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, 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.
Traditional chemical rockets work well in a thick atmosphere environment, but electric propulsion has higher propulsive efficiency than chemical propulsion. Ion thruster, for example, has an efficiency of 90 percent whilst chemical propulsion's efficiency is around 35 percent.
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.
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.
Artificial Asteroid Capture Missions can help scientists develop technologies that can be potentially useful for further exploration to other destinations in space:
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.
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.
The goal of proposed NASA Asteroid Redirect Mission 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.
A flyby is a spaceflight operation in which a spacecraft passes in proximity to another body, usually a target of its space exploration mission and/or a source of a gravity assist (also called swing-by) to impel it towards another target. Spacecraft which are specifically designed for this purpose are known as flyby spacecraft, although the term has also been used in regard to asteroid flybys of Earth.
Notable Mars flybys include the Mariner IV in July 1965, which returned more accurate atmospheric data about Mars and much closer views of its surface than previously available. Later, Mariner 6 and Mariner 7's flyby of Mars in 1969 caused another breakthrough in knowledge about the planet. Their infrared radiometer results showed that the atmosphere of Mars was composed mostly of carbon dioxide (CO2), and they were also able to detect trace amounts of water on the surface of Mars.
Cassini-Huygens, launched in 1997, which orbited Saturn from 2004–2017, performed flybys of many of Saturn's moons including Titan. It achieved 126 flybys of Titan, with its final close flyby on April 22, 2017 prior to its retirement.
International Cometary Explorer (ISEE-3) passed through the plasma tail of comet Giacobini-Zinner doing a flyby at a distance of 7,800 km of the nucleus on September 11, 1985. In 2010, the Deep Impact spacecraft, on the EPOXI mission, did a flyby of comet Hartley 2.
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis). The resulting drag slows the spacecraft. Aerobraking is used when a spacecraft requires a low orbit after arriving at a body with an atmosphere, as it requires less fuel than using propulsion to slow down.
When an interplanetary vehicle arrives at its destination, it must reduce its velocity to achieve orbit or to land. To reach a low, near-circular orbit around a body with substantial gravity (as is required for many scientific studies), the required velocity changes can be on the order of kilometers per second. Using propulsion, the rocket equation dictates that a large fraction of the spacecraft mass must consist of fuel. This reduces the science payload and/or requires a large and expensive rocket. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements. The use of a relatively small burn allows the spacecraft to enter an elongated elliptic orbit. Aerobraking then shortens the orbit into a circle. If the atmosphere is thick enough, a single pass can be sufficient to adjust the orbit. However, aerobraking typically requires multiple orbits higher in the atmosphere. This reduces the effects of frictional heating, unpredictable turbulence effects, atmospheric composition, and temperature.
Aerobraking done this way allows sufficient time after each pass to measure the velocity change and make corrections for the next pass. Achieving the final orbit may take over six months for Mars, and may require hundreds of passes through the atmosphere. After the last pass, if the spacecraft shall stay in orbit, it must be given more kinetic energy via rocket engines in order to raise the periapsis above the atmosphere. If the craft shall land, it must lose kinetic energy, also via rocket engines.
The kinetic energy dissipated by aerobraking is converted to heat, meaning that spacecraft must dissipate this heat. The spacecraft must have sufficient surface area and structural strength to produce and survive the required drag, The temperatures and pressures associated with aerobraking are not as severe as those of atmospheric reentry or aerocapture. Simulations of the Mars Reconnaissance Orbiter aerobraking use a force limit of 0.35 N per square meter with a spacecraft cross section of about 37 m2, equate to a maximum drag force of about 7.4 N, and a maximum expected temperature as 170 °C.[1] The force density (i.e. pressure), roughly 0.2 N per square meter,[2] that was exerted on the Mars Observer during aerobraking is comparable to the aerodynamic resistance of moving at 0.6 m/s (2.16 km/h) at sea level on Earth, approximately the amount experienced when walking slowly.[3]
Regarding spacecraft navigation, Moriba Jah was the first to demonstrate the ability to process Inertial Measurement Unit (IMU) data collected on board the spacecraft, during aerobraking, using an unscented Kalman Filter to statistically infer the spacecraft's trajectory independent of ground-based measurement data. Jah did this using actual IMU data from Mars Odyssey and Mars Reconnaissance Orbiter. Moreover, this was the first use of an unscented Kalman Filter to determine the orbit of an anthropogenic space object about another planet.[4] This method, which could be used to automate aerobraking navigation, is called Inertial Measurements for Aeroassisted Navigation (IMAN)[5] and Jah won a NASA Space Act Award for this work.
Many spacecraft use solar panels to power their operations. The panels can be used to refine aerobraking to reduce the number of required orbits. The panels rotate according to an AI-powered algorithm to increase/reduce drag and can reduce arrival times from months to weeks.[6]
Aerocapture is a related but more extreme method in which no initial orbit-injection burn is performed. Instead, the spacecraft plunges deeply into the atmosphere without an initial insertion burn, and emerges from this single pass in the atmosphere with an apoapsis near that of the desired orbit. Several small correction burns are then used to raise the periapsis and perform final adjustments.
This method was originally planned for the Mars Odyssey orbiter,[8] but the significant design impacts proved too costly.
Another related technique is that of aerogravity assist, in which the spacecraft flies through the upper atmosphere and uses aerodynamic lift instead of drag at the point of closest approach. If correctly oriented, this can increase the deflection angle above that of a pure gravity assist, resulting in a larger delta-v.