# ESA's Solar Orbiter sails safely past Earth despite orbiting debris concerns

ESA's Solar Orbiter has completed its flyby of Earth, collecting science data as it did so, and appears to be headed back into deep space without any close encounters with orbiting debris. The flyby took place on 26 and 27 November, with the spacecraft whizzing past the planet at 12km/s at an expected altitude of around 450km …

1. #### ESA 1, Russia 0

That little orbiter crossed my mind (so, a close call for mental debris) and this is good news.

Lucky that there's as much room to wiggle, what with such slovenly Russian space activity.

The only thing that could make this any better is the same debris completely disabling any future Russian space station.

(Resulting in a timely and successful rescue of any personnel onboard-- no need to be shitty about it.)

2. #### 10,000ths of a radian per second

I remember being enormously disappointed when I found out just how feeble standard ion thrusters are. They're much more exciting on Star Trek etc.

1. #### Re: 10,000ths of a radian per second

In space, it's the efficiency that counts. You'll typically have very long periods over which that thrust need be applied. For instance, if you could constantly accelerate something at 1G (around 100N), it would take less than a year to reach the speed of light (relativistic effects aside). With this in mind, the typical thrust of an ion thruster (0.5N apparently) isn't all that bad.

1. #### Re: 10,000ths of a radian per second

The force needed to accelerate something at 1G is dependent on something's mass. The Solar Orbiter's mass is 1,800 kg, so acceleration at 1G would require close to 18,000 N, not 100. Acceleration at .5N thrust would be less than 0.0003 m/s^2, so about 0.00003 G. It is a very tiny thrust.

1. #### Re: 10,000ths of a radian per second

That's the launch mass, including propellant. ESA's "fact sheet" says the "science payload" is 209kg. I'm assuming that is the actual orbiter, minus the launch vehicle, but I could be misreading it.

My point was that 1G is more than you might think, and there is no need to accelerate anything at that kind of thrust for any amount of time, as you can more efficiently use a much lower thrust for longer if you have plenty of time to do so. One of the characteristics of space is that things are a long away apart (cataclysmic events aside), so you do have plenty of time to do that acceleration.

2. #### Re: 10,000ths of a radian per second

*accelerating something with a mass of around 100kg at 1G is around 100N. Obviously the mass affects how much thrust to accelerate...

Apparently the solar orbiter is 209kg. From what I can find out, it has six ion thrusters. Assuming they are on different axes, and only one can produce thrust at a time, and using F=ma, then the acceleration this produces is about 0.0024 m/s2, it would take the orbiter something like 4,000 years to reach the speed of light using that thrust alone. To be fair though, I don't think they want it going that fast.

1. #### Re: 10,000ths of a radian per second

To be fair though, I don't think they want it going that fast.

Could be handy, if it spotted a funny looking photon, and wanted to take a second look....

3. #### Re: 10,000ths of a radian per second

Although in this case, to take full advantage of the Oberth effect, ideally you'd want to get all your delta-V change in while you were at closest approach.

2. #### Re: 10,000ths of a radian per second

It is a generally inconsidered practicality that if I want to stick a person in a space-tin can and send them somewhere fast that G forces are a big factor. To get anywhere near the speed of light takes a long long time without turning passengers into a layer of goo on the back wall of the ship. I also have to consider slowing the thing down when I'm halfway to the destination. Space is big, you won't believe how vastly mind-bogglingly big it is ;-) Even at the speed of light we are talking years of travel to get to the next interstellar chemist. I guess that is why no little green men have shown up yet (AFAIK), that or we are as boring as **** in the scheme of things.

1. #### Re: 10,000ths of a radian per second

At the speed of light it takes you no time at all to get anywhere. If you can sustain 1g (which you can't, anywhere near, but let's pretend we have Bussard ramjet or something: you need a jet not a rocket to get the specific impulse you would need) then you can get to Proxima Centauri in 3.6 years, to centre of galaxy in 20 years, to Andromeda in 28 years, stopping there when you arrive. All times as experienced by person in rocket: times for person watching rocket somewhat longer (somewhat...).

3. #### Re: 10,000ths of a radian per second

This is because nobody really cares about the thrust: you can make more thrust by making more engines: if you have an engine which produces a thrust T and you want nT, you need n engines, simple, just buy them at your nearest engine store.

If on the other hand you have an engine which has an exhaust velocity E, and you want to change velocity by a certain amount, V, then the amount of your vehicle which must be fuel is the exponential of V/E. If you want V to be large then you very, very much want E to be large too.

As example if you want V=100km/s and for your final spacecraft to have a mass of a tonne, then with a chemical rocket with E=4km/s, you must carry about 72 billion tonnes of fuel. If you use an ion drive with E=40km/s ... you need about 11 tonnes of fuel.

11 tonnes is a lot less than 72 billion tonnes.

3. #### Imaged on its way back out

An amateur astronomer in the western US imaged the spacecraft shortly after perigee, at a distance of about 17000 km. The following is available even to F__ebook refuseniks (including me).

Of course, all you get is a streak moving among the stars. (Or, when the telescope is tracking the spacecraft's motion, a solid dot and the stars become moving streaks. This image takes the first approach.)

Should note that amateurs (and some professionals) get images of spacecraft leaving the earth/moon system fairly regularly. (Exceptions usually involve the object's departure route putting it close to the sun in the sky.) It's the only tracking data we have for the boosters, and can come in handy when the asteroid surveys are wondering if they've found a rock or just some old bit of space junk.

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