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# Send up a satellite to zap space junk if you want Earth's orbit to be clean, say boffins

A group of scientists have proposed a new method to clear up space junk using a satellite that shoots out powerful beams of plasma. Researchers from the Tohoku University, Japan, and The Australian National University, think that potential debris disasters might be averted by sending up a cleaning probe. The satellite works by …

## COMMENTS

1. #### Orbital cleaning

Here, I thought that has already been done (so to speak):

https://www.youtube.com/watch?v=lTSWdHY9Ny4

1. #### I'll see your Spaceballs

and raise you Quark. (No not that one.)

2. #### It all seems a bit far fetched, to me

I've three objections to this idea.

1. As far as I'm aware, applying a force to an obect in orbit, merely changes the orbit of that object. It does not "knock it out of orbit" A satellite launch comprises two burns, The first to get it to the right height, and the second to impart the required velocity that it has to have to orbit at that height.

2. Applying the force by a plasma jet has to apply that force exactly at the objects centre of mass, wherever that might be, or all it will impart is a spin. Given that the object is bound to be asymmetrical, finding the centre of mass might be a tad difficult.

3. The de-orbiter is essentially linked to the object, and without extra expenditure of fuel and mass will simply follow the object down to a matching fate

1. #### Re: It all seems a bit far fetched, to me

Indeed. The idea as I understand it is to slow the debris down so it falls into a lower orbit, more affected by the outer borders of atmosphere.

To do so you'd need to put your plasma throwing satellite on the same orbit as the debris, a couple of meters in front of it. Then you start blowing your plasma at the debris, and as a result it slows down, thus falling back and down into a lower orbit. So, to keep slowing it down your plasma throwing satellite will have to follow it for a short while, slowing down by the same amount, which means it will descend into lower orbits too. If you don't want it to go plunging into the atmosphere too, you'll need to use some kind of booster to make it climb back up so it can go search for some other debris to kill.

Not impossible, but sounds definitely fiddly and very fuel intensive.

1. #### Re: It all seems a bit far fetched, to me

"So, to keep slowing it down your plasma throwing satellite will have to follow it for a short while"

The article suggests the impulse being applied for less than half a minute. Given that the device uses a balancing plasma beam suggests that the designers don't see a need to follow the target over this period of time.

1. #### Re: Re: It all seems a bit far fetched, to me

The article states 1800 seconds, so half an hour, not half a minute.

Agree with a later commentator that the physics seems a bit off

2. #### Re: It all seems a bit far fetched, to me

> The article suggests the impulse being applied for less than half a minute

Well, there is some vagueness in the description, but I read "1,800 seconds" as being 30 minutes during which the cleaning satellite will have to blow plasma onto the victim. Which would mean the cleaning satellite will have to follow its prey for at least half an hour, through what (should be) a rapid deceleration and radical change of orbit. I might be wrong though.

3. #### Re: It all seems a bit far fetched, to me

"The article suggests the impulse being applied for less than half a minute."

Funny, I read that as 1800 seconds.

1. #### Re: 1800 seconds.

Performance =/= requirement.

1800 seconds for 5 months of decaying orbit. So I assume 900 seconds for 12 months or more, 60 seconds for 5 years or so (probably not a linear scale due to differences in atmospheric density etc, but you get the idea).

In fact, most debris *is* already on a decaying orbit... it's just not decaying fast enough!

Or as the posts below note: "Reading the original article (always advisable before calling the authors idiots) reveals that the figure of 1800 seconds is in fact the specific impulse of the thruster not any actual time."

So Stop being idiots, and read, listen and learn before spouting off how clever you are!

4. #### Re: It all seems a bit far fetched, to me

The article suggests the impulse being applied for less than half a minute. Given that the device uses a balancing plasma beam suggests that the designers don't see a need to follow the target over this period of time.

@Doctor Syntax - think about it, both cleaning satellite and junk will be moving at high speed. So for anything more than a very short burst/shot, the cleaning satellite would need to follow the junk, in fact given the aiming problem, it would need to predict the course of the junk and effectively lead the junk down.

Given the size of much of the junk, it might be better to drift net stuff - either having your net run slightly faster than the junk or marginally slower. Obviously, with a carefully chosen orbit the net will automatically take the collected junk down, to be burnt up in the atmosphere.

