I What is the accuracy of parking spacecraft into Lagrange points?

[RobbyQ]

TL;DR Summary

What is the accuracy of parking craft into Lagrange points

I often wonder what the accuracy is when dealing with spacecraft and wondered how accurate a craft needs to be positioned into the Lagrange (L1,L2,L3) positions. Are we talking plus or minus 1km/100km/1000km etc. It's just that the system is so vast and what instruments are used to achieve a good L sweet spot?

 

[Vanadium 50]

There is a certain aspect of "how long is a piece of string" about this. How long do you need to be "parked"?

You can get an idea by looking at the orbits of Trojan asteroids. Obviously they have been there for a long time.

 

[pinball1970]

TL;DR Summary: What is the accuracy of parking craft into Lagrange points

I often wonder what the accuracy is when dealing with spacecraft and wondered how accurate a craft needs to be positioned into the Lagrange (L1,L2,L3) positions. Are we talking plus or minus 1km/100km/1000km etc. It's just that the system is so vast and what instruments are used to achieve a good L sweet spot?

You can have a look at this, a practical example.

https://webb.nasa.gov/content/about/orbit.html

 

[FactChecker]

TL;DR Summary: What is the accuracy of parking craft into Lagrange points

I often wonder what the accuracy is when dealing with spacecraft and wondered how accurate a craft needs to be positioned into the Lagrange (L1,L2,L3) positions. Are we talking plus or minus 1km/100km/1000km etc. It's just that the system is so vast and what instruments are used to achieve a good L sweet spot?

Those three Lagrange points are unstable, so there is a trade-off between staying close versus the fuel required to stay there. (L4 and L5 are stable). A big consideration is whether sunlight is needed for power. L2 is in the shadow of the Earth. JWST stays near, but orbiting the L2 Lagrange point so that its solar cells will get sunlight. The distance of JWST from the L2 point varies between 250,000 to 832,000 km. (see this). If you think that is a lot, remember that the Sun is over 100 million km away.

 

[RobbyQ]

Those three Lagrange points are unstable, so there is a trade-off between staying close versus the fuel required to stay there. (L4 and L5 are stable). A big consideration is whether sunlight is needed for power. L2 is in the shadow of the Earth. JWST stays near, but orbiting the L2 Lagrange point so that its solar cells will get sunlight. The distance of JWST from the L2 point varies between 250,000 to 832,000 km. (see this). If you think that is a lot, remember that the Sun is over 100 million km away.

Wow that's crazy. It actually orbits the L2

 

[FactChecker]

Wow that's crazy. It actually orbits the L2

Yes. It is roughly following the Earth's orbit but it is farther from the Sun than the Earth is. My understanding is that the combined gravity of the Sun and Earth (the component in the direction of the Sun) balance the centrifugal force of its orbit around the Sun so that part is in balance. Then it can essentially orbit the Earth at an offset distance around L2 (since there is a component of Earth gravity pointing toward L2). The orbit is a little unstable so it takes small corrections occasionally. There is a lot more to it that I am not knowledgeable enough to comment on.

 

[RobbyQ]

Yes. It is roughly following the Earth's orbit but it is farther from the Sun than the Earth is. My understanding is that the combined gravity of the Sun and Earth (the component in the direction of the Sun) balance the centrifugal force of its orbit around the Sun so that part is in balance. Then it can essentially orbit the Earth at an offset distance around L2 (since there is a component of Earth gravity pointing toward L2). The orbit is a little unstable so it takes small corrections occasionally. There is a lot more to it that I am not knowledgeable enough to comment on.

Thanks. This video I found very interesting too as it discusses the component forces leading up to the JWST orbit around L2. And even this is simplified.

 

[FactChecker]

Thanks. This video I found very interesting too as it discusses the component forces leading up to the JWST orbit around L2. And even this is simplified.

That is a great video! Thanks!

 

[RobbyQ]

That is a great video! Thanks!

I agree . Very informative. Also what's interesting (about 5mins into video) is that they keep it slightly Earth side of L2 so it has tendency to fall Earth wards and so the thrust burners to correct it back towards L2 are always Sun facing so as not to have to turn it around and get burnt up.

 

[pinball1970]

I agree . Very informative. Also what's interesting (about 5mins into video) is that they keep it slightly Earth side of L2 so it has tendency to fall Earth wards and so the thrust burners to correct it back towards L2 are always Sun facing so as not to have to turn it around and get burnt up.

Euclid joined L2 too last month.

https://en.m.wikipedia.org/wiki/Euclid_(spacecraft)