This drag sail could prevent spacecraft from turning into long-term orbiting junk. We spoke to its inventors ahead of launch Space-flight researchers are ready to test a prototype drag sail that could one day be used to prevent spacecraft turning into hazardous junk stuck for years in Earth's orbit. Here's the gist: academics at Purdue University in the US have built a device they called Spinnaker3 that will be attached to a rocket developed by …
Just burning propellant won't cut it.
The propellant must be burned while maintaining correct orientation and at the right time for helping the upper stage de-orbit.
Given that a used upper stage is basically a bunch of slightly torched, empty tanks, valves and pipes, the stage would need additonal thrusters for menneuvering, orientation via gyros or star tracker and additional command systems/ energy storage, etc.
Besides raising complexity considerably, this pretty fast would eat up the 15kg budget without adding much or any fuel to do the actual de-orbit burn...
A 15kg fully passive drag-chute sounds good to me.
KISS... :)
“Space, is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.”
Increasing the size of the satellite with the size of the sail means a very small bit in terms of cross sectional area and drag, but is absolutely negligible when it comes to increasing the chances of it hitting something. The increased risk from the added area of this sail is FAR outweighed by the massive decrease in making sure the satellite spends much less time in orbit.
Still:
Assuming the artist rendition in the article is accurate, the sail increases the footprint of the satellite by about 3 or 4 times. 3-4x is still 3-4x, no matter how negligible the initial risk is.
The added drag is expected to reduce deorbit from 25 days to 15 days, a 40% reduction.
So how much is the risk of collision reduced over those 15 days given the higher surface, compared to the risk of collision with the original size over 25 days?
"Assuming the artist rendition in the article is accurate, the sail increases the footprint of the satellite by about 3 or 4 times. 3-4x is still 3-4x, no matter how negligible the initial risk is."
That's true, but if you are within 2-3 rocket body diameters of another object, you're already seriously of course!
Imagine you are running at thousands of miles per hour in a straight line on the surface of an Earth size planet that's all land and smooth as a billiard ball. Imagine there are others doing the same, everyone running in their own direction at their own speed. How many people do you think would need to be doing this before you had even a 0.0001% (1 in a million) chance of colliding with one in a year spent doing this? How much greater would that chance of collision be if everyone was carrying a 3 meter square piece of cardboard with them?
In space it is much less likely to hit something because not only is the "surface" of that sphere larger due to the orbit being above Earth, but the "people running around" are at different altitudes making it even less likely they will collide even if their 2D paths cross.
Argument is more complicated than this. If you show that chance of collision is very low then you also must show why it is worth deorbiting spacecraft at all. What you actually must show is that chance of collision from much larger (with sail) object which is there for much shorter time falls.
If the drag sail doubled the chance of collision, for the sake of argument, it is worth it so long as it reduces the time it takes to deorbit by more than half.
It isn't a problem today, but if we keep increasing the number of satellites by an order of magnitude every decade or two as they become smaller and launches become cheaper, eventually it will be. It seems prudent to find a solution now than waiting until it becomes a real concern.
"Earth size planet that's all land and smooth as a billiard ball"
Incidentally, while there is some sort of urban legend that teh Earth is actually smoother than a billiard ball (false, particularly when compared to a new ball), it IS actually almost as smooth as a hard-worn billiard ball.
https://possiblywrong.wordpress.com/2011/01/03/is-the-earth-like-a-billiard-ball-or-not/
My thoughts on it is that this is just a test, and there might be a much larger reduction in orbit time for other vehicles later. And should one be so unfortunate as to hit the sail it would not be as catastrophic as other space collisions. A larger piece of debris such a piece of pipe would shatter a satellite, but just tear a hole through the sail.
how many satellites are actually operating close enough to earth? thought a good lot is stuck in geo-stationary orbit, which is too far out for space chutes.
nvmd i looked it up.
W> Approximately 63% of operational satellites are in low Earth orbit, 6% are in medium-Earth orbit, 29% are in geostationary orbit [..]
"Also, 'metre' and 'fiber' is making this sentence difficult for me to look at."
Metre, the unit of measurement is correct across the world, except in the US, as opposed to meter, a device to show a measurement, which is spelled the same all over the world (excepting those places which may use a different, local language word for meter). Other meaning and uses of the meter also apply, but no other meaning is ascribed to the word Metre.
Likewise, fibre and fiber are the same, the entire English speaking word spells is as Fibre, apart from, you guessed it, the US, who spell it Fiber :-)
It may sound counter-intuitive, but hear us out: slowing down an orbiting spacecraft decreases the time taken for it to deorbit and burn up.
Why would that sound counter-intuitive? Does anyone think that you speed up a spacecraft in order to land, or that all that atmospheric friction, ablative heat shielding and so on just prolongs re-entry?
Well having played Kerbal Space Program for the last couple of years, I discovered that the whole orbital dynamics thing is counterintuitive. If you slow down a spacecraft, it just puts it into a higher orbit (so, further away from the Earth). If you speed up the spacecraft then it reduces the orbit (thus bringing it closer to the Earth). This is what makes rendezvous between 2 spacecraft in orbit so tricky, because things work in the opposite way to what you would expect.
Paul
If you want to catch up to another spacecraft in the same orbit, you slow down your spacecraft so that it moves to a lower orbit, which being shorter also means that you are orbiting the earth faster than the higher orbit (this is why low earth orbits are c.90 minutes compared to geosynchronous ones c.24 hours) so travelling in your lower, faster orbit, you catch up with the other spacecraft and at the right time, you speed up and raise your orbit to intercept the other spacecraft.