2. #### Re: It all seems a bit far fetched, to me

Actually, for low earth orbits, almost any change of velocity (increase or decrease) will cause the orbit to intersect the atmosphere.

I would suggest launching a retrograde orbit satellite with a big can of compressed gas and let it out. The drag of the gas cloud would deorbit any small objects until the gas cloud dispersed. One could use several puffs of gas for multiple objects. I haven't done the calculations though.

1. #### Re: It all seems a bit far fetched, to me

The contents of how many Olympic swimming pools would it take per satellite?

Although, to put a damper on the idea, there are a lot of scientists who'd be real pissed at you putting water vapour clouds _above_ the atmosphere and destroying their 'viewing' in infrared and millimillimeter wavelengths.

1. #### Re: It all seems a bit far fetched, to me

The contents of how many Olympic swimming pools would it take per satellite?

A number that would take up an area roughly the size of Wales.

1. #### Re: It all seems a bit far fetched, to me

The contents of how many Olympic swimming pools would it take per satellite?

A number that would take up an area roughly the size of Wales.

====

Couldn't we just launch Wales ? :oP

what ever strikes it isn't going to make that much impact, and may well improve the vista :o)

2. #### Re: It all seems a bit far fetched, to me

So, if they sent up a thingie to p*ss on the unwanted object, you're suggesting they'd p*ss off everybody?

Maybe they should launch a mini olympic sized swimming pool. People probably p*ss in them all the time.

Thanks, mine's the smelly waterproof wetsuit on the end peg

2. #### Re: It all seems a bit far fetched, to me

I would suggest a very large ball of Blu Tak, also launched in a retrograde orbit.

1. #### Blu-Tak

I think a giant slice of buttered toast would be better - everything will stick to that, then just de-orbot the toast..... And if it burns up on re-entry - it'll be toast...

1. #### Re: Blu-Tak

JPL scientists proposed the giant buttered toast technique in the late 1980s. Unfortunately the project was abandoned after further research at NASA showed that the butter would inevitably rotate towards the earth, instead of the direction of the debris.

2. #### Re: Blu-Tak

I think a giant slice of buttered toast would be better - everything will stick to that, then just de-orbot the toast..... And if it burns up on re-entry - it'll be toast...

Just ensure there are no cats within the old satellite, or the thing will just spin in situ for ever!

2. #### Re: It all seems a bit far fetched, to me

How about a ginormous ball of sticky ballistics gel equivalent going in polar orbit. that way the orbit won't decay so quickly. kinetic energy remains the same after colliding, and "live" satellites can be steered out of its path. Anything NOT live gets to be part of the thing. launch it at an altitude that collides with most of the space junk.

it's as good as any other idea, and doesn't require propulsion, just enough time to stick to everything [like using tape on a suit to get the cat hair off]

3. #### Re: It all seems a bit far fetched, to me

Similar thoughts crossed my mind.

For this to work as described would surely call for military-grade precision beaming. Could that be a clue as to anyone's motivation?

4. #### Re: It all seems a bit far fetched, to me

>1. As far as I'm aware, applying a force to an obect in orbit, merely changes the orbit of that object. It does not "knock it out of orbit" A satellite launch comprises two burns, The first to get it to the right height, and the second to impart the required velocity that it has to have to orbit at that height.

So, by your own words, taking velocity back out must surely preclude it from orbiting at that height.

Bear in mind that taking velocity out of a circular orbit (One which is equal in altitude at all points) does not result in a circular orbit, it lowers the point on the opposite side of the current position.

>3. The de-orbiter is essentially linked to the object, and without extra expenditure of fuel and mass will simply follow the object down to a matching fate

It is in no way linked to the object being de-orbited, how did you get that idea?

It imparts deceleration to the object, so it lowers one side of it's orbit until it intersects the atmo.

The craft doing the imparting has beams firing both ways, so it does not alter it's own orbit.

5. #### Re: It all seems a bit far fetched, to me

I've three objections to this idea.

1. As far as I'm aware, applying a force to an obect in orbit, merely changes the orbit of that object. It does not "knock it out of orbit" A satellite launch comprises two burns, The first to get it to the right height, and the second to impart the required velocity that it has to have to orbit at that height.

