[HN Gopher] SpaceX lowering orbits of 4,400 Starlink satellites ...
___________________________________________________________________
SpaceX lowering orbits of 4,400 Starlink satellites for safety's
sake
Author : thread_id
Score : 49 points
Date : 2026-01-22 08:21 UTC (14 hours ago)
(HTM) web link (www.space.com)
(TXT) w3m dump (www.space.com)
| Jean-Papoulos wrote:
| From a comment :
|
| >The first move in the coming WWIII, where the emperors try to
| expand their empires militaril,y will be to wipe out any orbit
| with Starlink satellites.
|
| I find this highly unlikely, given Starlink is soon to reached
| 10k satellites and will continue to grow. Why expand 10 000
| ballistic missiles to bring down one of many communications
| networks ?
| xxs wrote:
| Starlink has already been used in Russian's war against
| Ukraine. Of course the satellites can take photos as a bonus.
|
| It's a massive spy network, if weaponized.
| DrScientist wrote:
| It's also been used for regime change attempts - part of the
| internet that's harder to shutdown, though apparently jamming
| GPS currently appears to be quite effective.
|
| https://www.independent.co.uk/news/world/middle-east/iran-
| in...
| GuB-42 wrote:
| What kind of pictures can starlink would take? When I look at
| pictures of starlink satellites, I don't see a camera. Maybe
| they have one, but if we can't see it, it is most likely
| useless for observation, except for taking pretty pictures of
| the Earth, or maybe other passing satellites.
|
| Spy satellites are more like space telescopes, but pointed at
| the Earth. As an example, Hubble is designed after a spy
| satellite, the "camera" is pretty massive and obvious.
|
| Starlink can probably be weaponized for a variety of thing,
| like for communication, obviously, but I don't think earth
| optical observation is one of them.
| NetMageSCW wrote:
| Perhaps Starlink can not (or wasn't designed for it) but
| Starshield includes cameras and other sensors on some of
| its satellites.
| TOMDM wrote:
| Because Kessler syndrome means you don't need to hit all 10k
| yourself.
|
| Lowering the orbits just means that we get back to normal
| faster, not that the it's impossible.
| lijok wrote:
| Does Kessler syndrome also mean ICBMs become nonviable?
| Dylan16807 wrote:
| No.
|
| It's not a wall. The risk from going through a dangerous
| orbit is much much less than the risk from staying there.
| goku12 wrote:
| That depends on how you define risk. If it means the
| probability of a collision, then you'd be correct. But if
| a collision does happen, the consequences will be worse
| than being in the same orbit. Based on an oversimplified
| model, debris in orbit is likely to have low relative
| velocities with respect to an intact satellite in the
| same orbit, since a large deltav would change the orbit.
| (It's not as simple as this, but it's good enough in
| practice.)
|
| This is actually what asat weapons take advantage of.
| They usually don't even reach orbital velocity, just like
| ballistic missiles (of course, there are exceptions like
| the golden dome monstrosity). The kill vehicle just
| maneuvers itself into the path of the satellite and lets
| the satellite plough into it at hypervelocity.
| gpderetta wrote:
| I remember a short story about Canada preventing total
| global annihilation in WWIII, by deliberately triggering
| Kessler syndrome. My google-fu is failing me though.
| iberator wrote:
| I would love to read it:)
| NetMageSCW wrote:
| Stop trying to make Kessler syndrome a thing. Kessler
| syndrome isn't a thing, and it will never be a thing.
|
| PS The original paper expects the cascade to take decades to
| centuries. No one can afford to shoot down Starlink except
| SpaceX.
| pixelpoet wrote:
| Stop trying to boss people around, and just make your point
| with some citations.
| bell-cot wrote:
| If it's WWIII, and you're using ballistic missiles against
| satellite constellations, then either:
|
| - You are not targeting _individual_ satellites; you 're
| setting off nuclear warheads in space, and relying on the EMP
| to disable all satellites within a large radius of the blast -
| https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse
|
| or
|
| - You're nuking the ground-based command & control centers for
| those satellites. Again, nothing like 10,000 missiles needed.
|
| (Or both.)
