Comet Capture | Research & Encyclopedia Articles

Comets are the most volatile-rich minor bodies in the solar system. It has been suggested that impacts with comets and asteroids provided Earth with much of its water. Although most comets are less accessible than near-Earth

asteroids, their high water content makes them an economically attractive resource for space mining. The possibility that some near-Earth asteroids are extinct or dormant cometary nuclei means that this water-rich resource may be more accessible than was once thought.

Recent spacecraftand ground-based studies of comets have confirmed and refined Whipple's "dirty snowball" model for cometary nuclei. Cometary material is composed principally of water ice and other ices (including CO, CO₂, CH₄, C₂ H₆, and CH₃ OH) mixed with cosmic dust grains. The passages of most Oort cloud comets through the inner solar system are not predictable. In addition, the highly elongated and inclined trajectories of these comets make them difficult targets with which to match orbits. In contrast, Jupiter-family comets tend to have predictable, well-determined orbits with short periods and low inclinations. Therefore, a future mining mission would most likely target a Jupiter-family comet.

The capture of an active comet as a source of water and other volatile elements is a difficult proposition. In the vicinity of Earth the jet-like gas that flows from a comet's nucleus would have a stronger influence on its trajectory than any force humans could apply to the comet. This behavior would make transporting an active comet into a suitable near-Earth orbit, and maintaining it there, very unlikely. The Earth-impact hazard posed by a sizable comet^* or comet fragment in an unstable near-Earth orbit would be unacceptable. For example, even if the trajectory of a cometary fragment could be manipulated to produce capture into a high-Earth orbit, bringing the material down to low-Earth orbit (e.g., to the space station) would be difficult. The Moon's gravitational pull would make the trajectory extremely difficult to predict and control.

Capture into a lunar orbit would also be problematical. Lunar orbits tend to be unstable because of gravitational influences from Earth and the Sun. Another difficulty that must be resolved is the current uncertainty about the consistency of cometary nuclei. Not only is the bulk density of cometary nuclei unknown (estimates range from 0.3 g/cm³ to greater than 1 g/cm³;liquid water has a density of 1 g/cm³), we do not know the cohesiveness of this material. Such uncertainties make it impossible to predict the mechanical properties of cometary material and the way a comet nucleus would react to a "nudge" to change its trajectory. A comet nucleus may or may not behave as a rigid object does; it might instead break up into fragments when a force is applied to change its orbit.

A more attractive approach to harvesting cometary material would be to send a robotic spacecraft to mine the comet. Returning fine-grained material and/or liquid water to Earth orbit would greatly lower the risks. A cargo spacecraft would be easier to control than a comet fragment, and even if an uncontrolled atmospheric entry occurred, the water and/or fine-grained material would vaporize or rain down harmlessly onto Earth's surface.

Comets are thought to have formed in the outer solar system. Two sources have been identified for the comets that are seen today: the Oort cloud and the Edgeworth-Kuiper belt (also known simply as the Kuiper belt). The Oort cloud is a roughly spherical shell located about a third of the distance to the nearest star. The Kuiper belt is a disk-like distribution of icy bodies extending beyond Pluto's orbit. Most bright new comets, such as Comet Hale-Bopp, come from the Oort cloud and have orbits that are highly inclined with respect to Earth's orbit. Most short-period or Jupiter-family comets have low inclination orbits (i.e., their orbits lie nearly in the same plane as Earth's orbit) and are believed to originate in the Kuiper belt.

Asteroid Mining (Volume 4);; Comets (Volume 2);; Kuiper Belt (Volume 2);; Living on Other Worlds (Volume 4);; Oort Cloud (Volume 2);; Natural Resources (Volume 4);; Resource Utilization (Volume 4);; Terraforming (Volume 4).

Whipple, Fred Lawrence. "A Comet Model. I: The Acceleration of Comet Encke."Astrophysical Journal 111 (1950):375-394.

^*A "sizable" comet in this context means greater than about 100 meters, depending on the density of the material.

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