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 satellites. 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’s Asteroid Redirect Missions from 2013. These efforts were canceled in 2017. But other asteroid-related missions remain functioning such as NASA’s OSIRIS-REx, which collected a sample of a near-Earth asteroid on October 22, 2020.
Natural occurrence of asteroid capture
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 relative to the planet. The asteroid's kinetic energy 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. 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. When the trajectory changes over time, asteroids may collide with each other. Considering the asteroid belt between Mars and Jupiter contains around 1.9 million asteroids, astronomers estimated that modest-sized asteroids collide with each other once a year. The impact of the collision can change the trajectory of an asteroid, and asteroids can enter a planet's sphere of influence.
Technology for capturing asteroids
Traditional chemical propulsion is great for a thick atmosphere environment, but electric propulsion has a superior efficiency over chemical propulsion. One of the main electric propulsions used, Ion thruster has an efficiency of 90 percent while chemical propulsion's efficiency is around 35 percent. In space, there is no friction between the environment and the spacecraft. Bringing a heavy asteroid requires an extremely efficient engine such as electric propulsion.
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 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
Artificial Asteroid Capture Missions can potentially allow scientists to make significant progress in many areas relative to planetary defense against near-earth objects:
- Anchoring. Artificial Asteroid 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. Asteroid 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 characterize NEO's surface material and structure, we will be able to use Kinetic Impact to redirect a NEO 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 Tractor and Conventional Rocket Engine.
Asteroid mining is a major reason to capture an asteroid. A relatively resource-poor LL chondrite 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.
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 us to acquire 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 the space will require much more robust docking capabilities to accommodate the utilization of 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 immediately 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.
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.
NASA redirect mission
The goal of NASA Redirect Mission is 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. In addition, the interaction with the asteroids would provide much helpful data regarding the internal structure of the asteroid and therefore solve long-lasting questions about asteroids’ material. 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. Yet, many major progress in development 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 continue to be used in the future because they are indispensable to human deep space explorations.
The goal of OSIRIS-REx(Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is operated by NASA to obtain a sample of a near-Earth asteroid named Bennu and learn about formation and evolution of the Solar System. Osiris-REx was launched on 8 September 2016 and reached the proximities of Bennu on 3 December 2018. On 20 October 2020, it reached Bennu and successfully collected a sample. Prior to the collection process, the spacecraft descended slowly to minimize thruster firings prior to contact in order to avoid asteroid surface contamination. During the collection process, a burst of nitrogen was released to blow regolith particles smaller than 2 cm into the sampler head. The process took only 5 seconds to avoid potential collision with the asteroid.
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