|
| To target 10,000 satellites directly, the "obvious" weapon
| would be a few satellite-launch rockets, lofting tons of BB's
| (or little steel bolts, or whatever) - which would become a
| sort of long-duration artillery barrage shrapnel in orbit.
| NetMageSCW wrote:
| The BB idea doesn't really work either- if they are in orbit
| they circle with the satellites and don't hit anything, if
| they are at different speeds they are in different orbits and
| fly above and below the satellites and miss, if they cross
| the orbit SpaceX just moves the satellites to miss.
| aucisson_masque wrote:
| You don't need 10k missiles. You need just one to blow up all
| of starlink satellites.
|
| This is like bowling, you hit one, it hits the other one
| etceteras.
| jdiez17 wrote:
| You would likely need at least one per orbital plane, of
| which there are about 24.
| goku12 wrote:
| Blowing up something in the same orbit as the targets isn't
| an effective strategy. The explosion disperses the
| fragments into different orbits that intersect the original
| orbit only at one or two points. And even if some of those
| fragments find their targets, the collision velocity will
| be low (relatively slow).
|
| It will be like getting hit with with shrapnels from a
| grenade. Depending on how they collide, the target may
| survive. If you think that grenade shrapnels are fast, you
| need to understand the 'hypervelocity impact' that happens
| when objects in different orbits collide, or when an
| interceptor hits a satellite. Hypervelocity impacts are
| impacts where the impactor moves faster than the speed of
| sound in the solid target. What that means in practice is
| that the debris/interceptor may have hit one end of the
| satellite and vaporized already, while the other end of the
| satellite doesn't yet feel the shock and vibration from
| that impact. That end doesn't yet know about the carnage
| that's about to hit it in a few milliseconds.
| NetMageSCW wrote:
| That is not how it works at all.
|
| Imagine using a rocket and blowing up one car on a highway -
| how many other cars will actually be affected? How many cars
| on other highways will be affected?
| LightBug1 wrote:
| What was that game on old PC's? ... Minesweeper ...
| tlb wrote:
| You could launch some missiles, blow a few satellites into
| smithereens, and gradually over the next few months they would
| take out the others. That's a poor kind of war weapon. An
| effective weapon is one where you can inflict damage
| continuously, and are able to stop immediately upon some
| concession. If you can't offer to stop in return for
| concessions, you won't get any.
| panick21_ wrote:
| Its not really that easy, to cause such a chain reaction,
| specially if the other person reacts.
|
| And its also really expensive, each sat you take down costs
| you far more then what you hit. So unless you can actually
| cause a chain reaction its a losing proposition.
| RealityVoid wrote:
| You don't take down satellites in order to force someone to
| negotiate, you take them down for denial of capabilities.
| ViewTrick1002 wrote:
| Not really. That's more science fiction than reality. You
| should try some Kerbal Space Program and explore how orbits
| are affected by thrust = collisions, in different directions.
|
| As soon as a satellite is hit the rest of the fleet can start
| thrusting and raise their orbits to create a clear separation
| to the debris field.
|
| Following such an attack the rest of the fleet would of
| course spread out across orbital heights and planes to
| minimize the potential damage done by each hit, leading to
| maximum cost for the adversary to do any damage. Rather than
| like today where the orbits are optimized for ease of
| management and highest possible bandwidth.
| ben_w wrote:
| Looking at the price of industrial lasers, right now the only
| thing stoping a random 3rd world terrorist cell from being able
| to afford to destroy all of them is the adaptive optics to
| compensate for atmospheric turbulence.
|
| Well, that and the fact that so much of the stuff on Amazon
| etc. that's listed as "welding laser" is actually a soldering
| iron.
| NetMageSCW wrote:
| I think you severely underestimate the amount of power you
| would need to damage some Starlink satellites in the 4
| minutes they would be visible while tracking them at
| ridiculous speeds.
| Cthulhu_ wrote:
| Or why try to shoot them down when you can also go to the
| command center and turn them off? Or do a targeted strike on
| said command center. The sattelites are plentiful and
| redundant, but the network will collapse very quickly when
| they're no longer controlled from the surface.
