Asteroids orbits

One more important property of Jupiter must be mentioned the Earth owes its relatively quiet periods (in geological terms) to the huge gravitational force of the giant planet. Jupiter attracts most of the comets and asteroids orbiting in its vicinity, thus protecting the Earth from impact catastrophes ... 

The heliocentric pattern of asteroid types, with thermally processed objects closer to the Sun (as inferred from spectra), persists despite subsequent dynamical stirring of asteroid orbits and ejection of bodies from the main belt. Differentiated objects appear to have formed earlier than chondritic bodies, and dynamical modeling suggests they may have accreted... 

The Hilton was a sixty-storey tower sticking out of the Monterey asteroid, orbiting a hundred and ten thousand kilometres above New California. Apart from Ldenist habitat starscrapers (which it was modelled on), there were few structures like it in the Confederation. Tourists could rarely look down on terracompatible planets in such a fashion. 

The first experiments with the thermal electric engine were conducted in Russia in 1929 by its inventor, Valentin P. Glushko, who later became a world-famous authority in rocket propulsion. For more than forty years, the United States and Russia have devoted many resources to research and development of various kinds of EREs. First tested in space by the Russians in 1964, these engines have found some limited applications in modern space technology. For more than two decades Russian weather and communication satellites have regularly used electric rocket engines for orbital stabilization. The first spacecraft to employ ERE for main propulsion was the American asteroid exploration probe Deep Space 1, launched in 1998. The performance of... 

After planetary accretion was complete there remained two groups of surviving planetesimals, the comets and asteroids. These populations still exist and play an important role in the Earth s history. Asteroids from the belt between Mars and Jupiter and comets from reservoirs beyond the outer planets are stochastically perturbed into Earth-crossing orbits and they have collided with Earth throughout its entire history. The impact rate for 1 km diameter bodies is approximately three per million years and impacts of 10 km size bodies occur on a... 

Asteroid mining has been seriously considered. A NASA design study produced a 10,000-ton mining vehicle to be assembled in orbit that would return a 500,000 ton asteroid fragment to geostationary orbit. 

The only in situ chemical data for asteroids are from the NEAR Shoemaker spacecraft, which orbited 433 Eros in 2000-1, and from the Japanese Hayabusa spacecraft, which visited 25143 Itokawa in 2003. NEAR obtained numerous measurements of the surface composition using X-ray fluorescence and gamma-ray spectrometers, and Hayabusa carried an XRF. The magnesium/silicon and aluminum/siUcon ratios for both asteroids are consistent with the compositions of chondrites. However, sulfur is depleted in Eros relative to chondritic compositions, possibly due to devolatilization by impacts or small degrees of melting. 

Potassium and thorium, which normally occur in minor or trace abundances, have been determined in surface materials on the Moon, Mars, and an asteroid by gamma-ray spectroscopy from orbiting spacecraft. 

Noble gas isotopes are also produced through irradiation by cosmic rays. These rays are mostly high-energy protons that produce a cascade of secondary particles when they bombard other target nuclei, in a process called spallation. Neon produced by spallation reactions has similar abundances of all three isotopes (Fig. 10.8). Cosmic-ray irradiation occurs on the surfaces of airless bodies like the Moon and asteroids, as well as on small chunks of rock orbiting in space. Using these isotopes, it is possible to calculate cosmic-ray exposure ages, as described in Chapter 9. 

Asteroids have been a focus of spectroscopic studies for decades. Spectra obtained from telescopes on the Earth can identify some of the minerals that make up asteroids, but do not measure asteroid chemistry. Nevertheless, spectroscopic matches can be used to link some meteorite classes to then probable parent bodies, and thus allow indirect assessments of then chemical compositions. A few asteroids have been visited and analyzed by spacecraft. Chemical analyses require long data integrations from orbit or actually landing on the surface, and analyses of only two small near-Earth asteroids have been reported. 

The orbits of asteroids in the main belt between Mars and Jupiter show gaps in both heliocentric distance and inclination (degrees from the ecliptic, the plane in which the planets... 

When asteroid collisions are especially violent, sufficient kinetic energy may be imparted to launch fragments at greater than escape velocities. In that case, separate asteroids are formed. These fragments share similar orbital characteristics and are referred to as families. The members of most asteroid families share the same spectral characteristics, further linking them together. Families composed of fragments of differentiated asteroids can potentially provide important information on their internal compositions. 

Several classes of asteroids are also thought to contain ices presently, or contained them at some earlier time. The D- and F-class asteroids occur in the outmost main belt, and the C-, G-, B-, and F-class asteroids are concentrated within the central part of the belt. These asteroids probably formed near their present locations, in which case they represent icebearing planetesimals that accreted inside the orbit of Jupiter. A few asteroids exhibiting cometary activity also occur within the asteroid belt. 

Some asteroids, thought to contain modest amounts of ices, might show cometary activity if they were to be perturbed into orbits that took them close to the Sun. Other asteroids originally accreted with ice components, which later melted. Such asteroids were then altered when fluids reacted with rock. Altered carbonaceous chondrites were discussed briefly in Chapter 6. Here we explore asteroid alteration in more detail. 

The D- and P-class asteroids dominate the outer main belt and Trojan asteroids located in Jupiter s orbit. With only a few exceptions, the spectra of these asteroids show no 3 pm absorption bands (Jones et al., 1990). The D and P asteroids are thought to contain ice that has never been melted. However, it is also possible that D and P asteroids could contain hydrated silicates, and that the 3 pm feature is masked by an increasing abundance of elemental carbon with heliocentric distance. The unique carbonaceous chondrite Tagish Lake has a reflectance spectrum quite similar to D-class asteroids, and it has been hypothesized to be a sample of this class. However, Tagish Lake shows a significant 3 pm absorption. 

Asteroids that formed beyond the snowline represent rock and ice accreted inside the orbit of Jupiter. The most distant asteroids may still contain ices, but many asteroids have been heated. Melting of ice produced aqueous fluids, which reacted with chondritic minerals at low temperatures to form secondary minerals (phyllosilicates, carbonates, sulfates, oxides). The alteration minerals can be discerned in asteroid spectra and characterized by analyses of chondrites derived from these bodies. 

Asteroid A nonicy object or small planet up to 1020 km in diameter, most of which orbit the Sun in the inner solar system. 

At the end of this section, two final points merit some attention. The first one concerns the location of the parent bodies of the chondrites. The asteroid belt between Mars and Jupiter is one possibility favoured by many authors, but some other possibilities exist (such as the families of asteroids crossing the earth s orbit). Asteroids themselves are of many different types and some of them are probably extinct comets (after too many passages at the perihelion). Some chondrites could be fragments of these extinct comets, but this hypothesis is not the most probable one. People interested in this problem will find information in Refs. 2, 5 and 9. 

---