With current costs of ~ $10,000 per kg to launch objects into space it can cost anywhere between $50M to $500M to get a satellite into orbit.

 

As mentioned in our previous asteroid mining article, mining and processing materials in space could lower our launch costs as we’d only need to strap essentials to the rocket, but at current tech levels, this seems infeasible – multiple trips between asteroids and Earth would take an extremely long time and cost a lot of money.

 

So instead of making repeated trips, why not just bring the asteroid to us?

Technically, an asteroid is captured when it does not have enough velocity to escape our planet’s gravity – this forms an orbit. The orbit radius of the asteroid has to be small enough to be able to reach easily. Once in a stable and safe orbit, we could do what we want with the asteroid and it’s precious materials.

 

But how do we actually catch an asteroid?

 

In 2016, NASA proposed the “Asteroid Redirect Mission” (ARM), they would construct a Asteroid Retrieval Robotic Mission (ARRM) spacecraft to rendezvous with a near-Earth asteroid and return with a ~4m boulder. During the mission, NASA was diligently searching for asteroids, and had picked out 5 potential targets for the mission: Ryugu, Bennu, Itokawa, and 2008 EV5. For testing and simulation purposes, 2008 EV5 – a potentially C-class asteroid that orbits relatively close to the Earth (the closest approach sitting at 8.4 lunar distances in 2008).

Initially NASA came up with 2 methods of capturing the chosen asteroid – method 1 consisted of deploying a 15 metre capture bag that would hold a small asteroid up to 8m in diameter with a mass of ~500t. The other option would have a vehicle land on a large asteroid and use robotic arms to lift a boulder up to 4m in diameter, and then park the boulder in a lunar orbit.

 

In the end, the second option was chosen because it was identified as more relevant to future technologies, like autonomous docking and planetary defence. NASA would land on 2008 EV5, latch onto a boulder and transport it into a lunar orbit.

The Keck Asteroid Retrieval Feasiblity Study suggested a total cost of an asteroid retrieval mission to be around $2.6bn, although the mission described in that report was larger in scale (the first option from above instead of just taking a boulder) – but it would definitely be expensive. Under the 2018 NASA budget this mission was cut.

 

There are other methods of asteroid capture littered around the internet – one of them being gravitational slingshots.

The objective of this method is to manipulate asteroids into certain astral objects in order to strip them of velocity – the final goal being an approach close to the Earth with low enough velocity allowing the moon to slow down and drop the asteroid into an orbit around the Earth.

 

Unfortunately, it seems that gravitational slingshots are as much art as engineering, especially if you need multiple slingshots across many bodies to get the asteroids into the right place at the right time. There are also many uncertainties with asteroids, many of them have only been observed over a few days – this adds potential error over orbital parameters like shapes, diameters and masses (which are important for our calculations).

 

The main selection criteria for this method are asteroids that have close proximity to gravitational keyholes – areas of space in which a planet’s gravity influences an asteroid such that it will collide with that planet on a future path.

By manipulating an asteroids orbit with some sort of tug spacecraft, we can move asteroids into keyholes to bring them close to the Earth, and with further manipulation we can set up an approach on the moon that will trap the asteroid into an Earth orbit.

 

Why capture asteroids?

 

The most desirable asteroids for return are carbonaceous C-type asteroids. C-type asteroids are the most diverse and contain a rich mixture of volatiles, complex organic molecules, dry rock, and metals. They make up around 20% of the known asteroid population, but since their albedo is low (they absorb more heat from the sun, so are easier to detect), they may be heavily biased against in optical surveys.

Carbonaceous asteroid material is easy to cut or crush because of its low mechanical strength, and can yield as much as 40% by mass of volatiles – roughly equal parts of water and carbon-bearing compounds. The residue after volatile extraction is about 30% native metal alloy, similar to iron meteorites.

 

A 500 tonne carbonaceous asteroid could contain up to 200t of volatiles, 90t of metals and 200t of silicate residue. It’s cool to note that getting 500t into deep space could cost $20B with conventional methods – as mentioned above NASA had the option to capture a 500t asteroid for a cost of around $2.6B – this would represent a 7.7x cost reduction (excluding the mass of the spacecraft).

 

The asteroid Apophis (a large s-type) could contain enough materials to construct around 150 five-gigawatt solar panel satellites at around 25,000 tons of steel and silicon each. Plus Kalpana One style habitats for 100,000 people, shielded from space by the slag remaining after the iron is smelted. Oxygen released during the smelting would be plentiful for the habitat needs, and could provide fuel mass for ion thrusters to move the habitat into a specific orbit.

Until 2014, there was a small chance the Apophis would pass through a keyhole and hit us on April 13, 2036 (there’s a 1 in 150000 chance it hits us in in 2068 currently). It’s worth noting that every potentially dangerous asteroid we remove from space is one less to hit us.

 

Exposure of astronauts to cosmic rays in space may represent a show-stopper for human exploration in deep space. The only known solution is to provide sufficient radiation shielding mass, and as we know, adding more mass to a rocket launch increases it’s cost to the eventual point of being infeasible.

 

One of the potentially earliest uses of the stolen asteroid would be for radiation shielding against cosmic rays. Astronauts could cannibalise the asteroid for material to upgrade their deep space habitat with radiation shielding.

Before embarking on asteroid capture missions, prototype scale experiments on processing the materials in the retrieved asteroid would validate our concepts and refine our techniques for the production of propellants, life-support materials, metals, and radiation shielding. The extraction of water would provide us with propellant, the use of solar power for electrolysis could supply hydrogen and oxygen for chemical propulsion and life-support.

 

 

Sources
NASA – https://www.nasa.gov/content/what-is-nasa-s-asteroid-redirect-mission
Asteroid Retrieval Feasibility Study – https://commons.erau.edu/cgi/viewcontent.cgi?article=1987&context=publication
Orbital Slingshots – https://space.nss.org/technologies-for-asteroid-capture-into-earth-orbit/