|
| In fact, if SpaceX can no longer do any launches due to
| whatever reason, Starlink will no longer be feasible after a
| few year - if I'm reading it correctly, the sattelites have a
| lifetime of only 5 years, meaning they will have to continually
| renew them at a rate of 2000 new sattelites a year.
| aucisson_masque wrote:
| There are so many satellites in orbit that there is a pretty good
| chance that if even one was to be hit by something and explode in
| many pieces, it would crash another one and then another one
| until there is nothing left.
|
| The nasa is pretty scared of it, so is SpaceX.
| fireflymetavrse wrote:
| There is huge increase of orbital launches in recent years [1]
| done mostly by SpaceX and China is also planning to double its
| numbers in the coming years. The risks will be even higher.
|
| [1] https://spacestatsonline.com/launches/country
| goku12 wrote:
| That's the Kessler Syndrome. But it's better if it happens in a
| lower orbit, irrespective of what assets are present there.
| Space will be free for exploration again in a few years since
| all the debris there would eventually decay and deorbit.
|
| The article mentions a few months at 480 km. I'm a little
| skeptical about this figure though, because the last tracked
| piece from an NRO satellite that was shot down at ~250 km by
| SM-3 missile in operation burnt frost, lasted 20 months in
| space before reentry. SpaceX is probably using a statistical
| cutoff percentage of fragments to calculate the time. But all
| the pieces are dangerous uncontrolled hypervelocity
| projectiles. Spain lost a military communications satellite a
| few days ago from a collision with a tiny undetermined space
| debris.
| _factor wrote:
| Two objects colliding can send debris into different orbits.
| Combined kinetic energy and mass differences can send debris
| to many different orbits.
|
| A golf ball hitting a bowling ball or basketball, both
| traveling at 30 units of speed can produce quite a fast golf
| ball. Not all of the debris will safely burn up.
| tlb wrote:
| At the speeds we're familiar with, basketballs and golf
| balls have elastic collisions. At orbital speeds,
| satellites are nearly inelastic. So fragment exit
| velocities lie between the two initial velocities, k _v1 +
| (1-k)_ v2 for some k that depends on where each fragment
| came from. If they're colliding, the velocities must be
| somewhat different, so the weighted average speed has to be
| lower than orbital speed. So fragments usually don't
| survive many orbits.
| WithinReason wrote:
| That's what I was thinking, Kessler syndrome should be
| impossible for objects in LEO since all debris orbits
| decay rapidly (probably 99.9% enter the atmosphere and
| burn up in minutes, the rest in hours)
| perilunar wrote:
| I guess if a collision ruptures a pressurised tank, or
| causes an actual explosion then you could end up with a
| higher-than-orbit speed?
| indoordin0saur wrote:
| Possibly. But more likely the thrust from escaping gas
| will push it in a direction to either slow the orbit down
| or make it more eccentric and unstable.
| tlb wrote:
| Right, if there's something like a small hole in a
| pressure tank, it's very unlikely to be aligned exactly
| with the CG, so the tank will spin around and the net
| thrust will be near zero.
|
| If a pressure tank splits in half, both halves will fly
| away but that's a very inefficient way of using the
| energy in the gas, so the added velocity will be a small
| fraction of the speed of sound in the gas, which is 1/6
| of orbital speed for hydrogen, less for any other gas.
|
| You can't really get much of a chemical explosion because
| the fuel and oxidizer both disperse very quickly in
| space.
| indoordin0saur wrote:
| Very well put. It also seems like there's a limit to how
| bad Kessler syndrome can get. The more debris there is
| the more collisions, but the more collisions the quicker
| the debris collides with itself and de-orbits.
| ViewTrick1002 wrote:
| The periapsis will always pass through where the collision
| happened.
|
| To circularize at a higher orbit you would need secondary
| collisions on the other side of the earth.
| goku12 wrote:
| You're right that all the fragments will pass roughly
| through the impact point in orbit. But it's not always
| the periapsis.
|
| 1. The normal or anti-normal delta-v imparted by the
| explosion/fragmentation (i.e, the velocity imparted
| perpendicular the plane of initial orbit) will cause the
| orbital plane of the fragment to change. The new orbit
| will intersect the old orbit at the impact point.
