Read out The cometary cradle on the track. Beyond the planets of Neptune and Pluto, there is a storeroom of billions of icy bodies that sometimes enter the inner solar system as comets. Thanks to new observations, astronomers have now gained a more accurate idea of ​​the structure of this reservoir. It was on the night of August 30, 1992, when a reddish patch of light in the sky suddenly brought life to the monotonous observation nights in the dome of the 2.2-meter mirror on Mount Mauna Kea, Hawaii. The discovery fever had hit Jane X. Luu (then at the University of California at Berkeley) and David Jewitt (University of Hawaii). They also observed the peculiar spot in the next nights: It changed its position, and track calculations soon revealed that in 1992 QB1, the provisional name for the object, is located beyond Pluto and is about 200 kilometers in size.

Beyond Pluto, the previous outsider in the Solar System - the reddish spot had opened the door to a new class of members of our solar system. "Georgesmiley" called it the two explorers - this is also the name of the agent in John Le Carré's novels, with which the scientists had evicted time during the observation nights.

For astronomers, this discovery was a welcome endorsement, as they had long since lost sight of the origin of comets. More than 750 tail stars, who only gave a one-time guest appearance on Earth, have already been recorded, but only one-third of them have enough track data. One distinguishes short-periodic and long-periodic comets.

The former account for about 20 percent and, by definition, need less than 200 years for a solar cycle. The comet Encke is the fastest with 3.31 years. An estimated 1000 short-period comets are known to date. They are thought to have originated from long-periodic events as a result of orbital deflections due to the gravitational influence of the planets, chiefly Jupiter. These have a typical circulation time of 1000 to 100000 years and often reach the inner solar system for the first time. Where are you from? display

The Dutch astronomer Jan Hendrik Oort (1900-1992) proposed a hypothesis in 1950, which met with a great response among experts. After evaluating the trajectory data available to him, he had found that the long-period comets projected into all directions in the inner solar system, not only in the plane of the planetary orbits, and that about the same number of clockwise and counterclockwise move.

In addition, estimates of the major half-axis of its highly elliptical orbits indicate that the extreme point of its orbit is 50, 000 to 150, 000 AU away from the Sun (an AE, an astronomical unit, is the mean distance between the Sun and Earth, 149.6 million kilometers). That's more than 7 to 22 trillion kilometers or 0.7 to 2.2 light-years.

Oort suspected that the solar system at that distance would be surrounded by a huge reservoir of cometary cores in a spherical shell, which, of course, can not be detected even with the most powerful telescopes. In honor of it, it has since been called the Oort Cloud. He put the number of bodies at 100 billion. Today one assumes at least tenfold. This means that in these outermost regions of the solar system at least a total mass of the order of magnitude of that of our earth should be distributed.

According to a 1978 proposal by Munich astronomers Ludwig Biermann and Reimar Lüst of 1978, every few hundred million-year-thick gas clouds moving between the stars could pass so close to the comet that some of the icy bodies are driven towards the sun (many others against it) then say goodbye to the solar system forever). Also by the influence of neighboring stars ice chunks are repeatedly pushed into the inner solar system.

The short-period comets must, that can be concluded from their orbits, but have a different origin. Gerard Peter Kuiper (1905-1973) of the University of Chicago therefore proposed in 1951 the existence of a second cosmic storeroom for comets. This reservoir should lie in the plane of the planetary orbits and begin beyond Neptune at a distance of about 35 AU. The discovery of the object Georgesmiley was the first confirmation of the existence of this so-called Kuiper Belt. Further confirmations were not long in coming.

Luu, Jewitt and several other astronomers have now discovered many of these new outsiders. By the end of 1997, there were nearly 60 objects, between 100 and 400 kilometers thick, whose orbits could be determined. But these are just the tip of the iceberg. According to computer simulations, the number of cometary cores in the Kuiper Belt was already estimated at 100 million to 10 billion in the 1980s, and initial observations of the Hubble Space Telescope confirmed this magnitude. About 10, 000 to 40, 000 members of the Kuiper belt are likely to have over 100 kilometers in diameter.

Again and again, there are collisions in the Kuiper Belt. After that, some of the pieces of debris arrive on chaotic paths, which lead them partly into the inner solar system - a new short-period comet appears.

The discovery of two unusual small bodies beyond Neptune now indicates that the outer solar system is even more complicated than previously thought. One has the catalog name 1996RQ20 and was discovered in September 1996 by Eleanor Helin of NASA's California Jet Propulsion Laboratory and her team. The object is about 300 kilometers, 47 AU from the Sun, and has a rather eccentric orbit inclined 32 degrees to the Earth orbit. The other small body was a group of American astronomers led by Jane Luu (now a Harvard-Smithsonian Center for Astrophysics, Massachusetts) with the 2.2-meter telescope of the University of Hawaii on the Mauna Kea in a solar range of 84 AE found. It is called 1996TL66, is about 490 kilometers tall and the outermost known object in the solar system.

Due to the large eccentricities and orbit inclinations, these two objects are clearly outside the Kuiper Belt. They provide the first direct indication of a diffuse group of small bodies between the Kuiper Belt and the Oort Cloud. So far, about 160 members of this family have been detected; According to projections, there will be around 500 altogether. Their tracks are however unstable. Over time, these objects are either catapulted out of the solar system by Jupiter's gravitational field or diverted so that they arrive as short-period comets in sunnier districts.

About the genesis of the outer solar system, the researchers still shatter their heads. It is clear that Jupiter and Saturn formed relatively early from the solar primordial nebula. From the gas and dust condensed also - up to a distance of a few hundred AU - also countless icy small bodies. The Kuiperg rtel is considered a relic of these events. Further inside formed the other two major gas planets of the solar system: Uranus and Neptune. Their gravitational field hurled the already existing small bodies around them both into sunnier regions and outwards, where over time they formed the Oort cometary cloud.

Because energy and angular momentum are always conserved according to a physical principle, the two planets were able to find their own way on sunnier tracks. Neptune's attraction probably also prevented the formation of other major planets in the Kuipergium. If there were planet-sized bodies there, they would have been caught long ago with modern telescopes.

=== R diger Vaas

science.de

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