Astronomy Chapter 13.1-13.4

Comet

A ball of frozen dust and rock that orbits the sun and has a tail that glows

Characterize the general physical appearance of comets

A comet is a relatively small chunk of icy material (typically a few kilometers across) that develops an atmosphere as it approaches the Sun. Later, there may be a very faint, nebulous tail, extending several million kilometers away from the main body of the comet. the meteor burns up in our atmosphere and is gone in a few seconds, whereas the comet may be visible for weeks in nearly the same part of the sky. times. Comets typically remain visible for periods that vary from a couple of weeks to several months.

Long-period comet

A comet with an orbital period of more than 200 years.

Describe trans-Neptunian and Kuiper-belt objects

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune The Kuiper belt is made up of ice-and-rock planetesimals, a remnant of the building blocks of the planets.

Interstellar asteroid

An interstellar object is an astronomical object (such as an asteroid, a comet, or a rogue planet, but not a star) in interstellar space that is not gravitationally bound to a star.

Trans-Neptunian Object (TNO)

Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune. Beyond the orbit of Neptune lies a cold, dark realm populated by objects called simply trans-Neptunian objects (TNOs). The first discovered, and best known, of these TNOs is the dwarf planet Pluto.

Describe the traits of the centaur objects

As more objects are discovered in these distant reaches, astronomers decided that they will be given the names of centaurs from classical mythology; this is because the centaurs were half human, half horse, and these new objects display some of the properties of both asteroids and comets. In 1992, a still-more-distant object named Pholus was discovered with an orbit that takes it 33 AU from the Sun, beyond the orbit of Neptune. Pholus has the reddest surface of any object in the solar system, indicating a strange (and still unknown) surface composition.

Describe the composition and classification of the various types of asteroids

Asteroids are as different as black and white. The dark asteroids are revealed from spectral studies to be primitive bodies composed of silicates mixed with dark, organic carbon compounds. These are known as C-type asteroids ("C" for carbonaceous). The second most populous group is the S-type asteroids, where "S" stands for a stony or silicate composition. Here, the dark carbon compounds are missing, resulting in higher reflectivity and clearer spectral signatures of silicate minerals. The S-type asteroids are also chemically primitive, but their different composition indicates that they were probably formed in a different location in the solar system from the C-type asteroids. Asteroids of a third class, much less numerous than those of the first two, are composed primarily of metal and are called M-type asteroids ("M" for metallic).

Discuss possible defensive strategies to protect our planet

Astronomers have urged that the first step in protecting Earth from future impacts by NEOs must be to learn what potential impactors are out there. there. In 1998, NASA began the Spaceguard Survey, with the goal to discover and track 90% of Earth-approaching asteroids greater than 1 kilometer in diameter. If we ever found one of these asteroids on a collision course with us, we would need to deflect it so it would miss Earth. The most straightforward way to deflect it would be to crash a spacecraft into it, either slowing it or speeding it up, slightly changing its orbital period. If this were done several years before the predicted collision, the asteroid would miss the planet entirely

Stony/silicaceous

Can make asteroids primitive

Asteroid deflection

Deflection is the gentler approach, which involves imparting a smaller amount of energy to the asteroid, keeping the object intact and pushing it onto a slightly different orbit with a slightly changed speed.

Basaltic

Describes dense, dark-colored igneous rock formed from magma rich in magnesium and iron and poor in silica.

Recognize the threat that near-Earth objects represent for Earth

In 2013, a small asteroid hit our planet, streaking across the sky over the Russian city of Chelyabinsk and exploding with the energy of a nuclear bomb (Figure 13.13). The impactor was a stony object about 20 meters in diameter, exploding about 30 kilometers high with an energy of 500 kilotons A much larger atmospheric explosion took place in Russia in 1908, caused by an asteroid about 60 meters in diameter, releasing an energy of 10 megatons, as large the most powerful nuclear weapons of today. At 1 kilometer or larger, the impact could blast so much dust into the atmosphere that the sunlight would be dimmed for months, causing global crop failures—an event that could threaten the survival of our civilization.

Summarize the discoveries of the Rosetta mission

In August 2014, Rosetta began a gradual approach to the comet nucleus, which is a strangely misshapen object about 5 kilometers across, quite different from the smooth appearance of Halley's nucleus (but equally dark). Its rotation period is 12 hours. The extent of the Rosetta images (and data from other instruments) far exceeds anything astronomers had seen before from a comet. The best imaging resolution was nearly a factor of 100 greater than in the best Halley images. At this scale, the comet appears surprisingly rough, with sharp angles, deep pits, and overhangs The spacecraft verified that the comet's dark surface was covered with organic carbon-rich compounds, mixed with sulfides and iron-nickel grains. 67P has an average density of only 0.5 g/cm3 (recall water in these units has a density of 1 g/cm3.) This low density indicates that the comet is quite porous, that is, there is a large amount of empty space among its materials. We already knew that the evaporation of comet ices was sporadic and limited to small jets, but in comet 67P, this was carried to an extreme. At any one time, more than 99% of the surface is inactive. The active vents are only a few meters across, with the material confined to narrow jets that persist for just a few minutes

