaust then you need to rephrase it, if you are genuinely asking if it's practical then you need to be clear on what you mean. $\endgroup$
    – GdD
    May 24 at 19:21
  • $\begingroup$ Cool question! I'm going to adjust "Would it be practical" to possible because a simple "No it won't because anything that interferes with the main mission adds unnecessary risk" answer could answer your original question while skipping all of the interesting orbital mechanical stuff. Please feel free to edit further. Thanks! $\endgroup$
    – uhoh
    May 24 at 19:49
  • $\begingroup$ only slightly related: How hard do you have to throw something off the ISS to make it deorbit? Answer is for something at about 400 km, tens of m/s to make a big difference in deorbit time, and ~100 m/s to do it immediately $\endgroup$
    – uhoh
    May 24 at 19:56
  • 1
    $\begingroup$ By "practical" I meant "change medium Earth orbit debris orbits sufficiently to significantly decrease orbital life". But I'm happy with the change already made. $\endgroup$ May 24 at 20:25

tl;dr: It will "blow away" too quickly to gain 100 m/s necessary to promptly deorbit, but you could at least make a dent in it's lifetime this way.

Starship says it will have 6 Raptor engines with a total thrust of 12,000 kN.

Let's say a 1000 kg satellite is absurdly close and can intercept 1% of that for 1 second as it immediately accelerates, which is the same as 0.1% of that for 10 second.

$$a = F/m$$

$$v = a t$$

give us an acceleration of 120 m/s^2 for 1 second, or a delta-v of 120 m/s.

But of course that means that by 1 second it's 50 meters away, so not intercepting 1% of the exhaust even within that one secon.

Answers to How hard do you have to throw something off the ISS to make it deorbit? tell us that 95 m/s delta v will promptly deorbit something in a circular 400 km orbit; it will hit the Earth's atmosphere a half-orbit (46 minutes) later.

So this is a weird idea but it is not likely to be even plausible because it will just blow away from the rocket before it can gain 100 m/s to promptly deorbit.

I also don't think that it is attractive because it poses some risk to the Starship; while unlikely, bits of the disintegrating spacecraft could fly back at the Starship somehow outside of the exhaust plume (there could be fuel on board) or something else could go wrong, so I don't think they will take on even a tiny amount of extra risk for something that could be done more easily with a dedicated space-cleaning mission.

But this is a cool stunt and it might be doable to impart say 10 m/s delta-v thereby decreasing the deorbit time, and if you did it at 200 km instead of 400 km, a smaller kick will make a much bigger dent in deorbit time; a Starshp could orbit for a few hours or a day or so at 200 km if it really, really wanted to.

  • $\begingroup$ Wouldn't they only be using the 3 vacuum raptors? $\endgroup$ May 25 at 6:32
  • 2
    $\begingroup$ @user2702772 3=6 within an order of magnitude $\endgroup$
    – uhoh
    May 25 at 9:50
  • $\begingroup$ It's a fair point :) $\endgroup$ May 25 at 10:53
  • 1
    $\begingroup$ Also, you're pretty much going to have to deliberately rendezvous with the piece of space junk you're going to try to deorbit, otherwise the relative velocities are going to be significant and make it an even worse proposition. Space junk is not known for hanging around in low earth parking orbits, waiting in the precise position for an efficient Mars departure burn. $\endgroup$
    – notovny
    May 31 at 12:08

We need to find out what mechanisms may work in space to stop the satellite and de-orbit it.

In LEO, Drag is a valid de-orbit method. You can take a look here and here.

The other effects can be :

  1. The jet impacting on the satellite itself. In vacuum, the Weber number will approach infinite, as there is no surface tension. A large Weber number makes the plume unstable. In this regime, the flow will go out of hand. However, not all is lost. Impingement force of 800 Pa is observed at a distance of 40 mm, where a control thruster of 10 N was fired at a flat plate. The ambient pressue is 80 km. I am not adding any picture, as those are not my work. So if you are at that distance, then you are good to go. I do not have the computational resources to scale up the simulation for a spaceX thruster.

  2. The radiation pressure of the hot gas is irrelevant. Again, the Weber number says that the jet will very quickly dissipate.

That brings us to the effect of total aerodynamic drag, and how this will affect the de- orbiting. We can take a look at solar events that change the drag at that height, and see how that is impacting the de-orbiting. This is a very difficult calculation to make. But fear not, for we have some steps in that direction.

In the study period, the proton density spiked by 40 particles per m³, and the satellite decayed by 0.52 km. This is NOT to say that the additional particles were the ONLY cause, but that gives you some ballpark idea. Similar ideas can be found here

So to conclude : The orbit will decay, but it may not be enough to de orbit the satellite within a short time span of few days. If there is sufficient decay, and no one recovers it, then over a few months it will go down.

References : ( I have not made them consistent sorry)

  1. Bijiao He, Jianhua Zhang, Guobiao Cai, Research on vacuum plume and its effects, Chinese Journal of Aeronautics, Volume 26, Issue 1, 2013, Pages 27-36, ISSN 1000-9361, https://doi.org/10.1016/j.cja.2012.12.016.

  2. Jennifer L. Rhatigan, Wenschel Lan, Drag-enhancing deorbit devices for spacecraft self-disposal: A review of progress and opportunities, Journal of Space Safety Engineering, Volume 7, Issue 3, 2020, Pages 340-344,

  3. R.W. Fenn III and S. Middleman, Newtonian jet stability: The role of air resistance, AIChE J., 15: 379-383, 1969, https://doi.org/10.1002/aic.690150315

  4. David Vallado & David Finkleman, A Critical Assessment of Satellite Drag and Atmospheric Density Modeling, Acta Astronautica, 95, 2014, 10.1016/j.actaastro.2013.10.005.

