Near Earth Object Comets

by Anthony C. Zuppero, Idaho National Engineering Laboratory

Bowell discovered during July 1992 (Bowell, 1992 References) that at least one of the near Earth objects (NEOs) is in fact a comet, and not a "rock in space," as are most of the more than 250 known NEOs. Shoemaker had suggested that up to 40% of the NEOs should be water objects of some kind. Most of these NEOs would contain hydrated minerals holding between 5% and 25% water of hydration. Some percent of NEOs should be comets or spent comets. Spectral data of the dark surfaces of this class of object are like that of carbonaceous objects. The data from comet Haley data showed that comets are "off" most of the time. A very black, less than a meter thick, carbonaceous fluff on the outside of the NEO insulates and covers the ice within. Comets do in fact appear like some NEOs most of the time.

One can nearly always find a comet or NEO whose orbit passes close to the orbit of any other object. A refueling station needs this ubiquitous location.

Comet Encke

Encke, first discovered during 1820, had a very faint comet signature, the "tail." The tail is barely visible because the comet is active only when near the Sun and difficult to observe. A 1980 radar signal from its nucleus suggests it has a 1 to 4 km diameter. Enough water evaporates in jets from its surface, characteristic of comets, to push it and change its orbit. The IRAS sky survey showed it spews a very pronounced dust trail. It is known to slowly spin with a period somewhere between few hours and a few days. Comet Encke is accessible about once every 3 and 1/4 years. It passes as close to the Sun as Mercury and swings out past Ceres at aphelion.

Comet Wilson Harrington

From 1979 to 1992 the NEO named "1979 VA" was thought to be just another rock in space whose orbit comes alarmingly close to that of Earth. It is now classified as a comet. Its orbit is nearly tangent to that of Earth's orbit around the Sun, passing within 20 Earth-Moon distances of Earth orbit once every 4.3 years. Its exact size is unknown and somewhere between .5 and 2 km. Its water content is certain, but the amount is unknown.

Photometric data of 1979 VA had always been erratic. Interpretation attributed "poor observation technique" to the bad luck people had with measuring its luminosity. The puzzle was solved when observers realized their electronic sensor designed to view a point of light was viewing different parts of a comet tail some of the time, resulting in sometimes erratic light output.

Astronomers found it exceptionally interesting because it verified their theory that some "rocks in space" were not rocks at all, but comets. It "became" a comet when Bowell discovered its orbit was identical to that of the Comet Wilson-Harrington, first observed during 1949. Because its orbit comes so close to that of Earth, the delta_V required to move payloads from it to a captured orbit around Earth are "reasonable," as defined by what a steam rocket could achieve. The steam rocket would use water taken from the comet.

Other "Nearby" Periodic comets

A few dozen comets are accessible. They belong to a group of about 150 comets that are captured in orbits related in some way to that of Jupiter. Almost all have orbits within about 15 degrees plane of Jupiter's orbit. Most have orbits with period similar to that of Jupiter. We classify them "accessible" because of relatively achievable total delta_V needed to bring payloads back to captured Earth orbit. (Zuppero, 1991, References). The required delta_V is less than to go to comet Encke or to Mercury. Some of them have never been seen again. They may have broken up and evaporated. Most of these never come closer to the Sun than Mars. Virtually all are rich in hydrocarbons and water ice. The closest of these, based on a 6500 m/s delta_V for payload capture into an orbit around Earth itself are: P/du Toit-Hartley, P/Neujmin 2, P/Finlay, P/Tuttle-Giacobini-Kresak, P/Howell, P/Haneda-Campos, P/Schwassmann-Wachmann 3, and P/Wirtanen. The "P/" in front of each of these indicates that it is "Periodic" and in the Jupiter formation.

Those with delta_V less than about 7500 m/s are: P/Churyumov-Gerasimenko, P/Wild 2, P/Forbes, P/Tritton, P/Kopff, P/Clark, P/du Toit-Neujmin-Delporte, P/Tempel 1, P/Helfenzrieder, P/Boethin, P/Kohoutek, P/Reinmuth 2, P/Bowell-Skiff, P/Neujmin 3, P/Gehrels 2, P/Schwassman-Wachmann 2, P/de Vico-Swift, P/Shajn-Schaldach, P/Chernykh, P/Van Biesbroek, P/Kojima, P/Kowal-varova, P/Kowal-Mrkos, P/Gehrels 3, and P/Oterma.

Comet Halley

Satellite fly-through of the tail of Comet Halley during 1986 showed that comets consist of massive amounts of an organic material almost identical to high grade oil shale (kerogen). (Huebner 1990, References). A close-up picture of the comet nucleus revealed 4 times more black than white. Only the white was visible from Earth. The factor of 4 excess black implied 4-cubed more comet mass than had been thought.

The mass collectors flying through the tail found a mixture of carbon, hydrogen, oxygen and nitrogen compounds, labeled "CHON." The particle size and blackness explain why a comet is inactive most of time and for a majority of its orbits. The inactive part of the comet surfaces are the darkest objects known. The comet interior consists of 1/3 water ice, 1/3 hydrocarbons, 5-10% CO2, some % CO, and about 1% nitrogen compounds, like ammonia or urea.

Kuiper Belt

Past the orbit of Neptune, inaccessible mostly because of the 30 year, one way travel time humans would suffer, lies a small universe of comet objects whose mass may exceed that of Pluto, our Moon and all other planetary satellites combined. During 1992 a team including Jane X. Luu of Harvard, some University of Hawaii astronomers, David Jewitt, Jun Chen, and others, found 17 bodies larger than 100 km in the "Kuiper belt." The statistics of the observed population of these objects suggests the Kuiper Belt must contain at least 35,000 similar comet objects larger than 100 km. This would be several hundred times the estimated number of main-belt asteroids in that size range. Senay and Jewitt of the University of Hawaii in Honolulu measured heavy CO emission from a comet in that vicinity. This suggests these objects may provide quite poisonous environments. They will surely spew dust everywhere. These represent more carbonaceous material than has been processed by our carbonaceous life form since the formation of the solar system. Low gravity makes virtually all their material minable and usable by us.

Anthony C. Zuppero, Idaho National Engineering Laboratory, Idaho Falls, ID 83415 208 526 5382, FAX 208 526 7146, zca@inel.gov