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Not exactly. The first burn is usually to establish the satellite in orbit, and if the first burn hasn't already done this, the second burn is to change the eccentricity of the orbit. The process you describe would be energetically inefficient.

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2. Applying the force by a plasma jet has to apply that force exactly at the objects centre of mass, wherever that might be, or all it will impart is a spin. Given that the object is bound to be asymmetrical, finding the centre of mass might be a tad difficult.

=============================================================

Not correct. Given that the plasma jet does not deliver all its force at a single point, it would be very difficult (probably impossible) to avoid a deorbiting vector in the acceleration of the target. Depending on the distribution of the force, and the mass, there may also be a rotational force, but you don't care about that. All you have to do is slow down the satellite enough that it will enter a region of increased atmospheric drag in some portion of its orbit... which will tend to become more eccentric, with a lower perigee.

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3. The de-orbiter is essentially linked to the object, and without extra expenditure of fuel and mass will simply follow the object down to a matching fate

=============================================================

I presume that is the reason for a high efficiency plasma thruster setup - the deorbit satellite will be maneuvering a lot, including slowing down and speeding up as it deals with targets and moves on. An interesting question is whether it can be refuelled by linking with a fuel tanker, which would substantially increase its operational life.

6. #### Re: It all seems a bit far fetched, to me

It's a shame these researchers are rocket scientists and not internet commentators otherwise they'd have spotted all these potential flaws in their plan ages ago.

3. #### Centrifugal force?

Let's apply Newtonian physics,

Newtons 1st law - The satellite is moving in a straight line with just gravity pulling it into orbit due to the altitude & velocity being nicely balanced.

Newtons 2nd law - The Plasma beam is an outside force changing the velocity.

Newtons 3rd law - gravity is also pulling earth towards the satellite.

Centrifugal force would sort of prevent all the floaty fun astronauts get up to.

Having said that, the basic principle of using balanced beams to hold satellite A in place while knocking satellite B out of orbit works, and satellite A has the ability to change it's own orbit at will, it could also be easily refuelled. A good name for this would be a hunter-killer satellite to get budget funding.

1. #### Re: Centrifugal force?

>Having said that, the basic principle of using balanced beams to hold satellite A in place while knocking satellite B out of orbit works

It works better by having the hunter-killer satellite in a slightly higher orbit than the junk as then the beam would be working with gravity to hasten the junk's descent.

4. #### Centrifugal force

Centrifugal force, space borne weaponry, hmmm.... I sense a white cat being stroked.

https://xkcd.com/123/

1. #### Re: Centrifugal force

There is an entirely new propulsion system that is based on the redirection of the reaction of centrifugal force. It is called a Centrifugal Propeller. It will be capable of launching from Earth and returning, 5 or 6 times in a single day. This will make the clean-up of near space, a simple matter. The Centrifugal Propeller will put all rocket and jet engine manufacturers out of business. It will continually accelerate in space so there will be gravity in the spacecraft while accelerating. At the halfway point of the journey you will simply turn the spacecraft around and re-engage the Centrifugal Propeller, and you will have gravity in your spacecraft again. It will mean a trip to Mars will be little more than a day-trip, and people will be leaving for day-shifts mining on the moon and return home for supper on the same day. There is a working proof-of-concept prototype.

1. #### Re: Centrifugal force

Centrifugal Force? .... an engineering phantom. There is no such thing. An object undergoing circular motion will be subject to a centripetal acceleration. The "force" outwards is a reaction to the circular motion.

[Takes off her pedant hat before she feels it necessary to refute the assertion "It surrounds us and penetrates us; it binds the universe together"]

1. #### Re: Centrifugal force

Centrifugal force is quite correct - although often misunderstood...

It is what happens when you solve newtonian mechanics in a rotating frame of reference.

If you solve in a static frame the centrifugal force won't appear, but you will have a pile of other things to consider (like very high lateral velocity), and the diagram becomes much more complex.

An orbit can be easily viewed in a rotating frame of reference (just assume the earth is a perfect sphere, so you don't mind it slipping backwards).

