Asteroid

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Asteroid

Science > Astronomy
Technology > Application > Manufacturing
Technology > Application > Materials Technology
Culture and Society > Economics > Occupations, Avocations, and Work

Image from Copyright Lilly Harper
A Rockhopper mining ship approaches an asteroid

Asteroids, also known as planetesimals, are astronomical bodies too small to undergo gravitational compaction and eliminate internal porosity, or too cold to undergo significant planetary differentiation. Products of the protoplanetary disc and building blocks of planetary bodies, asteroids can be found in great numbers around nearly every star. In mature planetary systems, they are most abundantly found gathered in belts and clusters, shepherded by gravitational influences of larger worlds. These bodies may be ejected from their original planetary system, becoming interstellar asteroids.

1. Formation and Dynamics
2. Composition
3. Uses
3.1 Asteroid Mining
3.2 Asteroid Habitation

Formation and Dynamics

During the early stages of the formation of a planetary system, substances condense and produce dust grains as the protoplanetary disc cools, and these dust grains collide and stick to grow into pebbles. Clusters of such small solid particles can rapidly collapse via processes such as streaming instability to form kilometre-sized asteroidal bodies.

Some asteroids continue to grow more massive through collisions with other objects. The largest of these grow rapidly, eventually becoming planetary bodies. The gravitational influences of such massive planets clear out smaller bodies, whether through accretion or ejection, as well as scattering others into more eccentric and inclined orbits across the planetary system; surviving asteroids are usually found in reservoirs in stable regions, forming asteroid belts or clusters that are in stable resonances with planets.

Asteroids colliding with each other at sufficiently high speeds can break apart and produce fragments. These numerous fragments generally continue to follow similar orbits and feature similar compositions, and can be grouped into asteroid families. Collisions between asteroids in an asteroid belt cause the size distribution of objects within it to evolve with time: larger objects are destroyed, and smaller asteroids (including dust) are produced. Due to shorter orbital periods, asteroid belts closer to the star generally evolve faster, while belts located further may contain many pristine objects largely unaltered since formation.

The orbits of asteroids change over time due to effects such as the Yarkovsky effect. For some asteroids, this can move them into unstable zones, where gravitational influences of massive planets can scatter them out of the belt and into regions occupied by major planets. As a result, asteroid belts "leak", continually supplying asteroidal bodies into chaotic orbits. Eventually, such bodies will be removed through planetary perturbations, either ejected out of the planetary system or collide with another celestial body.

Additionally, asteroids do not survive long in close orbits around stars, as various factors such as stellar radiation-related processes like the Yarkovsky effect and YORP effect, as well as faster orbital (and hence impact) velocities and higher thermal stresses from diurnal temperature cycling, all conspire to rapidly erode and destroy such objects. This means vulcanoid asteroids are rare in most mature planetary systems.

Composition

An asteroid's composition depends on its formation location and evolutionary history. Asteroids can be categorized into two major compositional types: primitive bodies and non-primitive bodies.

Primitive bodies

Primitive bodies are objects whose bulk elemental abundance patterns are similar to an object condensed directly from the protoplanetary disc. The degree of depletion of any given element in such bodies can be described by devolatilization: elements that preferentially form more volatile species are more depleted from the body in question.

Siliceous: Primitive bodies with bulk composition consistent with bodies formed in a high temperature environment, interior of the water ice line. Depleted of volatiles and organics, they contain refractory materials: silicates and metal oxides/sulfides. Siliceous asteroids generally have relatively bright, slightly yellowish surfaces.

Carbonaceous: Primitive bodies with bulk composition consistent with bodies formed in a moderate temperature environment. They contain refractory and organic materials, and are enriched in volatiles, although not as much as glaciaceous asteroids. Carbonaceous asteroids generally have relatively dark, greyish surfaces. Some larger carbonaceous asteroids undergo limited differentiation, where water melts and percolates towards the centre of the body, ultimately producing a core consisting of rocky materials and ice, surrounded by a dry outer layer.

Glaciaceous: Primitive bodies with bulk composition consistent with bodies formed in a low temperature environment, exterior of the water ice line. As the low temperature enables condensation of most species, they contain refractory, organic, and volatile materials such as water ice. Glaciaceous asteroids tend to have relatively dark surfaces, and their surface colours can range from greyish to brownish.

Non-primitive bodies

Non-primitive bodies are objects whose bulk elemental abundance patterns are significantly altered by planetary differentiation. They are usually fragments of planetary bodies, and hence are classified by the planetary layer they originated from.

Sideritic or Metallic: These bodies originated from the core or the core-mantle boundary of a planetary body. They contain high concentrations of siderophilic elements such as iron, nickel and cobalt, along with impurities such as sulfur. Many also contain significant amounts of silicate materials. Due to their composition, sideritic asteroids tend to be relatively dense compared to other types of asteroids.

