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Planetary Systems


Comets

Kuiper Belt Comets are formed of rocky material, dust, and water ice. A few have highly elliptical orbits that bring them very close to the sun and swing them deep into space, often beyond the orbit of Pluto. The most widely accepted theory of the origin of comets is that there is a huge cloud of comets called the Oort Cloud (see Figure 07-21a), of perhaps 1000 comets orbiting the sun at a distance of about 50,000 AU (in the Solar System halo). These comets are near the boundary between the gravitational forces of the sun and the gravitational forces of other stars with which the sun comes into interstellar proximity every several thousand years. According to the theory, these stellar passing perturb the orbits of the comets within the Oort cloud. As a result, some may be captured by the passing star, some may be lost to interstellar space, and some of their orbits are modified from a relatively circular orbit to an extremely elliptical one coming close to the sun. These are the long period comets with period more than 200 years.

Figure 07-21a Kuiper Belt and Oort Cloud

Comet Hale-Bopp Comet Another reservoir of comets is the Kuiper belt (see Figure 07-21a), a disk-shaped region about 30 to 100 AU from the sun beyond Neptune. This is considered to be the source of the short-period comets. The orbit of a Kuiper belt object is sometimes perturbed by gravitational interactions with the Jovian planets causing it to cross Neptune's orbit, where eventually it may have a close encounter with Neptune, either ejecting the comet or throwing it deeper into the solar system. These are the short period comets with period less than 200 years.

Figure 07-21b Comet Hale-Bopp [view large image]

Figure 07-21c Comet Structure [view large image]

Comets are invisible until they come near the sun and develop an extended structure. These structures are diverse and very dynamic, but they all include a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the sun. The dense, inner coma often appears pointlike, but the actual nucleus is rarely seen from Earth because it is too small and dim. The coma and the nucleus together constitute the head of the comet.

When far from the sun, the nucleus is very cold and its material is frozen solid. In this state the comets are sometimes referred to as "dirty icebergs" or "dirty snowballs," since over half of their material is ice. As the comets approach the sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the sun. Approaching within a few AU of the sun, the surface of the nucleus begins to warm, and the volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust. When a coma develops, dust reflects sunlight, while gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the sun, fluore-scence usually becomes more intense than the reflected light.

As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by the atmosphere, but it has been detected by spacecraft. The sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the sun. Comet Hale-Bopp's 1997 encounter with the inner Solar System is shown in Figure 07-21b with its yellow dust (curved) and blue ion (straight) tails. The structure of a comet approaching the Sun is shown in Figure 07-21c.

Each time a comet visits the sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.


During its five-year-long cosmic journey, NASA's Stardust spacecraft (see Figure 07-22a) has examined solar system objects including Comet and asteroid. On January 2, 2004 Stardust successfully fly into the coma of Comet Wild 2. It has returned the most detailed images yet of the center of a comet. The icy centers of comets are usually hidden from Earth-bound telescopes by opaque dust and gas that boils off during
Stardust wild2 approach to the Sun. Figure 07-22b shows the nucleus of Comet Wild 2 taken by Stardust when passing within 500 kilometers. Clearly visible are numerous craters and hilly terrain. Stardust has also captured particles from the coma and will jettison them to Earth in 2006. Analyses of the images and returned particles will likely give fresh information about our Solar System back near its beginning, when Comet Wild 2 formed. Discovered in 1978, Comet Wild 2 takes 6.39 years to orbit the Sun, traveling nearly as close to the Sun as Mars is and as far away from the Sun as Jupiter. The space capsule that contains dust particles from the comet has returned to Earth on January 15, 2006.

Figure 07-22a Stardust [view large image]

Figure 07-22b Wild 2
[view large image]

Detailed analyses of the comet dust in the aerogel collector (Figure 07-22a insert) reveal that there is a vast array of different of particles - some from the heart of the Solar System, some from a dying star in pre-solar times. It seems to indicate that this comet was formed very early before the formation of the Solar System with an eccentric trajectory, which allowed the comet to collect different kinds of material along the way from the edge to the inner part of the Solar System.

On July 4, 2005 NASA smashes a 372 kg spacecraft into the comet Tempel1 in the believe that the debris from under the comet's surface would reveal the composition of the solar system 4.5 billion years ago at the time of its formation. Figure 07-22c shows the hardwares for the mission and the sequence of events near the time of impact. Click here to see a movie showing the approach of the impactor (toward Tempel 1).
Tempel1 Initial analysis indicates that the comet appears to have a soft, dusty surface with crater-like features. Trapped ice seems to be below the surface, possibly containing the primordial ingredients of the solar system. Click here to see the impact as observed by the HST. Further analysis shows that the comet's dust and ice grains form a fluffy structure of fine particles held together loosely by a weak gravitational pull. Materials in the debris include tiny grains of silicates, iron compounds, complex hydrocarbons, and clay and carbonates thought to require liquid

Figure 07-22c Tempel 1 Impact [view large image]


water to form. The existence of a substantial amount of organic material means that the comets might have brought such material to Earth early in the planet's history at a time when asteroid and meteor strikes were common.


Tempel1, Second Visit Another spacecraft called Stardust-NEXT paid a second visit to Tempel 1 on February 2011. Figure 07-22d shows an image of the closest approach and the impact area six years later. There is a small mound at the center of the impact crater, which appears to be rather subdued - indicating a fragile cometary nucleus. The space craft also went through a shower of disintegrating cometary particles coming out in chunks.

Figure 07-22d Tempel 1, 2nd Visit [view large image]


Comet 67P Rosetta Spacecraft By August 2014, the European Space Agency's comet-chasing Rosetta spacecraft (launched in 2004) has reach a distance of about 100 km to the comet 67P (Figure 07-21e). Rosetta had moved into the same orbit around the Sun as the comet. Scientists will have the opportunity to observe its activities as it goes around the Sun. A probe will be released to land on the comet in November, 2014. See scheduled events in Figure 07-21f.

Figure 07-21e Comet 67P [view large image]

Figure 07-21f Rosetta Spacecraft
[view large image]

The lander Philae was released from Rosetta on 12 November 2014. It bounced 3 times to stop on a rough terrain with not much sunlight to recharge the battery. It operated for about 3 days returning data to indicate :
Figure 07-21g shows series of images of Comet 67P approaching the perihelion. It was captured by Rosetta's OSIRIS narrow-angle camera on
Comet 67P Approaching Perihelion Comet 67P at Perihelion Comet 67P Surface 12 August 2015 from a distance of about 330 km just a few hours before the comet reached the closest point to the Sun along its 6.5-year orbit. An image at perihelion on August 13 (Figure 07-21h) portrays the spewing of water vapor at the peak rate of 330 kg/sec (roughly 2 bathtubs every second). Along with gas, the nucleus is also estimated to be shedding up to 1000 kg/sec of dust, creating dangerous working conditions for Rosetta which is too close to reveal the cometary tail. (see more from "Big Day in the Sun")

Figure 07-21g Comet 67P Approaching Perihelion

Figure 07-21h Comet 67P at Perihelion
[view large image]

Figure 07-21i Comet 67P Surface
[view large image]

As the spacecraft has flown beyond Jupiter with not enough sun light to power its operation, it was sent crashing to the comet on 30 September, 2016 in an attempt to study the
comet's gas, dust and plasma environment very close to its surface. Figure 07-21i is a picture taken about 16 kilometres from the comet's surface.

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