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Comment by TeMPOraL | Hacker News Reader

Rule 1 of orbital mechanics: if you're about to dock with something, you're already on the same trajectory as that thing.

Might make a good radiation shield, though.

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Except this is not an orbit around any body. If you can do what the parent proposes, you can add the chemical energy of fusing the hydrogen to your kinetic energy and defeat the rocket equation

That's what my comment's GP proposed. I understand my comment's parent to be asking about just attaching yourself to an asteroid.

You could always impact/land violently with it without having to match its trajectory. If your instruments are designed to survive the impact , it could work.

You can, but you only save as much velocity as the difference during the impact. Can't think of the case where that is, at the same time, both a useful saving and not fast enough to melt your equipment from a high-velocity impact.

Well is it not cheaper(by payload mass) to have higher protection to your equipment than more fuel ? Any benefit is useful , and this could be substantial depending on the strength of materials used

I imagine that, mass for mass, it isn't (of the extra fuel you carry, each bit you expend means the rest has that much less mass to push). Also, I can't think of means of protection that would withstand a high velocity collision with space gravel, that wouldn't be at the same time prohibitively heavy.

I'm not into orbital mechanics at all, just an interested layman, but how does a space tug work in relation to your 'Rule 1 of orbital mechanics'?. What is the point of a space tug then?

One of the purposes of a proposed 'space tug' is to use slow low thrust but high delta-v, high efficiency engines to do things such as:

Raise the orbit of satellites in LEO which are affected by atmospheric drag. Or re-boost things like the international space station, proposed future Chinese space station, etc.

Extend the lifetime of geostationary satellites which are still electrically functional, but out of station keeping propellant. One such thing docked for the very first time with a satellite earlier this year. https://en.wikipedia.org/wiki/Mission_Extension_Vehicle

On unmanned missions, slowly move cargo from low earth orbit to destinations at the Moon or Mars. If you can use ion and hall effect type thrusters for your missions to move cargo around, you can establish a logistics supply chain for essential supplies consisting of unmanned craft.

https://en.wikipedia.org/wiki/Delta-v_budget

https://en.wikipedia.org/wiki/Specific_impulse

An interesting example of using ion engines to maintain low earth orbit, through long continual thrust was this mission:

https://en.wikipedia.org/wiki/Gravity_Field_and_Steady-State...

I try to follow interesting space missions, but I've never heard about either of the two you linked to here. Thank you!

Others have provided excellent answers explaining both regular tugs and gravity tractors. So let me add a justification of my "1st rule".

It boils down to the fact that in space you can, with a very good degree of approximation, determine the trajectory of an object not under thrust, knowing only these two things: it's position at a given time, and it's velocity at that same time. These two vectors give you a single trajectory through space.

It follows from two physical laws that you may remember from high school:

- Newton's second law, or F=ma

- Newton's law of universal gravitation, or F=GMm/r^2

If you put them against each other, you get ma=GMm/r^2, or consequently a=GM/r^2. The acceleration (and thus, future position and velocity) of an inert object in space is independent of its mass. So all you need to tell where it will go is to know where it is, and what is it's current speed and direction of movement. And, of course, what other bodies influence it with their gravity.

Of course, in practice, there are other considerations in which the mass may become relevant. For instance, solar radiation acts on the surface, impacting force that's scaled by object's mass. The same is true for collisions with various stray atoms, especially prevalent near planets with atmospheres.

But discounting above factors (and, again, you can go pretty far just ignoring them), if you have two objects very close to each other and not moving relative to each other, they'll just go together along the same path for a long, long time.

The space tug is there to change the trajectory of the other thing:

1. Tug gets in same position and velocity as its target, and docks.

2. Tug performs some kinda of burn to put both it and its target on the new trajectory.

3. Tug undocks and performs a burn to head off to where ever else it's needed.

If I understand correctly, it's really just a way to avoid putting engines and fuel tanks on things that don't need to change trajectory much.

The space tug and its load / target are on the same trajectory at the time of rendezvous. Once they're synced up, they're drawn toward each other, with the tiny gravity of each affecting the trajectory of the other. But while the load is passively traveling through space, the tug can reposition itself. This way, the tug determines the ultimate trajectory of the load while also having some control of its own trajectory, which it uses to continually bend the load's trajectory.

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