| Meanwhile, the eccentricity (the stretch of the orbit),
| semi-major axis (the size of the orbit) and displacement
| of periapsis from the impact point (the orientation of
| the orbit) remains the same as the initial orbit.
|
| 2. The prograde and retrograde delta-v (velocity imparted
| tangential to the orbit) will cause the diametrically
| opposite side of the orbit to rise or fall respectively.
| Here too, the new orbit intersects the old orbit at the
| point of impact. But since the impact point isn't
| guaranteed to be the periapsis or apoapsis, the above
| mentioned diametrically-opposing point also cannot be
| guaranteed to be an apsis.
|
| 3. The radial and anti-radial delta-v (this is in the
| third perpendicular axis) will cause the orbit of the
| fragment to either dip or rise radially at the point of
| impact. Again the impact point remains the same for the
| new orbit. So the new orbit will intersect the old orbit
| either from the top or the bottom. The new orbit will
| look like the old orbit with one side lowered and the
| other side raised about the impact point.
|
| So none of three components of delta-v shifts the orbit
| from the impact point. You can extrapolate this to all
| the fragments and you'll see that they will all pass
| through the impact point. The highest chance of recontact
| exists there. However the perturbation forces do disperse
| the crossing point (the original impact point) to a
| larger volume over time.
|
| Edit: Reading the discussion again, I get what you were
| trying to say. And I agree. The lowest possible altitude
| of the fragments in orbit (i.e the periapsis) is the same
| that of the impact point. So if the impact point is low
| enough to cause drag, the orbit will decay for sure.
| There is nothing that demonstrates this better than a
| Gabbard plot [1][2] - the best tool for understanding
| satellite fragmentation.
|
| [1] Gabbard Plot Discussion (NASA Orbital Debris Program
| Office): https://ntrs.nasa.gov/api/citations/20150009502/
| downloads/20...
|
| [2] Satellite Breakup Analysis (Australian Space
| Academy): https://www.spaceacademy.net.au/watch/debris/co
| llision.htm
| ViewTrick1002 wrote:
| No worries. I think I could have been more precise in my
| wording. :)
|
| My comment is based on the hunch concerning physical
| calculations and interactions from an engineering physics
| degree and way to many hours in kerbal space program a
| decade ago.
| goku12 wrote:
| Thanks! I figured that you had a reasonable understanding
| in this subject. But I still couldn't help just laying it
| out. I have some background too - as a professional.
| xoa wrote:
| > _But it 's not always the periapsis._
|
| > _But since the impact point isn 't guaranteed to be the
| periapsis or apoapsis, the above mentioned diametrically-
| opposing point also cannot be guaranteed to be an apsis._
|
| You're correct on the generalized case of the math here,
| no argument at all, but this also feels like it's getting
| a bit away from the specialized sub-case under discussion
| here: that of an existing functional LEO satellite
| getting hit by debris. Those aren't in wildly eccentric
| orbits but rather station-kept pretty circular ones
| (probably not perfectly of course but +/- a fraction of a
| percent isn't significant here). So by definition the
| high and low points are the same and which means we can
| say that the new low point of generated debris in
| eccentric orbits will be at worst no lower then the
| current orbit of the satellite (short of a second
| collision higher up, the probability of which is
| dramatically lower). All possible impact points on the
| path of a circular orbit are ~the same. And in turn if
| the satellite is at a point low enough to have
| significant atmospheric drag the debris will as well
| which is the goal.
| FranOntanaya wrote:
| Solar pressure would be a small factor too, though I
| assume it's not a big deal compared with orbital speeds.
| goku12 wrote:
| Just to elaborate the correct reply given by the others,
| the perigee of all fragments will be less than or equal to
| the altitude at impact point. If that's low enough, they
| will all eventually decay and deorbit. Even the fragments
| in elongated high-eccentricity orbits will have their
| orbits circularized by lowering apogee (the perigee is
| never going to rise) due to air drag. It will eventually
| spiral into the atmosphere. Here is the best visualization
| for this phenomenon - the Gabbard plot.
