[HN Gopher] OneWeb, SpaceX satellites dodged a potential collisi...
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OneWeb, SpaceX satellites dodged a potential collision in orbit
Author : advpetc
Score : 53 points
Date : 2021-04-11 15:08 UTC (7 hours ago)
(HTM) web link (www.theverge.com)
(TXT) w3m dump (www.theverge.com)
| intrepidhero wrote:
| Given that starlink is in LEO does that mean there is enough
| friction to deorbit fragments of a collision in a relatively
| short time frame? Does anyone know? Kessler syndrome is scary
| because it could render certain orbits unusable for a long time
| and starlink ups the risk by having a huge constellation but I've
| wondered how much of the risk is mitigated by being in LEO.
| wongarsu wrote:
| If a Starlink satellite fails the drag alone will deorbit it
| within five years or so. (this actually happens, they used to
| have 5% failure rate)
|
| Of course a collision event is chaotic. The debris is
| catapulted in all directions, changing the orbit. But because
| of how orbital mechanics works their new orbit will still pass
| through their old orbit, so every piece of debris will deorbit
| eventually (unless it impacts something else first).
|
| Being so low reduces the risk considerably, and even with a
| couple thousand satellites more we are talking about mostly
| empty space.
| nabla9 wrote:
| Not all of them.
|
| Practically all space debris collisions in LEO will be
| hypervelocity collisions (the kinetic energy of the impact is
| greater than the explosive energy released by the same mass of
| TNT)
|
| When collision happens in LEO, some fragments gain more
| velocity than the colliding satellites. They may end up in
| higher orbits where they can stay for a very long time.
| Gwypaas wrote:
| That is not correct. To actually end up in an orbit with both
| higher apoapsis and periapsis another collision would need to
| happen higher upp, which then would raise the periapsis.
| Otherwise it would simply be an elliptic orbit still passing
| through the altitude which the collision happened at.
|
| An elliptic orbit may slow the decay though, but contrast
| this with the larger surface to weight ratio of smaller
| pieces and it should not really matter.
| nabla9 wrote:
| How come?
|
| If the satellites collide close to 90 degree angle the
| horizontal velocity of some fragments can be more than 10
| km/s and the orbit increases.
| Gwypaas wrote:
| See the regular Hohmann transfer [0]. Adding Dv at one
| point causes the orbit to become elliptic and will still
| pass through the same altitude, the yellow line in the
| picture. To increase the lower altitude, also called the
| periapsis, you need to add Dv higher up, which is the
| opposing side of the orbit for 100% efficiency and
| through that you create the circular red line.
|
| [0]: https://upload.wikimedia.org/wikipedia/commons/thumb
| /d/df/Ho...
| nabla9 wrote:
| Ah. The _lowest point_ of the orbit is the previous
| orbit.
|
| The fragments can still have more energy and decay slower
| than the original satellite. Especially when they don't
| have drag generating solar panels anymore.
| modeless wrote:
| The perigee of an orbit originating from a collision
| cannot be higher than the collision point and is very
| likely to be lower. Apogee could be higher, but it won't
| be for most of the debris. And even if it is, the low
| perigee will still cause the orbit to decay. And as
| Gwypaas points out, the pieces will in general decay
| faster than the whole satellite would due to higher
| surface area to mass ratio.
| LeifCarrotson wrote:
| Orbits in LEO are nearly circular. By giving a fragment a
| single instantaneous change in velocity, you change that
| orbit to an ellipse, which could have a higher apogee,
| but will always return to the initial altitude.
|
| To turn that ellipse into a circle with a larger radius
| than the LEO orbit, you need an additional maneuver when
| it is at that larger radius.
| Anunayj wrote:
| this man plays KSP.
| [deleted]
| tmarthal wrote:
| > collision probability of 1.3 percent, with the two satellites
| coming as close as 190 feet (50m)
|
| LEO orbits have speeds around 7.8 km/s (rounding up to ~8000m/s
| for quick calculations) - this avoidance detection is saying that
| the two satellites both traveling at 8000m/s would be in the same
| 50m box at the same second.
