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Asteroid capture is an orbital insertion of an asteroid around a larger planetary body. When asteroids, small rocky bodies in space, are captured, they become
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
s, specifically either an irregular moon if permanently captured, or a temporary satellite. All asteroids entering Earth's orbit or atmosphere so far have been natural phenomena; however, U.S. engineers have been working on methods for telerobotic spacecraft to retrieve asteroids using chemical or electrical propulsion. These two types of asteroid capture can be categorized as natural and artificial. * Natural asteroid capture is ballistic capture of a free asteroid into orbit around a body such as a planet, due to gravitational forces. * Artificial asteroid capture involves intentionally exerting a force to insert the asteroid into a specific orbit. Artificial asteroid retrieval may provide scientists and engineers with information regarding asteroid composition, as asteroids are known to sometimes contain rare metals such as palladium and platinum. Attempts at asteroid retrieval include
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
’s
Asteroid Redirect Mission The Asteroid Redirect Mission (ARM), also known as the Asteroid Retrieval and Utilization (ARU) mission and the Asteroid Initiative, was a space mission proposed by NASA in 2013; the mission was later cancelled. The Asteroid Retrieval Robotic ...
s from 2013. These efforts were canceled in 2017.


Naturally captured asteroids

Asteroid capture happens when an asteroid "misses" a planet when falling towards it, but it no longer has enough velocity to escape from the planet's orbit. In that case, the asteroid is captured, entering a stable orbit around the planet which does not pass through the planet's atmosphere. However, asteroids occasionally strike a planet. Small asteroids are estimated to hit Earth every 1,000 to 10,000 years. The size and physical characteristics of an orbit depend on the planet's mass. An approaching asteroid will almost always enter a planet's sphere of influence on a
hyperbolic trajectory In astrodynamics or celestial mechanics, a hyperbolic trajectory or hyperbolic orbit is the trajectory of any object around a central body with more than enough speed to escape the central object's gravitational pull. The name derives from the ...
relative to the planet. The asteroid's
kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...
when it encounters the planet is too great for it to be brought into a bounded orbit by the planet's gravity; its kinetic energy is greater than its absolute potential energy with respect to the planet, meaning its velocity is higher than
escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for an object to escape from contact with or orbit of a primary body, assuming: * Ballistic trajectory – no other forces are acting on the object, such as ...
. However, an asteroid's trajectory can be perturbed by another mass that could reduce its kinetic energy. If this brings the asteroid's velocity below the local escape velocity, its trajectory changes from a hyperbola to an ellipse and the asteroid is captured. Triton is thought to have been captured in this way, as are some of the outer
moons of Jupiter There are 97 Natural satellite, moons of Jupiter with confirmed orbits . This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that ...
. When the trajectory changes over time, asteroids may collide. Considering the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
between Mars and Jupiter contains around 1.9 million asteroids, astronomers estimated that modest-sized asteroids collide with each other about once a year. The impact can change the trajectory of an asteroid, sending it into a planet's sphere of influence.


Technology for capturing asteroids


Electric propulsion

Traditional chemical rockets work well in a thick atmosphere environment, but
electric propulsion Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generating thrust to modify the velocity of a ...
has higher propulsive efficiency than chemical propulsion.
Ion thruster An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. An ion thruster creates a cloud of positive ions from a neutral gas by ionizing it to extract some electrons from its atoms. The i ...
, for example, has an efficiency of 90 percent whilst chemical propulsion's efficiency is around 35 percent. In space, there is no
atmospheric drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...
. Since carrying propellant to an asteroid is expensive, fetching a heavy asteroid requires an extremely efficient engine such as an electric one, or one that uses the asteroid's own mass as reaction mass.


Robotic arms

Based on NASA's Asteroid Redirect Mission, a satellite would grab a boulder and return to predetermined orbit. Robotic arms are used for various purposes including grabbing a boulder. Canadarm 2 is an example of an advanced robotic arm used in space. Canadarm 2 not only helps docking cargo spacecraft to the International Space Station but also performs station maintenance. Advancement in robotic arms helps artificial asteroid capture to perform precise collection of samples on the asteroid's surface.


Lunar flyby

Lunar flyby can also be used to capture an asteroid. The orbits of an asteroid before and after lunar flyby have different Jacobi constants. When the Jacobi constant of its orbit reaches a certain value, the asteroid will be captured. The capture regions of different pre-flyby Jacobi constants can be represented numerically, and these capture regions can be used to determine whether the asteroid can be captured by lunar flybys, which will finally be validated through the ephemerides model.