|
| [1] Gabbard Plot Discussion (NASA Orbital Debris Program
| Office): https://ntrs.nasa.gov/api/citations/20150009502/do
| wnloads/20...
|
| [2] Satellite Breakup Analysis (Australian Space Academy):
| https://www.spaceacademy.net.au/watch/debris/collision.htm
| Cthulhu_ wrote:
| It's one reason why space should be regulated (but globally /
| internationally), the systems in place are kinda loose and
| more of a gentleman's agreement insofar as I understand it. A
| plan for decomissioning / de-orbiting stuff should definitely
| be mandatory. I know there's an area for geostationary
| sattelites to park themselves after their lifespan, for
| example.
|
| But the LEO ones like Starlink will see their orbit decay in
| about five years (if I'm reading things correctly) even if
| they run out of fuel / can no longer be controlled, according
| to e.g. https://space.stackexchange.com/a/59560. But it's
| exponential, at 600 km it takes 10 years, at 700 25 years, at
| 800 100 years, etc. Between 500-600 km seems to be ideal for
| things to naturally decay in case of issues.
|
| But also, it won't be a hard and fast "we are confined to the
| earth now"; the simplest model is a "the risk of being hit by
| debris is now x%", more advanced is "there are debris clouds
| in these altitudes / inclinations so best to avoid those at
| these times of day".
| vermilingua wrote:
| Given that the previous world police are presently treating
| international law as toilet paper, how do you propose
| global regulation of space would work or be enforced?
| wongarsu wrote:
| There are tentative signs that this is happening right now. As
| in: each collision causes debris that on average causes more
| than one additional collision, causing collision rates to go up
| exponentially.
|
| But so far it's not anything like in Hollywood movies, it's
| just a graph slowly going up. There are about 12000 satellites
| orbiting earth. That looks like a lot on a map, but 12000
| objects spread over an area larger than the surface of the
| earth isn't all that much
|
| Like all exponential processes it will become a major issue if
| we don't address it, but this is one that starts pretty slow
| and is well monitored
| spiritplumber wrote:
| Yep. That's the things about exponential curves, it's a graph
| slowly going up until it's no longer "slowly".
|
| https://www.thefrogdoctrine.com/p/the-29th-day
| indoordin0saur wrote:
| This can't go up forever. There is only so much mass up
| there in orbit, and much of it is in low earth orbit so
| will fall back into the atmosphere quickly as it's
| trajectory gets knocked off course.
| childintime wrote:
| > 12000 objects spread over an area larger than the surface
| of the earth isn't all that much
|
| People keep saying this, but the only way to assure there is
| no collision is to have non-intersecting orbits, but that is
| not going to work: not enough space.
|
| It's a tell that SpaceX is now lowering the orbits, even
| though their satellites mostly move in flocks that maintain a
| formation relative to each other: because the other ways are
| exhausted.
|
| Of course if they do cause a (low orbit) Kessler syndrom,
| then they don't have a business any more, and SpaceX will
| have achieved the opposite of its stated goals.
|
| The major reason to lower these orbits is likely the risk of
| a terrorist state turning these constellations into a weapon,
| by willingly causing the Kessler syndrome. SpaceX isn't going
| to tell you that, just as it doesn't tell you it's the USA's
| most important military asset.
| notahacker wrote:
| > The major reason to lower these orbits is likely the risk
| of a terrorist state turning these constellations into a
| weapon, by willingly causing the Kessler syndrome.
|
| Hard to see how the repositioning appreciably alters this
| risk, since there are still thousands of satellites in the
| original plane to get hit by shrapnel from intentionally
| caused collisions, and the satellites in the lower orbit
| aren't invulnerable to it either
|
| Suspect there's a rather more practical calculation that
| the extra thruster firings needed to main position in a
| lower orbit with more atmospheric drag are offset by the
| smaller number of conjunction avoidance manoeuvres they
| need to undertake in less congested space (the cost of
| lowering the orbit is simply deducted from their original
| delta-v budget for end of life deorbiting). In simple terms
| they get lower accidental collision risk without operations
| in the lower orbit shortening satellite lifetime.