So how exactly does the host spacecraft command it to deploy, if the host spacecraft is inactive? Perhaps and automatic failsafe if it doesn't receive heartbeat from the spacecraft for more than some arbitrary period, but that carries its own risk of failure and prematurely deorbiting a working bird.
No good being deployed all the time since a bird would need constant low burn to remain on-orbit, or periodic burns to boost it back to desired orbit (more than already needed for LEO and MEO). Either way it'll use up the fuel a lot faster, requiring bigger tankage and more mass to stay orbiting for the desired lifetime.
Please give the engineers credit: they will find the way. This is only a test of a system after all.
As for slowing down in space using thrusters, simply rotate the craft 180 degrees from the direction of travel and accelerate. Simple physics.
I like to think that it will have its own, independent, radio etc,so main satellite can expire and this bit still be alive.
Then I also like to think that one day when many of these things are up there, Dr Evil (who will, of course be comedy Russian) will get access to private keys for command system to tell all these to deploy. Expect James Bond will then have to get involved in movie version. In real version all the satellites will just be deorbited, will be expensive, catastrophic, but boring.
"So how exactly does the host spacecraft command it to deploy, if the host spacecraft is inactive? Perhaps and automatic failsafe if it doesn't receive heartbeat from the spacecraft for more than some arbitrary period, but that carries its own risk of failure and prematurely deorbiting a working bird."
This initial iteration, the prototype, is designed to help speed up the return and burn up of used launch vehicles, so deployment at a set time or under set conditions very early in its life is the expected action. Later iterations for use on satellites will no doubt have other conditions which must be met before it deploys. Possibly a separate and independent receiver solely for receiving the deployment signal.
Conductive in magnetic field.
So they'll be doing a reverse spider* manoeuvre. Nice.
.
* Spiders fly by spinning electrically-charged silk which interacts with the earth's magnetic field. In case you've ever wondered why the silk doesn't just fall down plop on the ground.
First formally identified by Charles Darwin aboard the Beagle way out at sea: he noted spiders arriving from nowhere, then later taking off again and when they did, the strand stood out from the boat rather than in direct line with the wind. He suspected the cause was the well-known local deviation ships make in the local magnetic field, even wooden ones, and that the silk was charged. He turned out to be right.
This. Electrodynamic tethers are multi-purpose miracles for satellites in any Earth orbit, needing only to be made longer as the orbit gets higher.
1. Dumb-simple mode: Provide a closed loop with some resistance to dissipate magnetically induced electrical energy as heat. Some will point to the problem of making the plasma connection for the return circuit, which can be avoided by using dual tethers in opposition. This passively ensures deorbit. You could attach it to a rock.
2. Emergency power mode: When a satellite becomes unable to properly point it's solar cells, the loss of power would trigger tether deployment/activation, which can provide adequate power to permit a fully-controlled deorbit maneuver.
3. Active Propulsion Mode: An electrodynamic tether can directly be used as a thruster against the ambient magnetic field by pumping power into it, and/or as a power source for an ion thruster. Meaning not only can a desired orbit be maintained, but the disposal options also increase, including moving expired satellites to parking orbits.
There are other factors and benefits, chief among them being: Is the tether spinning or dangling?
Satellites needing to be Earth-oriented often use a "gravity gradient boom" for coarse pointing, augmented by thrusters and torquers for fine pointing. Such a boom can be designed to also be an electrodynamic tether, where it's default mode would be an open-circuit, so as to avoid degrading the orbit.
Tether mass is also a factor, with the weight changing from ounces to tons for electrodynamic tethers, depending on the magnetic field flux at the desired orbit. However, electrodynamic tethers are not the only option: Tethers can be useful beyond LEO when their mechanical properties are emphasized.
A spinning tether can grasp an object in a higher orbit and throw it to a lower orbit (steal velocity from it) while raising its own orbit and/or increasing its spin. This is an extremely efficient way to actively deorbit dead junk in LEO. However, if the tether has its own power source and is also electrodynamic, it become a self-sustaining means to toss payloads to higher orbits, even to the Moon. If there is roughly symmetric bi-directional traffic, the tether's onboard propulsion need only provide enough thrust to control its orbit, with the inbound payloads spinning up the tether for the outbound payloads.
Such "flinger" tethers can move payloads back and forth between lunar orbit and the surface, with zero velocity at the place and moment of lunar contact. In essence, such a tether would be in lunar orbit, and "walk" along the surface, where each point of contact can be a pickup/drop-off location.
Sure, tethers can be used to deorbit satellites. But that's literally just half the story.
Slightly off-topic but I've always been curious about what happens to these satellites when they "burn up"? Where do they go? As in - stuff doesn't just burn away to nothing, but usually leaves some residue or ash and often exhausts toxic gases into the air. A few little cooking fires a few centuries ago didn't make much of a mark on the planet's atmosphere but after the industrial revolution so much pollution was dumping into the air that Beijing 2008 almost wasn't visible. So at the moment there are relatively few old satellites "burning up" but when the space revolution comes what's going to happen to all the metal and other exotic materials "burning up" as the latest Starlink fleet starts approaching its end-of-life?
“At lower altitudes, deorbit could occur in days, while at higher altitudes it could take tens to hundreds of years”
I suspect that geostationary satellites, out at 36,000km, would stay up an awful lot longer than that. At that kind of altitude, space is an extremely hard vacuum.
Typically, geostationary and geosynchronous satellites are moved to a "graveyard orbit", several hundred kilometers further out. Although there are plans now to require decommissioning used-up sats means moving them to a "disposal orbit" from which point they further slow down due to atmospheric drag, within 1 year.
Yes, both methods use retro thrust.
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