2. #### Re: Centrifugal force

"There is no such thing"

1. #### Re: Centrifugal force

"There is no such thing"

Wrong.

If it's just inertia viewed from a certain frame of reference, then it isn't a separate thing, is it? Why call it something different when it isn't?

1. #### Re: Centrifugal force

Why call a web page with writing on it an article, or a piece of paper with writing a letter?

Why have male and female instead of having "Humans with parts that go inside another, and humans with parts designed to take other humans parts inside them?"

Its the definition of a term to define a common situation, in this case an apparent force, which allows for succinct communication.

5. #### Song Title - Missed Headline

"Sweeping Satellite"

1. #### Re: Song Title - Missed Headline

Yes - as sung by Barry Kripke!

6. #### Enemy junk

Interesting that someone thinks they have a better chance of funding if they present the 1-tonne enemy satellite as "junk".

Somebody should tell them that the most dangerous space junk is the 99.99% - the millions of tiny and invisible pieces, not the handful of monsters you can see coming and avoid.

1. #### Re: Enemy junk

"Somebody should tell them that the most dangerous space junk is the 99.99% - the millions of tiny and invisible pieces, not the handful of monsters you can see coming and avoid."

It's both. A big dead satellite crashing into another object creates a storm of little bitty pieces. They tell two friends and they tell two friends until there is a fine mist of high velocity particles and no more satellite weather photos on the evening news.

Using high energy light is easy than trying to capture junk in orbit, but it's power intensive and it will take a lot of computing power to figure slow deceleration orbits so the clean up doesn't just cause more collisions. The power required might take something as large or larger than ISS to collect and store enough solar power to accomplish much cleaning.

7. #### Idiots

"The team has calculated that in order to send a one to two tonne object to reach the Earth’s atmosphere in about 80 to 150 days would require a thruster performance of 60 milliNewtons for 1,800 seconds."

Has it? Has it really? This team of boffins has calculated an impulse of 10J (0.006 x 1800) is enough to deorbit a two tonne satellite? A delta v of 5mm per second (divide the impulse by the mass) on its 9.4km/sec orbit speed will knock it out of orbit?

Am I the only person with an intuitive enough understanding of GCSE physics, that I didn't just glaze over when those numbers were mentioned, but actually had a mental 'this is bullshit' warning go off? You're all a bunch of fucking innumerate idiots. This is exactly like a newspaper breathlessly reporting that a farmer has grown a 10 trillion tonne turnip the size of Wales as truth, and not even bothering to clarify or have any 'these numbers aren't reasonable' warnings go off.

1. #### Re: Idiots

"A delta v of 5mm per second (divide the impulse by the mass) on its 9.4km/sec orbit speed will knock it out of orbit?"

Seems highly doubtful to me. But:

* How much delta-V is required to adjust a typical satellite in LEO into a more eccentric one that will naturally decay at a more acceptable rate due to increased aerobraking?

* How much delta-V is required to avoid an impending collision between two dead satellites in LEO? (overly simplistic question here, we also need to know how reliably and how far into the future we can predict the orbits of the two dead satellites, and how quickly we can get the "zapping" satellite launched and on station).

Those questions aside: if 10J isn't enough, then take the XKCD what-if approach and build a zapper that can do 100 J (repeat as necessary, or until the physics become silly).

2. #### Re: Idiots

"Am I the only person with an intuitive enough understanding of GCSE physics, that I didn't just glaze over when those numbers were mentioned, but actually had a mental 'this is bullshit' warning go off? "

You missed that all you need to do is get the orbit elliptical enough to start dragging in the upper atmosphere.

1. #### Re: Idiots

"You missed that all you need to do is get the orbit elliptical enough to start dragging in the upper atmosphere."

What, and you think a delta v of 5 cm/s, while it zips around at 9.4km/s is enough to do that?

I haven't missed anything, and I repeat my assertion. You're an idiot with zero intuition about what these numbers, what half an hour of a 60mN force on a two tonne mass moving at 9.4 km/sec actually *means* to it. Answer. Fuck all.

1. #### @Aquilus

47 posts since 2007.

Just got out of jail?

2. #### Re: Idiots

"You're an idiot with zero intuition about what these numbers, what half an hour of a 60mN force on a two tonne mass moving at 9.4 km/sec actually *means* to it. Answer. Fuck all."