| ben_w wrote:
| > Hard to see how the repositioning appreciably alters
| this risk, since there are still thousands of satellites
| in the same plane to get hit by shrapnel from
| intentionally caused collisions, and the satellites in
| the lower orbit aren't invulnerable to it either
|
| Yes, but the lower the orbit, the faster atmospheric drag
| (which isn't zero, just low) cleans up a cascade.
| childintime wrote:
| Exactly. And this is likely to be the only valid reason
| for the orbit change.
| notahacker wrote:
| Feel like I'm repeating myself here, but _they 're moving
| less than half of them_, which is going to have a
| negligible impact on a state with sufficient ASAT
| weapons' ability to create a massive mess with the many
| thousands of Starlink satellites operating in their
| original plane. Not even like the satellites in the
| lowest orbit are insulated from the effects of debris
| cascades set off in higher reaches of LEO either
|
| Plenty of operational reasons to want a large fraction of
| your constellation in a slightly lower orbit, none of
| them involve "terrorist states"
| tonyhart7 wrote:
| small price to pay for global internet
| ben_w wrote:
| When it happens, it no longer provides global internet.
| tonyhart7 wrote:
| interstellar internet ???
| ben_w wrote:
| If you smash up your router, your router does not
| magically get better, it simply fails to provide any
| internet.
|
| The same happens with orbiting routers, e.g. Starlink
| satellites.
| tonyhart7 wrote:
| and we would fix that shit, you acting like its
| impossible problem
| ben_w wrote:
| A Kessler cascade necessarily requires the density of
| shrapnel destroys basically everything in that orbit.
| Below the density where this happens, it isn't a Kessler
| cascade in the first place.
|
| You would be forced by (currently around) 2000 tons worth
| of bullet-mass shrapnel to wait for that shrapnel to de-
| orbit. Depending on the orbit, this takes months to
| millenia, because it's determined by atmospheric drag.
|
| The lower the orbit, the better. Starlink's orbits got
| lowered, this is better vs. that particular issue.
| inglor_cz wrote:
| I think the maths is counterintuitive here and that 10-20-40
| thousand objects, give or take, isn't that much. The volume of
| space around our planet is HUGE.
|
| Let us say that you had 10 thousand people running around on
| Earth, including all the oceans and Antarctica, and that
| collision of any two would release a hail of small deadly darts
| into the troposphere lasting, for, at 2 years or so. Which is
| approximately how long debris will last on LEO, though the
| actual values vary.
|
| You still wouldn't expect all those 10 thousand people to
| obliterate themselves like that, as the Earth's surface is
| pretty darn big.
|
| The volume of the LEO-relevant space is much bigger than the
| volume of the entire troposphere on Earth, because a) it is
| further away from the Earth's center than the troposphere, b)
| it is much deeper.
|
| Now, 10 million objects, that would be a different story. So
| would be some specific peculiar orbit which is overcrowded. But
| tens of thousands of objects spread all over the entire planet
| isn't that much. That would be like 2-5 people in total roaming
| the entire Czechia, how often would they come into contact? Not
| very often.
| NetMageSCW wrote:
| There is no chance at all of that happening, and especially not
| at the orbital height of Starlink.
| choeger wrote:
| I think it's important to note that not all collisions are
| equally dangerous. Consider a sat on a polar orbit colliding with
| one on a equatorial orbit. Or two satellites on different
| directions. _That_ is going to be spectacular. Otoh, these kind
| of collisions are unlikely and should be manageable by just
| assigning certain shells (say 5km) for every possible direction
| and orientation.
|
| If two Starlink satellites collide that go roughly in the same
| direction, it's not exactly a huge problem.
|
| I think the biggest issue is to coordinate this and potentially
| disallow some excentric orbits.
| bell-cot wrote:
| Not quite how it works, unfortunately.
|
| Once you've got even hundreds of satellites in non-equatorial
| orbits, trying to provide global coverage - their ground tracks
| very frequently cross each other. Even if they're all at the
| same orbital inclination. While those mostly won't be 90 degree
| crossings - the great majority will involve several km/s
| relative velocity. And you'd run out of (say) 5km LEO shells
| _very_ quickly.
| nelox wrote:
| What's the plan as the solar maximum returns?