  5. Victor U. J. Nwankwo, William Denig, Sandip K. Chakrabarti, Muyiwa P. Ajakaiye, Johnson Fatokun, Adeniyi W. Akanni, Jean-Pierre Raulin, Emilia Correia, and John E. Enoh, Atmospheric drag effects on modelled LEO satellites during the July 2000 Bastille Day event in contrast to an interval of geomagnetically quiet conditions, Ann. Geo. Dis., 2020, https://doi.org/10.5194/angeo-2020-33

  6. S Khodairy, M Sharaf, M Awad, R Abdel Hamed and M Hussein,Impact of solar activity on Low Earth Orbiting satellites, International Symposium on Space Science : Journal of Physics: Conference Series, 1523 (2020) 012010, IOP Publishing, 2020, doi:10.1088/1742-6596/1523/1/0120101 .


TLDR: Yes, but they aren’t going to. Perhaps 52-71m/s dV is practical.

My model of a rocket exhaust (in a vacuum) in as cone, that extends out behind the rocket while it fires, which applies pressure on the flat circle of the cone, as the gas molecules slam into whatever they impact.

By my measurement, this vacuum raptor engine bell https://twitter.com/SpaceX/status/1302038129990279168/photo/1 is about 14cm tall and 11cm wide – note that we care about the ratio here, not the absolute measurement. That gives us a radius to height for this engine’s exhaust plume at about 14:5.5, or 2.5:1. According to the wikipedia entry, Starship will have ~12MN of thrust in a vacuum.

So – at some distance away the engine bell there will be a point where the plume would apply 1N per square meter, or one pascal. There’s effectively no chance that this could cause any risk to the starship in question – it’s safe. Using area =πr2, this gives us 12,000,000 = πr2, or r = 1954m. How far away is that? Using the cone from earlier, that’s about 4886m away.

What does a pressure of 1Pa give us? Best case, we’re trying to push something like a centaur upper stage – large surface area, but low mass – travelling in an identical orbit, but a little distance behind the Starship. According to wikipedia, a Centaur III is about 3.05m diameter, 12.68m long and has a dry mass of 2247KG.

Assuming optimal positioning without rotation, that’s 3.05x12.68=38.7m2. So 38.7N exerted on a 2247KG object gives us, um, 0.017ms-2 – that’s...not much. If sustained for a whole minute – which is a brave assumption, given the starship would be accelerating away – would give us 1m/s of deltaV. Per minute.

Now I’ve demonstrated methodology, here’s a table I calculated, which ends when the target object is subject to about one atmospheric pressue - which given it was constructed on Earth should be be fine - well, at least not catastophic. Atmospheric pressure assumed to be 100,000Pa, not 101325Pa.

Pressure (Pa) Force (N) Area (m2) Radius (m) Distance Target area Target Force Target acceleration
1 12000000 12000000 1954.41004761168 4886.0251190292 38.7 38.7 0.0172229639519359
10 12000000 1200000 618.038723237103 1545.09680809276 38.7 387 0.172229639519359
100 12000000 120000 195.441004761168 488.60251190292 38.7 3870 1.72229639519359
1000 12000000 12000 61.8038723237103 154.509680809276 38.7 38700 17.2229639519359
10000 12000000 1200 19.5441004761168 48.860251190292 38.7 387000 172.229639519359
100000 12000000 120 6.18038723237103 15.4509680809276 38.7 3870000 1722.29639519359

Huh. So as long as SpaceX was willing to let the target get within 150-500m of the Starship, a meaningful amount of DeltaV could be imparted to our perfect target. Outside of 500m the thrust is negligible,

Next question – how long would a Starship be close enough to do this. After all, it’s burning to goto Mars. Thrust of 12MN, mass of around 1320T. 12,000,000/1320000 = 9.1ms-2. Assuming constant mass for the first few seconds, and that once the starship is 500m away the thrust is negligible, starting at 150m - you have meaningful thrust for the first 350m of the Starship’s burn. Ignoring orbital mechanics, as this will be over in seconds – the formula s = ut + 0.5at2 gives a total time of… 8.8 seconds until it’s out of meaningful range.

I don’t trust my maths to integrate this entire mess to give a better estimate, but back of the envelope excel work gives me an estimate of range 52-71m/s, based on a series of calculations based on 1s intervals as they move apart.

52-71m/s isn’t enough to de-orbit from many orbits – but it’s enough to significantly decreased the orbital life of an object at 400km height, as per previous answers.

All you need is persuade SpaceX to let their rocket get within 150m of orbital spacejunk about as big as it is…

  • 1
    $\begingroup$ Downvoter-without-comment, any chance you could explain why this answer isn't helpful, or is wrong? $\endgroup$ May 31 at 12:03
  • $\begingroup$ This analysis assumes that pressure causes most of the thrust when momentum does. IIRC nozzle exit pressures on upper stage engines are on the order of ~1000 Pa (and quickly expanding to vacuum pressures). $\endgroup$ May 31 at 13:45
  • $\begingroup$ I'm not seeing a difference here. The momentum is transfered when the gas impacts the target. Pressure simply being the spread of force over an area - a way to model the expansion of the plume over distance. $\endgroup$ May 31 at 19:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.