Outline the story of the discovery of asteroids and describe their typical orbits

In the late 1700s, many astronomers were hunting for an additional planet they thought should exist in the gap between the orbits of Mars and Jupiter. The Sicilian astronomer Giovanni Piazzi thought he had found this missing planet in 1801, when he discovered the first asteroid (or as it was later called, "minor planet") orbiting at 2.8 AU from the Sun. His discovery, which he named Ceres, was quickly followed by the detection of three other little planets in similar orbits. The asteroids all revolve about the Sun in the same direction as the planets, and most of their orbits lie near the plane in which Earth and other planets circle. The majority of asteroids are in the asteroid belt, the region between Mars and Jupiter that contains all asteroids with orbital periods between 3.3 to 6 years In 1918, the Japanese astronomer Kiyotsugu Hirayama found that some asteroids fall into families, groups with similar orbital characteristics.

Explain the proposed fate of comets that enter the inner solar system

It may, of course, survive its initial passage near the Sun and return to the cold reaches of space where it spent the previous 4.5 billion years. At the other extreme, it may collide with the Sun or come so close that it is destroyed on its first perihelion passage (several such collisions have been observed with space telescopes that monitor the Sun). Sometimes, however, the new comet does not come that close to the Sun but instead interacts with one or more of the planets. A comet that comes within the gravitational influence of a planet has three possible fates. It can (1) impact the planet, ending the story at once; (2) speed up and be ejected, leaving the solar system forever; or (3) be perturbed into an orbit with a shorter period. In the last case, its fate is sealed. Each time it approaches the Sun, it loses part of its material and also has a significant chance of collision with a planet. Once the comet is in this kind of short-period orbit, its lifetime starts being measured in thousands, not billions, of years. A few comets end their life by dramatically breaking apart For comets that do not meet so dramatic an end, measurements of the amount of gas and dust in their atmospheres permit us to estimate the total losses during one orbit. Typical loss rates are up to a million tons per day from an active comet near the Sun,

Near-Earth Asteroids (NEA)

Near Earth Asteroids

Discuss what was learned from spacecraft missions to several asteroids

On the way to its 1995 encounter with Jupiter, the Galileo spacecraft was targeted to fly close to two main-belt S-type asteroids called Gaspra and Ida. The Galileo camera revealed both as long and highly irregular (resembling a battered potato), as befits fragments from a catastrophic collision The greatest surprise of the Galileo flyby of Ida was the discovery of a moon (which was then named Dactyl), in orbit about the asteroid Phobos and Deimos, the two small moons of Mars, are probably captured asteroids (Figure 13.7). They were first studied at close range by the Viking orbiters in 1977 and later by Mars Global Surveyor. For a year, the NEAR-Shoemaker spacecraft orbited the little asteroid at various altitudes, measuring its surface and interior composition as well as mapping Eros from all sides (Figure 13.8). The data showed that Eros is made of some of the most chemically primitive materials in the solar system. In 2003, Japan's Hayabusa 1 mission not only visited a small asteroid but also brought back samples to study in laboratories on Earth. The target S-type asteroid, Itokawa (shown in Figure 13.9), is much smaller than Eros, only about 500 meters long. This asteroid is elongated and appears to be the result of the collision of two separate asteroids long ago. The most ambitious asteroid space mission (called Dawn) has visited the two largest main belt asteroids, Ceres and Vesta, orbiting each for about a year (Figure 13.11). Their large sizes (diameters of about 1000 and 500 kilometers, respectively) make them appropriate for comparison with the planets and large moons. data from the NASA Dawn spacecraft indicated that Ceres has (or has had) a subsurface ocean of water, with occasional eruptions on the surface.

Short-period comet

Only a few comets return in a time measureable in human terms (shorter than a century or two), like Comet Halley does; these are called short-period comets. Many short-period comets have had their orbits changed by coming too close to one of the giant planets

Oort Cloud

Oort suggested, therefore, that the new comets we were seeing were examples of objects orbiting the Sun near the edge of its sphere of influence, whose orbits had been disturbed by nearby stars, eventually bringing them close to the Sun where we can see them.1 The reservoir of ancient icy objects from which such comets are derived is now called the Oort cloud.

Nucleus

Since the gravity of such small bodies is very weak, the atmosphere is rapidly escaping all the time; it must be replenished by new material, which has to come from somewhere. The source is the small, solid nucleus inside, just a few kilometers across, usually hidden by the glow from the much-larger atmosphere surrounding it. The nucleus is the real comet, the fragment of ancient icy material responsible for the atmosphere and the tail

Kuiper Belt

TNOs are a part of what is called the Kuiper belt, a large area of space beyond Neptune that is also the source of many comets.