| yanis_t wrote:
| Can anyone explain how does one technically lower a satellite?
| frumiousirc wrote:
| Eject mass in the forward direction of its current tangent of
| motion. Slow down to go down.
| pandemic_region wrote:
| So, for this they have a bit of expendable extra mass on
| board? What material is it, would it not cause even more
| debris then?
| alecco wrote:
| https://starlink.com/technology
|
| > Efficient argon thrusters enable Starlink satellites to
| orbit raise, maneuver in space, and deorbit at the end of
| their useful life. Starlink is the first argon propelled
| spacecraft ever flown in space.
|
| And you can see "How Ion Engines Work in Under 60 Seconds"
| https://www.youtube.com/shorts/_MUv28Yf_4g
| goku12 wrote:
| The 'expendable mass' is almost never a solid or liquid.
| It's the gaseous combustion exhaust or plasma exhaust from
| the satellite's thrusters. The advantage of gases is that
| they just expand and disperse fast enough to be too wispy
| to cause anything on impact.
|
| However, there are a few systems that do use solid masses
| for obtaining a reaction force. A remarkable example is
| called a 'Yo-yo despinner' [1]. It was used in missions
| like Phoenix (Mars mission) and Dawn (Asteroid belt proto-
| planet mission). And yes, it does create space debris. But
| those space debris are probably somewhere in orbit around
| the sun. Nothing that those guys are going to be too
| worried about.
|
| [1] https://en.wikipedia.org/wiki/Yo-yo_de-spin
| goku12 wrote:
| Let me see if I can. Before we go to space, let's try something
| on the ground. Imagine pitching a ball horizontally. What do
| you expect if you pitch it too slow? The ball will curve more
| towards the ground and meet it early, won't it? (In other
| words, it doesn't go very far and doesn't stay airborne for
| long). Going from ground to space, this action remains the
| same. You need to 'lower an orbit'? Reduce its forward
| velocity. It will curve more towards the planet and reach
| closer to the ground.
|
| However, there is a bit more detail involved here. Why doesn't
| the satellite just fall to the Earth? (Please excuse me and
| disregard this part if you know this already. I'm trying to
| maintain conceptual continuity.) So, when something is flying
| horizontally (no aerodynamic forces), we know that its
| trajectory will curve towards the Earth due to the pull of
| gravity. If the ground (on Earth) curves as fast as, or even
| faster than the trajectory's curve, the object will never get
| an opportunity to even reach the ground. This is 'orbiting'.
|
| Now assume that the satellite is initially in a circular orbit.
| The gravitational force acting on the satellite at any point in
| the orbit is _perpendicular to the satellite 's velocity
| vector_ and tangential to the orbit. The satellite will
| maintain a constant speed at this point, since its velocity and
| the force are always perpendicular [1]. So, what happens when
| we reduce the satellite's forward velocity? Just as we've seen
| with the ball, the satellite's trajectory (orbit) starts to
| curve more towards Earth. Now a subtle, but important change
| occurs. The velocity and the gravitational pull are no longer
| perpendicular! They start to align! And when that happens, the
| speed MUST increase. So, the satellite is now _losing altitude_
| and _speeding up_ simultaneously [2]. At some point, the
| satellite will pick up enough speed again to 'straighten its
| curve' and avoid falling to the ground. In effect, the
| satellite had to compensate for the lost velocity in order to
| remain in orbit, and it did so by exchanging some of its
| altitude (gravitational potential energy) for velocity (kinetic
| energy) [3].
|
| So our satellite 'fell' from where we slowed it down, until it
| had enough velocity again to maintain orbit. At that point, the
| gravity and the velocity are parallel again, since it will keep
| falling otherwise [4]. But since it 'fell from a higher
| altitude', it's speed is now too high for it to remain at that
| altitude. The orbital curvature is a bit 'too straight' now and
| it starts to curve away from Earth. So now we're in the exact
| opposite situation of what was explained in the last paragraph.
