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Comment by fred_is_fred | Hacker News Reader

If you had a thousand (or million) satellites doing this, could you actually measurably modify the orbits of planets? Mercury is small enough that presumably 1000 satellites each doing 9 gravity assists could change it in some way?

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Yes, you can do this with Mercury. Mercury's orbit is chaotic, so even modifications in its position on the order of millimeters can build up to produce completely different long-term dynamics. In particular, there's a few percent probability that Mercury will get ejected from the Solar System before the Sun dies. [1] It's also possible that Mercury will collide with Venus.

[1]: https://www.nature.com/articles/nature08096

Isn't space exploration then a bit like playing with fire? Like, we could mess up some delicate balance by attempting too many fly-bys

Chaotic systems like mercury orbiting the sun have zone of stability where all the potential orbits lay. The energy from a space probe doing a flyby won't impart enough energy to pop it out of that zone. I'm not going to do the math but probably other planets have a larger effect on Mercuries orbit than a space probe does.

No. Everything gets hit by big rocks all the time and are subject to all sorts of perturbations which dwarf anything a little spacecraft can do to a planet.

How would you ever know? It would be like identifying a butterfly that caused a given hurricane.

Especially given that there are much bigger things than butterflies dashing around everywhere.

Cycling a large asteroid between Earth and Jupiter could be used to slowly shift the orbit of Earth, most of the energy coming from Jupiter. Astronomical engineering: a strategy for modifying planetary orbits [https://arxiv.org/abs/astro-ph/0102126]

There's a congressman that would like to have a converstation with you about this to save Earth from climate change.

We could solve climate change in barely a billion years this way. Genius.

That's a great paper, thanks for the link!

This is literally impossible to tell without more information.

According to Wikipedia Mercury weighs about 3.285 × 10^23 kg, about 10^20 times more than most satellites. You'd need much more than 1000 satellites (or have much heavier satellites) to significantly impact the orbit. OTOH, if you'd have sensitive enough instruments you could measure the effect of even a single gravity assist.

Mercury's mass is 3.285 × 10^23 kg, so if we assume each satellite is 328.5 kg and there are 1,000 of them, then that's 1/10^18 of Mercury's mass.

So, if each satellite changes speed by 10 km/s, then Mercury's orbital speed will change by 10^-14 m/s, or about 0.03mm/century.

Doesn't seem like we'll notice it.

Because comets and asteroids have been zooming around for billions of years, it's clear that the effect would be negligible.

Not negligible at all. Comets and asteroids do not target to move a planet in some other direction, but a swarm of satellites can do it in a coordinated way. Of course you will need millions of satellites performing such dives over million of years.

But if your goal is to move the planet farther from the sun because it becomes more luminous and threatens to scorch the earth and you need to move away to cancel it, it is probably doable.

Let's take Voyager as an example. It weighs under 1000kg. 1 million Voyagers then weighs 1x10*9 kg.

A 1km radius rock asteroid, which is nowhere close to the maximum, weighs an order of magnitude more than this.

Solar sails give you essentially free power to fly around the solar system. So it is just a matter of time. With millions of years (and sun burning out is not exactly a fast process), this is quite doable.

There's kind of an XKCD What If for this, though with Jupiter: https://what-if.xkcd.com/146/

Summary is no for Jupiter, even the entire planet Earth wouldn't affect it that much. Mercury is much smaller though, so it might be possible to make a significant change, though you're probably still talking about sending a significant fraction of the mass of the whole planet Earth.

You might be able to alter the orbit like that without destroying the Earth if you figure out how to gather a ton of mass already in space, asteroids or something maybe. But now you've got to figure out how to redirect their orbit into the exactly right one without throwing most of their mass around, and do it with a reasonably-sized mission from Earth.

Of course by slowing down the speed of the Mercury it would affect is ability to be stable and it might end up be sucked into the Sun :-(

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