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I'm working on a SciFi story set roughly in 2100 that takes place both on the moon and Mars. One plot point hinges on terraforming Mars, the biggest stumbling block being its lack of water and atmosphere. Moving comets and icy asteroid bodies are old ideas, but I wanted to think big. Saturn's moon Hyperion is roughly 300km in diameter, composed of mostly ice, and is a great deal closer than anything found in the Kuiper Belt or Oort Cloud. It's perfect.

The problem of course is moving it to Mars. Most ships in use have bimodal Nuclear NTP/NEP engines, and we have already set foot on Titan. Fusion ships are on the drawing board, but the issue is neutrons. Neutron radiation is proving to be a difficult problem for manned vessels, hence the reason they are not (yet) in use. However, if we placed them on Hyperion, using its own ice as a fuel source, who cares if they spit out deadly radiation? :)

So my question is, how feasible is this? What problems can I expect? Can it even be done?

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    $\begingroup$ How, uh, controlled do you want the mars-hyperion impact to be? $\endgroup$
    – Ingolifs
    Sep 29, 2023 at 1:36
  • $\begingroup$ I'd like to avoid any lithobraking if possible. Having it break up in orbit so that the pieces would burn up on reentry would be ideal. Granted, difficult to pull off in Mars's thin atmosphere. $\endgroup$ Sep 29, 2023 at 2:10
  • $\begingroup$ @Hewholooksskyward another option would be to mine the ice on Europa (a moon of Jupiter) since it is closer and has a lot of ice. If your story is happening in the 22nd century, then skyhooks could help the transportation and save a lot of fuel. This video here is explains how skyhooks work. m.youtube.com/… $\endgroup$ Sep 29, 2023 at 4:29
  • $\begingroup$ "is a great deal closer than anything found in the Kuiper Belt or Oort Cloud": it may be closer, but has around double the delta-v cost of a Kuiper belt object. It would take longer for the ice to start arriving, but in the long run ice shipments from the Kuiper belt and beyond could easily dwarf those from the gas giant moons. And the first ice packets of a steady stream launched from Kuiper belt bodies might well start to arrive before you can maneuver all of Hyperion into Mars orbit. $\endgroup$ Sep 29, 2023 at 14:13
  • $\begingroup$ @TheRocketfan I'm not sure you'd even need a skyhook on Europa. With no air, 0.13 G, and a relatively flat surface, you should be able to lay a long linear accelerator track and just huck buckets full of ice into orbit directly from the surface. It may not be enough to get them out of Jupiter's gravity but a skyhook would have that issue also. $\endgroup$ Sep 29, 2023 at 21:15

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A very quick answer based on some quick back-of-the-envelope stuff.

The delta-V required for an object to escape Saturn from Hyperion's orbit is about 2000 m/s. To get an intersecting orbit with Mars would require a bit more, but I don't think it's that much more (Please correct me on this if I'm wrong). You could do some low energy transfer shenanigans to get it down further, but that would mean a potentially very long time getting the asteroid to Mars.

Getting it to stop in a timely way in Mars orbit would require quite a bit more delta-V.

A quick look at fusion rockets on wikipedia tells me that an exhaust velocity of between 100 km/s and 700 km/s might be theoretically possible. So let's go with those values for the calculation.

By the rocket equation, using a fusion rocket to accelerate to 2000 m/s requires somewhere between 2 and 0.2 % fuel mass fraction. I.e. it would take about 0.2 - 2% of the mass of Hyperion (so $10^{16}$ to $10^{17}$ kg) in fuel alone. That's about 1% of the mass of all water on earth.

This is a huge amount of mass. It's probably more deuterium than Hyperion has locked inside of it. For reference, deuterium makes up 0.01% of all the hydrogen in the water on Earth. Also consider that the thrust to weight ratio is going to absolutely tiny, which means the transfer will be more inefficient than what I stated.

So yeah, you probably already knew this, but moving Hyperion to Mars would be a multi-generational effort using insane quantities of fuel (that you would have to transport to Hyperion), even if you had really, really good fusion engines.

You're still better off mining near-mars Asteroids and bringing the water back that way.

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    $\begingroup$ The problem with your fusion engines is that you're trying to run them exclusively using the fusion fuel as propellant. As you found, this requires unreasonable amounts of deuterium for this purpose. However, you get far more thrust from the same fusion energy if you use some of the ice as propellant. Yes, this reduces specific impulse and increases the propellant mass fraction, but getting 90% of Hyperion to the destination is more useful than exhausting its deuterium before escaping Saturn orbit. However...you're right about there being many easier targets. $\endgroup$ Sep 29, 2023 at 3:31
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Yes it's possible, but there are some shattering implications - quite literally - that would make great plot elements. Let's see how Dr Verina von Braun will enable humankind to make its next great leap to becoming a starfaring species.

Let's assume she uses clever orbital mechanics to avoid brute force and "nudge" Hyperion into an unstable orbit that slingshots it via Saturn with just enough energy to get to a Mars transfer. This BTW is the solution to your SciFi problem - Hyperion (purple) has an chaotic orbit just outside of Titan's orbit (light blue) about Saturn (dark blue dot), in a 4:3 resonance lock with that large moon.

Chaotic orbit of Hyperion (purple) outside Titan (blue)

In principle a small delta-V is required, say a few m/s (no trivial effort given Hyperion's mass), to begin gravitational interactions with Titan, which after several orbits would start to involve Saturn. A few more orbits and you're on a Hohmann transfer orbit to Mars.

From Mars Transfer to the surface of Mars is where your real problem begins. MT to the surface of Mars is still a delta-V of 5.7 km/s, with 3.6 km/s from a 200 km orbit to the surface. No conceptual fusion rockets of 2100 could possible slow Hyperion into any orbit around Mars. The minimum delta-V is 673 m/s. (See @Ingolif's answer.) Before proposing a solution, let's consider the effect of a direct impact with Mars.

Hyperion has a mass of 5.5E18 kg. Hyperion would have a kinetic energy of nearly 1E27 Joules when it hits Mars' surface. It takes 300 kJ to melt a kg of granite, or 3E8 Joules to melt a tonne, which would be at 1500 Celsius when molten. You just melted about 3E17 tonnes of rock, or 1E17 m^3. With a surface area of about 150 million km^2 or 1.5E14 m^2, you've melted a layer of Mars surface about 700 metres thick.

No doubt you've created several new moons, but it will take millennia for Mars to cool.

Not ideal, so the protagonist in this story, Dr Verina von Braun, a brilliant rocket engineer, has devised a solution. Without slowing Hyperion, orbital insertion is impossible. Hyperion will just zoom past Mars into a solar orbit. Verina proposes crashing Hyperion into Phobos as a braking maneuver to achieve orbital insertion. Of course, asserts Verina, the two bodies will merge and form a highly elliptical orbit. Hyperion's foam-like structure is ideal for absorbing the impact of a smaller body. Verina insists there will be "no risk" to the tens-of-thousands of colonists on Mars, who, unsurprisingly, are desperate to stop Verina's madness. A major plot element!

Verina counters, that this is the only way to terraform Mars. It will be safe and she has done the calculations to prove it! Once in a (highly elliptical) Mars orbit the merged Hyprion-Phobos can be mined at leisure over hundreds of years to "gently" make Mars a habitable world. Verina; "I know that scientists occasionally do things that upset people, but there doesn't seem to be any alternative open to the human race if it is to survive."

Of course things go wrong. Verina humbly apologises, "I have very deep and sincere regret for the victims..."

So moving Hyperion to Mars with fusion rockets can be done "without much trouble" using fancy astrodynamics, but that's the least of your problems.

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