Motivations for capture


Planetary defense

Asteroid capture missions can potentially allow significant progress in many areas relative to planetary defense against
near-Earth object A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun ( perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit a ...
s: # Anchoring: Capture missions will enable the development of more reliable anchoring capability, which helps spacecraft attach to asteroids better, thus providing more options for the deflection of near-earth objects (NEO). # Structural Characterization: Capture missions will help engineers to improve structural characterization capability. One of the most mature NEO deflection technologies is through Kinetic Impact, but its effectiveness is highly unpredictable due to the lack of knowledge on the condition and structure of the NEO. If we can better understand a NEO's surface material and structure, we can use a Kinetic Impact to redirect it with greater certainty. # Dust Environment: Scientists will gain knowledge on the dust environment of NEOs, and better understand forces that can trigger dust levitation and settling behaviors. This knowledge will help with the design of some NEO redirection approaches, such as gravity tractors and conventional rocket engines.


Asteroid resources

Asteroid mining Asteroid mining is the hypothetical extractivism, extraction of materials from asteroids and other minor planets, including near-Earth objects. Notable asteroid mining challenges include the high cost of spaceflight, unreliable identification ...
is a major reason to capture an asteroid. A relatively resource-poor
LL chondrite LL may refer to: * Ll or ll, a digraph that occurs in several natural languages Arts and entertainment *LL, the production code for the 1967 ''Doctor Who'' serial ''The Evil of the Daleks'' * ''Labyrinth Lord'', a fantasy role-playing game * ''L ...
asteroid contains 20% iron, as well as a significant quantity of volatiles in the form of water, minerals and oxygen. Although it is possible to bring these resources back to Earth, the high cost of transport and the abundance of resources on Earth means the primary goal of asteroid retrieval in the near future will be for immediate use in space. Asteroid mining is expected to be cheaper than sending those resources from Earth. Using conventional chemical propulsion, it is estimated by NASA that delivering one kilogram of mass to a high lunar orbit costs $100K. That would mean a $20B cost to deliver 500 tons. An Asteroid Capture Mission that delivers the same amount of material to a high lunar orbit, would ideally only cost $2.6B.


Further exploration

Artificial Asteroid Capture Missions can help scientists develop technologies that can be potentially useful for further exploration to other destinations in space: # Trajectory and Navigation. From the experience of maneuvering a large mass such as an asteroid, scientists can gain knowledge on how to navigate in the gravity fields of different celestial bodies. Artificial Asteroid Capture Missions can also help perfect capability to deliver large amounts of resources required for further space exploration. # Sample Collection and Containment Techniques. Artificial Asteroid Capture Missions will require samples from asteroids. This can help with the development of techniques for sample collection and containment, which will be useful for all types of space exploration missions. # Docking Capability. Further explorations into space will require much more robust docking capabilities to accommodate vehicles, habitats and cargo modules. Asteroid Capture Missions will help engineers improve these capabilities.


Base for habitation

If scientists can find an efficient way to utilize resources such as water, oxygen and metal collected from captured asteroids, these asteroids also have the potential to become bases for human habitation. The abundant mass of an asteroid can be valuable to a habitat due to its radiation shielding properties. Metals and other materials excavated from the asteroid can be used for construction of the habitat. If the asteroid is large enough, it could even provide some amount of gravity, which would be preferable for human habitation.


International cooperation

An international panel can oversee all asteroid retrievals and studies on collected materials and provide balanced, fair distribution of retrieved materials. Nations without an expensive space national program can still conduct research.


Proposals


NASA redirect mission

The goal of proposed NASA
Asteroid Redirect Mission The Asteroid Redirect Mission (ARM), also known as the Asteroid Retrieval and Utilization (ARU) mission and the Asteroid Initiative, was a space mission proposed by NASA in 2013; the mission was later cancelled. The Asteroid Retrieval Robotic ...
was to send a robotic spacecraft to a large near-Earth asteroid and then collect a multi-ton boulder from its surface. The astronauts would take samples of the boulder and bring them back to Earth for further scientific study, and finally they will redirect it into orbit around the Moon so that it would not hit the Earth. This mission integrates robotic and crewed spacecraft operations and, if successful, would demonstrate key capabilities necessary for NASA's journey to Mars. However, White House Space Policy Directive 1 canceled the mission on Dec. 11, 2017 to accommodate increasing development costs. Technologies developed for this mission, such as solar electric propulsion, detection and characterization of small near-Earth asteroids, and the capability to capture large non-cooperative objects in deep space, will be used in future missions. File:Asteroid Redirect Mission Option B.jpg, Asteroid grippers on the end of the robotic arms are used to grasp and secure a 6 m boulder from a large asteroid. File:Asteroid Redirect Mission-Option B.jpg, Rendering of the Asteroid Redirect Vehicle departing the asteroid after capturing a boulder from its surface. File:Asteroid capture.jpg, The 'Option A' was to deploy a container large enough to capture a free-flying asteroid up to in diameter.


References

{{DEFAULTSORT:Asteroid Capture Capture Moons Asteroid mining