| The satellite is now climbing back up again! As it happens, the
| satellite actually climbs back up to the point where we slowed
| it down! And when at that point, its velocity is exactly the
| same as what it was, after we had slowed it down! [5] So the
| satellite did the inverse of what it did earlier - it exchanged
| kinetic energy to get back its altitude (potential energy). The
| satellite is now living in cycles juggling kinetic energy and
| potential energy back and forth. The final effect is that the
| point in orbit that's diametrically opposite to where you
| slowed it down, is now at a lower altitude. And thus you've
| effectively 'reduced the orbit'!
|
| One more detail to pin down. How do we slow down a satellite in
| the first place? Easy! Push the satellite in the opposite
| direction of its velocity [6]. This is called 'retrograde
| thrusting' or 'retro burn'. But that's about as easy as it
| gets. Remember that unlike on Earth, you don't have a surface
| (a wall or the ground) to lean against. Imagine pushing
| something heavy on an ice rink. The good news is that you can
| still push things on an ice rink. The only catch is that the
| push force will set both the item and you in motion in opposite
| directions [7]. And that's exactly what we do in space. We
| throw out mass from the satellite in the form of super-fast
| gaseous of plasma exhaust. The key is to throw out the mass
| with as much momentum as possible. But the mass is limited by
| how much you can carry - it's a depleting resource. So you're
| basically left figuring out how to throw it out with ever
| increasing speeds. And that's how we slow down the satellite in
| space - fire your thrusters!
|
| And finally to lower an orbit entirely, instead of just one
| point on it, you have to do multiple firings. There are bunch
| of these 'orbital maneuvers'. The most common one is the
| Hohmann Transfer [8]. If you could understand what's given
| above, most orbital maneuvers including Hohmann Transfer will
| feel very intuitive to you.
|
| [1] Speed is the magnitude of velocity and it remains steady in
| a circular orbit. However, the perpendicular force will keep
| bending the velocity vector, thus constantly changing its
| direction.
|
| [2] This is the from-the-first-principles explanation of
| conservation of angular momentum. This is how the ballerina
| spins faster by pulling in her arms.
|
| [3] If this sounds like a 'negative feedback' phenomenon to
| you, that's because it is. Feedback is a mathematical
| construct. Nobody ever said that a feedback mechanism must be
| implemented separately. Some systems have them inherently
| built-in.
|
| [4] This is the lowest point of the orbit - the periapsis.
|
| [5] Yes. There is quite a bit of hand waving here. I didn't
| explain why the satellite went back to its original position
| with the exact same speed. But that's what actually happens. It
| might take a lot more 'mathematical sense' to explain just
| using words. One thing I know is that this has something to do
| with the fact that the gravitational field is one of those
| 'conservative fields'. If you take a trip inside a conservative
| field, and return to the location where you started, you will
| be left with the exact same (kinetic) energy as you started
| with. You may exchange your energy during the trip, but you
| always regain it back when you get back to the starting point,
| no matter what path you took. As far as I understand, the
| 'conservative' part refers to the part that the energy is
| conserved and stored, and never lost. Unfortunately, the force
| field that we're most familiar with - frictional force - isn't
| conservative at all. If you're going on a trip, be ready to
| spend some energy!
|
| [6] One matter that confuses a lot of people is why the
| satellite's position changed at the opposite side of the orbit,
| instead of the point where we applied the force. The answer is
| in the Newton's second law. Force changes momentum, not
| position - at least not directly. The direct effect of
| application of retro thrust is that the velocity reduces at
| that point. The change of position on the other side of the
| orbit is only a consequence of that velocity change.
|
| [7] Yes, the Newton's vengeance law.
|
| [8] https://en.wikipedia.org/wiki/Hohmann_transfer_orbit
|
| [9] Every so often, someone comes along and argues that gravity
| is not a real force and all these explanations are wrong. If
| you want to deal with this in terms of relativity and space
| time curvature, be my guest. But for all practical purposes,
| the old faithful Newtonian physics works just fine, even as a
| special case of relativity.
|
| [10] This should probably have been a blog post. Please don't
| shout at me if it annoys you. This is one of my favorite
| subjects and I just got carried away. I used to teach and train
| many students and junior professionals in these topics.
| ChrisArchitect wrote:
| 3 week old news OP?
|
| Previously: https://news.ycombinator.com/item?id=46457454
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