# Is it feasible to put a comet into a Lagrangian point?

How feasible would it be to put a large comet, like the one that Philae landed on, into a Lagrangian point, or some other spot that would make it convenient & SAFE for mining, colonization, experiments, etc?

• I cannot see how a Lagrangian point would be safer than in orbit around the Sun. A Lagrangian point would tend to collect objects besides the one of interest, which are then more likely to crash into it. – Andrew Thompson Nov 13 '14 at 0:42
• The primary barrier to space mining is the cost, specifically the cost of transfer of bulk materials. So, bringing lots of dead weight with little resources inside, versus bringing mined resources near Earth would be terrible cost-wise. Not to mention delta-V for stopping at lagrangian point, versus aerobraking you could use to dump mined resources to Earth (...and the immense risk of the comet slipping from the lagrangian point and falling to Earth... talk about SAFE...) – SF. Nov 13 '14 at 10:17

I'm going to interpret your question as "is it feasible to change a large comet's orbit to one we can easily access?"

The short answer is no, we cannot with current technology. In order to move something as massive as Chury (100 Billion metric tons) in a controlled way you need a rocket that can produce a great deal of power, and you'd need an enormous amount of fuel to power it. Fuel is extremely expensive to lift to orbit, for every unit of fuel you put into orbit you expend 20 getting it there. Then you need to move all this engine and fuel out of orbit to meet the comet. That would be challenging to say the least, even if it is possible at all. There's simply be no benefit from that approach, even if the entire comet was made of valuable materials.

Realistically it is possible to nudge a comet off its orbit a bit with current technology. It may be necessary to do this if there's a comet or asteroid which could collide with the earth. This could be done by using a nuclear explosion, or series of explosions to heat one side of the comet, causing some of it to ablate with enough force to alter its trajectory. Painting or coating the body is also an idea, as is attaching a solar sail. None of these would be particularly precise, and would certainly not produce enough force to move a comet into an orbit where it could be reached, unless it was almost in that orbit anyway.

If there's to be any moving of celestial bodies then technology needs to move forward. You need a better engine technology that produces much more power for the amount of fuel consumed, most likely nuclear rocketry. You would also need to develop the ability to make fuel from material found in space. Comets have a significant amount of water on them, if you could manufacture fuel from that water on the surface of the comet itself you would then only need to get your powerful engine and your fuel factory there. It would still take a long time to do it, probably several decades, but once you are in space you need to take the long view.

• I would imagine that we could do this over the course of thousands or millions of years by using the same principles we use to "slingshot" probes and whatnot. What would be some rough calculations to do this with Chury, using present day technology? Or is that a better question for the physics exchange? – TruthOf42 Nov 13 '14 at 15:43

Parking a nice juicy body full of volatile ices in a high lunar orbit is one of my favorite daydreams. Lots of CHON - Carbon, Hydrogen, Oxygen, and Nitrogen. Stuff necessary for life support. Cometary volatiles could also be used to make propellent.

Departing from earth's surface for any destination beyond earth orbit takes 13 km/s or more. But EML2 has a C3 much closer to zero than earth's surface. Departing for Mars or other destinations from EML2 would be more like 1 km/s. A body of life support consumables and propellent at such a location would be a game changer.

But is this daydream plausible? Let's look at some numbers.

Earth moves at about 30 km/s relative to the sun. Comets falling from the Oort or Kuiper Belt are moving about 42 km/s when they're in our neighborhood. If they happen to be moving the same direction as earth, the relative velocity would be 12 km/s. However it is more likely the comet's orbit would intersect earth's orbit at an angle. In which case the velocity relative to earth would be more than 12 km/s. These are out.

Then there's short period comets. A lot of these have aphelions of around 5 astronomical units. When in earth's neighborhood these are moving 38 km/s relative to the sun. Speed relative to earth would be 8 km/s or more.

If the braking burn were done deep in earth's gravity well, a healthy Oberth benefit could be enjoyed. At a 300 km altitude perigee it'd take a 3 or 4 km/s braking burn to slow the comet into an earth capture orbit. So long as we're indulging in pleasant daydreams, let's imagine we've already established infrastructure on the comet for mining and manufacturing propellent. Exhaust velocity of hydrogen/oxygen bipropellent is about 4.4 km. Propellent mass fraction is $1-e^{-delta V/exhaust velocity}$. Half to 3/5 of the comet's mass would need to be burned within the few minutes the comet spends near perigee. Burning such an enormous amount of reaction mass in a very short time would require implausibly large rocket engines.

With a braking burn so close to earth there is significant risk of impact. A small error could result in a major catastrophe.

Short period comets are also out.

There are bodies with more earth like orbits that could plausibly be parked in our neighborhood. See the Keck paper on asteroid retrieval. But these are orbits closer to the sun. Warmer rocks would lose volatile ices by sublimation in which case they are no longer comets. It is possible an accessible asteroid could hold onto some water in the form of hydrated clays. But this form of water would be more difficult to extract than the water ice found in comets.