Lithic: These bodies originated from the mantle or the crust of a planetary body. They contain silicate materials, rich in lithophilic elements such as oxygen, silicon, magnesium, aluminium, and calcium, and poor in siderophilic elements.

Carbonitic: These bodies originated from the carbon-rich layer of a planetary body. These bodies generally contain graphite, diamond, or organic materials, sometimes mixed with silicate or icy materials.

Pagosic: These bodies originated from the icy layers, such as the ice shell or the subsurface ocean, of a planetary body. The common types of ices these bodies can be made of include water, carbon dioxide, methane, and nitrogen. They can be very bright if fresh ice is exposed on their surfaces, but they may also have dark surfaces if covered by tholins.

Image from MrOnyx
A sophont overlooks a deep valley on a recently-captured large asteroid. The mineral-rich deposits will soon be extracted, and transferred to a nearby Orbital.

Uses

Asteroid Mining

In a burst of magnetic acceleration and a shimmer of laser light a thousand seeds shot from the habitat. Each one comprised of a carefully measured protective coating around a soft, semi-organic core. After two weeks climbing the gravity well the fist-sized cloud impacted the target.

Not all of them survived, but that was to be expected. In this case quality had diminishing returns that quantity lacked. From those seeds that didn't deflect into space or were totally destroyed upon lithobreaking, stirrings of artificial life began.

It took a few months to notice any change. Energy in those tiny craters was not abundant. But eventually even an unaugmented eye would notice the dark, moss like growth upon the asteroid. Solar fibers soaked up stray photons, powering newly hatched synsects to spread layers of smartgel. The process was slow at first, but not too long after first sighting an observer would find the entire rock wrapped in film.

The fronds came next. Long, spindly trees of black leaves stretched over kilometers by the asteroid's slow spin. With a thousand fold increase in power the real work could begin. Focusing on the infrared an observer would note the increasing build-up of heat. Lines of fresh material, dug from deep within by tunneling bots, were squashed into blisters upon the envelop. In baths of ice-mined water and molecular machines precious resources were separated from waste.

Eventually the final stage began. Fruit budded on the trees. Slick-skinned coconuts packed with the refined products of the blisters. In puffs of gas and shivers of branches ten thousand pods were released. In a lazy arc they escaped the ghostly pull of the rock, new fruit growing in their place on the slowly shrinking celestial body. In a matter of months their artificially evolved bodies would meet the mother hab. The first reaping of a simple sewing.

Nanobiotechnology: The harvest collection

Despite their small size and comparatively sparse distribution, asteroids are the most common source of raw material within Terragen civilization. Readily accessed and with negligible gravity, they can be mined and transported more easily than a similar volume of mass extracted from a planetary body. While asteroids can be found freely orbiting almost anywhere, gravitational interactions tend to collect them into denser groupings such as asteroid belts or planetary rings.

Because of the ease with which resources can be accessed, and habitats created, asteroids are very important in the development of new solar systems, and are frequently the first part of a solar system to be settled. The nature of any general group of asteroids, including but not limited to their location, orbit, consistency, density, and mass, along with the inhabiting civilizations' potential intentions, desires, culture, and technology, leads to innumerable ways that the material might be extracted and utilized. Despite this, there are several general methods which deal with the advantages and disadvantages that asteroids offer.

One main advantage of asteroids is their inherent orbital momentum and low mass, making extracting their resources or moving them to a different orbit a trivial feat for even midtech civilizations. The main disadvantages of asteroid mining are the challenges associated with the debris which collects naturally on their surface as well as that generated by the mining process. Even small asteroids possess layers of loose surface material ranging from particulate dust to large boulders, held together delicately in an otherwise undisturbed micro-gravity environment. The escape velocity of these bodies can be fractions of a kilometer per hour, meaning any contact with their surface, or any changes to their rotational momentum, will result in clouds of debris which can escape and float away, enter orbit, or fall back to the surface. Asteroids also tend to have irregular rotation, tumbling through space about random axes. This rotation must be stopped or matched in order to have easy access to their material, however the process of slowing down the rotation may result in parts of the surface flying off into space.

This issue is usually solved by enveloping the asteroid before material extraction, a method inherent to the process regardless of surface debris as it can ensure no desired material is lost to space. If the target is located initially in an area intended to receive regular traffic, such as a waypoint for a larger mining operation, it is likely to be completely enclosed prior to mining operations. Mid-tech mining units suited to this task utilize a bag which opens and closes mechanically, while higher-tech versions consist of utility fog able to form surface-area efficient sealed environments around the body and perform the material extraction itself. The debris issue can also simply be ignored; for asteroids far from other infrastructure, it may not be detrimental to lose the debris within a localized area, as it will continue more or less in the same orbit it always had. If the asteroid is to be sent to another location, the debris may be intentionally cast off before departure.