Whatever makes you think they're going to be using this to being down 2 tonne objects?

A 2 tonne mass is easy to latch onto and bring down with other means. More to the point it's also easy to SEE, calculate trajectory on and AVOID.

The problematic shit is all 10cm or smaller and impossible to latch onto - which is where getting into the same orbit as the debris cloud, parking in front the leading edge and using a balanced ion thruster comes into its own - effectively firehosing the crud into the gutter (atmosphere)

Of course the laser broom solution is probably more practical in all cases, even with atmospheric blooming taken into account.

3. #### Re: Idiots

First, your numbers are out by an order of magnitude. 60mN is 0.06 not 0.006. (Innumerate?) So the product is 108.

Secondly, you don't get to energy by multiplying force and time. The equation you want is Newton's second law, F=Δp/Δt, which we can rewrite as Δp = FΔt to show that a force applied over a period of time gives a change in momentum. Dividing this figure by the mass will calculate the change in velocity, but because you were out by an order of magnitude, it's actually 54cm/s.

Now we can calculate the change in kinetic energy of the satellite: it will be ½m(v+Δv)² - ½mv² which is mvΔv + ½mΔv² . (This should look familiar as it's the Galilean equation for acceleration multiplied by mass. We could have gone that route directly using the "F=ma" form of the second law, but we've got a deltaV so we'll stick with this.)

We'll use your velocity of 9.4E3m/s. And as Δv is tiny we'll discard the Δv² squared term and call the change in energy mvΔv. Which is `2E3 * 9.4E3 * 0.054 = ~1MJ (~50kW)` This figure is a bit high because, I think, you've used the launch velocity not the orbital velocity. But it's the right ballpark. This energy is big because most of the transferred megajoule comes from the kinetic energy the plasma particles have from already being in orbit.

Anyway, detailed analysis suggest the impulse supplied will be enough to get an object in 1000km orbit down to 300km orbit, where the atmosphere will do the rest. There's an in-depth analysis here.

So, on the new numeric GCSE grading system, I reckon I'd give you -1.

1. #### Re: Idiots

@Brewster's Angle Grinder:

...

Blimey, you're clever!

Sorry, that was all just like rocket science to me....

2. #### Re: Idiots

@Brewster's Angle Grinder

I don't understand what you're doing with your energy calculation but 0.06 m/s is a tiny delta V. One article I looked at suggests a delta V of around 200m/s is required to put a satellite in geosynchronous orbit into a Hohmann transfer orbit that would intersect the atmosphere.

1. #### Re: Idiots

most satellites are NOT geosynchronous - that is the super high value real-estate/parking spot directly in front of the shops, that is roughly 3 times further out than most satellites bother with.

Sure it is high value and limited in quantity? - which would make it a prime candidate for clean up, but again, easy to disrupt.

Pushing something from geosynchronous will either drop it into a faster lower orbit (difficult), a slower higher orbit.. or a most likely an eccentric orbit which will eventually graze atmosphere and produce extremely expensive fireworks. Which is the desired outcome.

Lower orbits are easier to get to, but will decay more easily as well - and its always easier to push something in the direction it wants to go.

Something the article stated was that the process could take up to 150 days - almost half a year before atmospheric contact, They arent talking about something that will kick satellites out of the sky immediately.

Damn kids these days, always expecting instant gratification....

2. #### Re: Idiots

"One article I looked at suggests a delta V of around 200m/s is required to put a satellite in geosynchronous orbit into a Hohmann transfer orbit that would intersect the atmosphere."

99% of the junk is not at GEO, it's at LEO, with most of the remaining fraction at SEO

1. #### Re: Idiots

That may be technically true but objects in LEO require constant reboosting to stay in orbit. Spack Junk in LEO doesn't remain a problem for long. (Not that it matter since we're talking about something like a 50% difference in orbital and the error in my figure is greater than that.)

3. #### Re: Idiots

"Dividing this figure by the mass will calculate the change in velocity, but because you were out by an order of magnitude, it's actually 54cm/s."

108 / 2000 = 0.054 m/s = 5.4 cm/s

You're out by an order of magnitude.

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