|
| A quick calculation shows that the collision avoidance is
| operating at least the millisecond level to predict this
| collision (50m/(8000m/s) ~.005 seconds.
|
| One thing someone once mentioned to me is that space is big and
| things travel fast. It's hard to believe that the two satellites
| (most likely each <1m in diameter) came "close" to colliding,
| when a half second later they would be 8000 meters apart.
| mhh__ wrote:
| Starlink satellites are a bit bigger than that from what I can
| gather on google at least.
|
| > came "close" to colliding, when a half second later they
| would be 8000 meters apart.
|
| So is a bullet if it goes through you. Space is big but the
| orbits these satellites operate in aren't as big as space
| itself.
|
| Why does it matter if it's hard to believe anyway, they gave a
| figure of 1.3%, which is pretty low anyway, and they didn't do
| it on the back of a napkin.
| satyanash wrote:
| > they gave a figure of 1.3%, which is pretty low anyway,
|
| 1.3% chance of collision is _not_ low at all!
|
| Especially when you start considering the chaotic cascading
| effects that a collision can result in!
| cesarb wrote:
| > SpaceX's automated system for avoiding satellite collisions has
| sparked controversy, raising concerns from other satellite
| operators who say they have no way of knowing which way the
| system will move a Starlink satellite in the event of a close
| approach.
|
| For aircraft, we have TCAS RA, which tells each pilot in which
| direction they should move, with the knowledge that the other
| aircraft has been told to move in the opposite direction. It
| might be time to consider something like that for satellites.
| cptskippy wrote:
| SpaceX has an automated system, that can be overridden to
| coordinate with others, which is exactly what they did in this
| case. I don't see why that's bad? If they refused to override
| and coordinate maneuvers, that's one thing, but they didn't.
|
| Having a system that can maneuver itself in the absence of
| outside intervention seems like a good thing when time to
| respond is measured in milliseconds under certain
| circumstances.
|
| This seems like collision avoidance systems in cars, easily
| overridden but functioning when the driver isn't paying
| attention.
| 627467 wrote:
| I saw this earlier:
| https://twitter.com/ProfHughLewis/status/1380647894705930242...
| mylons wrote:
| "Currently, there's no national or global authority that would
| force satellite operators to take action on predicted
| collisions."
|
| Exciting.
| dylan604 wrote:
| "Space Force's urgent alerts sent OneWeb engineers scrambling
| to email SpaceX's Starlink team to coordinate maneuvers that
| would put the two satellites at safer distances from one
| another."
|
| This is exactly how it should work though.
| jagger27 wrote:
| These companies, NASA, the Space Force, and other space
| agencies need to have big scary red telephones on their desks
| for exactly this situation.
| Gwypaas wrote:
| Not really. Coordination takes resources, doesn't scale and
| increases the risk of human error. A predefined set of rules
| making encounters predictable are way better. Like we have
| done for ships [0] and aircraft [1]. Any time you have to
| actively communicate with the other party you have not made
| your intentions clear enough.
|
| I.e. a transferring satellite gives way to an operational
| which gives way to a disabled. (Of course it's more involved
| than this, including predictable avoidance maneuvers, but
| that's would be the basic gist of it.)
|
| [0] https://en.wikipedia.org/wiki/International_Regulations_f
| or_...
|
| [1] https://planefinder.net/about/visual-flight-rules-vfr-of-
| the...
| bryanlarsen wrote:
| AFAICT NASA and SpaceX already have such an agreement, and
| is basically that the NASA satellite will hold steady and
| the SpaceX one will maneuver.
| JoeDaDude wrote:
| I once met with Iridium [1] engineers not long after the
| collision of one of their satellites with a Russian weather
| satellite [2] in 2009. The meeting was for another purpose
| entirely, but since the collision was fresh news, it naturally
| came up in conversation. The Iridium folk told me that they
| would receive warnings from NORAD of imminent collisions,
| defined as two satellites coming with 500m. They were receiving
| warnings to the tune of 10 or 12 a day, so many that they
| basically started ignoring them.
|
| [1] https://www.iridium.com/ [2]
| https://en.wikipedia.org/wiki/2009_satellite_collision
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