For larger bodies, extraction may be done from the inside out. Small access ports are drilled within small bubbles on the surface, leading to the interior where the asteroid is hollowed from the inside out. Inhabitants may live on their surface, or utilize the interior to create numerous types of enclosed habitats. The material on the surface will still be disturbed and must be collected or cast off.

Image from Alex Mulvey
A neumann swarm envelopes an asteroid-sized object within a planet's rings

The actual processing methods themselves can take many forms, any of which may be most desirable for a given logistical system. Asteroids may be collected and moved as a whole towards central processing locations within the field. Gravity tugs can be used to pull them. Small seedships may land on the surface and construct small thrusters and processors on the surface from material mined there, turning the asteroid itself into a ship which flies to its destination. Asteroids can also act as radiation shields, keeping a cool shadow over orbital infrastructure.

Image from Copyright Lilly Harper
A mass driver attached to an asteroid can be used to deliver mined materials to a distant location, and may also be used to accelerate the asteroid itself.

Because of their small and easily manageable size, individual asteroids are very valuable. They have negligible gravity wells, making it relatively easy to move materials to habitat constructions in orbit. Even a small asteroid of a few hundred meters diameter can contain billions of tons of raw material. For this reason, individual prospectors (the so-called belters) are willing to gamble life-savings against the cost of relativistic transport, mining rights, and so on, and quite a few do make it rich. Both small and large development corporations are also very keen to get their hands on asteroids. Many asteroids, most particularly nickel-iron rocks, contain varying amounts minerals: platinum, iridium, and sometimes radioactives, while carbonaceous chondrites are prized for being a rich source of volatiles and very occasionally fullerenes and amino acids). Water ice and ammonia are also very useful.

The main challenges in Asteroid Mining

The main difficulties facing an asteroid miner include

A lack of atmosphere for aerobraking; landing on Earth is made easier because of this useful deceleration aid.
Minimal Oberth Effect making it expensive in delta-vee to decelerate into orbit round an asteroid,
Irregular shape, making any orbit around an asteroid chaotic and potentially dangerous ( try plotting a close orbit around Kleopatra without hitting one or other of the protuding ends).
Rapid rotation- for asteroids smaller than 100m the rotation may be fast enough that you could be thrown off the surface.
Tumbling- many asteroids have unstable rotation (i.e. their period and axis of rotation vary over time), which also makes it difficult to land.

Envelope Mining

Image from Steve Bowers
A Pacman miner manoeuvring around an asteroid, after matching its rotation. The spherical bag will envelop the object and will become an enclosed environment allowing mining activities to be carried out with minimal losses

The small gravity field of an asteroid also causes problems, as mined material may drift away and volatiles can be lost to evaporation. For this reason most mining occurs inside flexible, sealed domes or in sealed tunnels inside the object. Very small asteroids can be entirely enclosed by an envelope miner vessel, known for historical reasons as a Pacman Miner or Pacminer. The rapid rotation of small asteroids also causes problems when they need to be moved or deflected - various kinds of gravity tug can be used for this purpose without contact with the rotating object.

Image from Steve Bowers
This envelope miner system encases the asteroid in smart gel, which refines the ores and contains any volatile gases

Optical Mining

Another process which is often used is optical mining: using concentrated sunlight to break up asteroids. This begins with enclosing a target asteroid in a bag attached to the mining ship. Next, sunlight is concentrated onto the asteroid using a system of mirrors, fracturing it and releasing volatiles such as water. The volatiles are then collected and frozen for storage. The remaining solid slag may be discarded or retained for further processing, depending on the capabilities of the individual mining ship.

Not all asteroids are amenable to optical mining: this technique is best used on asteroids that are relatively small and have a high water content.

Image from Steve Bowers
Neumann vecs replicating near Dactyl. Many von Neumann self-replicating systems replicate by producing miniature copies of themselves which then grow larger over time

Many individual asteroids and whole asteroid belts have been exploited by self-replicating von Neumann devices, which sometimes develop various degrees of autonomy or sentience (becoming Neumann vecs) and establish civilisations of their own.

Occasionally an asteroid or asteroid belt will be discovered with an unusual composition that makes it valuable for scientific or even aesthetic purposes. Even more rare, the hulk of an ancient ship or a neumann may be recovered, often centuries or millenia old, drifting in a belt. The salvage rights of such a find can make a belter rich for a life. Rarest and most prized of all are alien artifacts, although the possibility of such a find is more a part of myth and legend than pragmatic reality.

Image from Todd Drashner

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