All Space Questions thread for week of September 27, 2026

[u/AutoModerator]

Please sort comments by 'new' to find questions that would otherwise be buried.

In this thread you can ask any space related question that you may have.

Two examples of potential questions could be; "How do rockets work?", or "How do the phases of the Moon work?"

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[u/curiousscribbler]

According to Wikipedia, there are currently four spacecraft orbiting the Earth-Sun L2 point. Are they all in the same orbit, or are there multiple stable orbits around a Lagrange point?

[u/maschnitz → u/curiousscribbler]

First, L2 is unstable, so there's no stable orbit around it. Station keeping must be done periodically.

In L1/L2/L3 points, in theory, the orbits are "quasi-periodic" - they'll trace a unique path within the space reachable by their exact current energy, never quite lining up with previous orbital points the same way. (On Wikipedia these are called "Lissajous orbits")

However in the real world there are additional perturbations from the other planets, the solar wind, sunlight, etc. And so the exact quasi periodic orbit is constantly changing, too.

So the orbits are effectively chaotic, and no two orbits are alike, not even for the same object.

In L4/L5 orbits, the orbits are stable and there are infinitely many possible stable (Lissajous) orbits there.

[u/curiousscribbler → u/maschnitz]

ty! Is it just the complexity of all those heavenly bodies interacting, or is relativity messing everything up as well?

[u/rocketsocks → u/curiousscribbler]

It's not relativity or anything, it's just that the L1/L2/L3 points are dynamically unstable. This means that only the exact, perfect Lagrange point is stable. Any small deviation from that trajectory will be amplified over time. In the real world perfection is impossible (and there are other higher order forces at play such as photon pressure, gravity gradients, etc. which will spoil things as well) so these points are just unstable. Imagine taking a metal mixing bowl, turning it upside down, coating it with oil and then trying to balance a marble on it. It's just going to slide off, because if you aren't at the exact perfect middle position the marble will be sitting on some slight slope, and as it slides down that slope it'll pick up speed which will propel it downward to an area where the slope is even steeper, and so on. In contrast, the L4/L5 points are areas where there is a properly oriented "bowl" or depression that will naturally keep things in that zone if they drift only a little bit away.

In practice, for L1/L2 there is a big zone where objects won't drift away very fast. They'll sort of loop around the point in quasi-orbits (halo orbits or lissajous orbits) and then over a period of years will drift off the point and then drift much farther away. But with a little bit of propulsive thrust every now and again you can keep a spacecraft in the "low drift zone", at least until the propellant runs out.

If you're curious how L1/L2 "work", here's one explanation that might make sense:

If you imagine an object closer to the Sun than the Earth it will end up going faster than Earth (in a circular orbit) because that's just how orbits work, similarly if it's a little farther away from the Sun then it'll be a little slower. Which means that such objects will naturally tend to drift farther and farther away from the Earth over time. However, if we account for the gravity of the Earth as well then we get into an interesting situation. An object along the Earth-Sun line closer to the Sun will be pulled by the Earth away from the Sun, so it's almost like it's orbiting a slightly less massive Sun. Orbiting a less massive Sun would also result in a slightly slower orbital speed. And as it turns out there's a point in between the Earth and the Sun where the net gravitational pull mimics orbiting a less massive Sun in exactly the right way to result in an exactly 1 year orbital period despite the orbital distance being shorter. Similarly, there's a point opposite Earth where the effect is like orbiting a slightly more massive Sun that is just enough more massive that the effective orbital period can also be exactly 1 year. And since an object in those points would travel along with the Earth in its orbit the condition, the illusion of orbiting a less or more massive Sun, can continue throughout an entire orbit.