Dust tail

The brightest part of the tail is called the dust tail, to differentiate it from a fainter, straight tail made of ionized gas, called the ion tail.

Ion/gas tail

The ion tail is carried outward by streams of ions (charged particles) emitted by the Sun.

Discuss the atmospheres of comets

The spectacular activity that allows us to see comets is caused by the evaporation of cometary ices heated by sunlight. Beyond the asteroid belt, where comets spend most of their time, these ices are solidly frozen. But as a comet approaches the Sun, it begins to warm up. recent observations of many comets indicate that the evaporation is not uniform and that most of the gas is released in sudden spurts, The diameter of a comet's head is often as large as Jupiter, and it can sometimes approach a diameter of a million kilometers A comet's tail is an extension of its atmosphere, consisting of the same gas and dust that make up its head. The brightest part of the tail is called the dust tail, to differentiate it from a fainter, straight tail made of ionized gas, called the ion tail. The ion tail is carried outward by streams of ions (charged particles) emitted by the Sun.

Near-Earth Objects (NEO)

Together with any comets that come close to our planet, such asteroids are known collectively as near-Earth objects (NEOs).

Coma

When we look at an active comet, all we normally see is its temporary atmosphere of gas and dust illuminated by sunlight. This atmosphere is called the comet's head or coma.

Describe the size and composition of a typical comet's nucleus

When we look at an active comet, all we normally see is its temporary atmosphere of gas and dust illuminated by sunlight. This atmosphere is called the comet's head or coma. Since the gravity of such small bodies is very weak, the atmosphere is rapidly escaping all the time; it must be replenished by new material, which has to come from somewhere. The source is the small, solid nucleus inside, just a few kilometers across, usually hidden by the glow from the much-larger atmosphere surrounding it. The nucleus is the real comet, the fragment of ancient icy material responsible for the atmosphere and the tail the nucleus is a solid object a few kilometers across, composed in substantial part of water ice (but with other ices as well) mixed with silicate grains and dust. This proposal became known as the "dirty snowball" model. It seems that much of the "dirt" in the dirty snowball is dark, primitive hydrocarbons and silicates, rather like the material thought to be present on the dark, primitive asteroids. Since the nuclei of comets are small and dark, they are difficult to study from Earth.

Spaceguard Survey

an international plan to survey all NEO's

Asteroid

any of numerous small celestial bodies composed of rock and metal that move around the sun (mainly between the orbits of Mars and Jupiter)

Asteroid families

asteroids are members of these when they have similar orbits and composition or move in groups, possibly due to collision

Centaur

composition. As more objects are discovered in these distant reaches, astronomers decided that they will be given the names of centaurs from classical mythology; this is because the centaurs were half human, half horse, and these new objects display some of the properties of both asteroids and comets.

Carbonaceous

consisting of or containing carbon or its compounds

Gravitational sphere of influence

gravitational sphere of influence—the distance within which it can exert sufficient gravitation to hold onto orbiting objects

Explain the range of cometary orbits

he noted that the orbits of the bright comets that had appeared in the years 1531, 1607, and 1682 were so similar that the three could well be the same comet, returning to perihelion (closest approach to the Sun) at average intervals of 76 years. If so, he predicted that the object should next return about 1758. Although Halley had died by the time the comet appeared as he predicted, it was given the name Comet Halley Only a few comets return in a time measureable in human terms (shorter than a century or two), like Comet Halley does; these are called short-period comets. Many short-period comets have had their orbits changed by coming too close to one of the giant planets—most Its aphelion (furthest point from the Sun) is beyond the orbit of Neptune.

Metallic

made of metal

Tail

nebulous tail, extending several million kilometers away from the main body of the comet.

Chronicle the discovery and describe the composition of the Oort cloud

the aphelia (points farthest from the Sun) of newly discovered comets typically have values near 50,000 AU (more than a thousand times farther than Pluto). This clustering of aphelion distances was first noted by Dutch astronomer Jan Oort, who, in 1950, proposed an idea for the origin of those comets that is still accepted today It is possible to calculate that a star's gravitational sphere of influence—the distance within which it can exert sufficient gravitation to hold onto orbiting objects—is about one third of its distance to the nearest other stars. Stars in the vicinity of the Sun are spaced in such a way that the Sun's sphere of influence extends a little beyond 50,000 AU, or about 1 light-year. Oort suggested, therefore, that the new comets we were seeing were examples of objects orbiting the Sun near the edge of its sphere of influence, whose orbits had been disturbed by nearby stars, eventually bringing them close to the Sun where we can see them.1 The reservoir of ancient icy objects from which such comets are derived is now called the Oort cloud. Astronomers estimate that there are about a trillion (1012) comets in the Oort cloud. These objects remain undiscovered because they are too faint to be seen directly and their orbits are too stable to permit any of them to be deflected inward close to the Sun. The total number of icy or cometary objects in the outer reaches of our solar system could thus be on the order of 10 trillion (1013), a very large number indeed.

Asteroid Belt

the region of the solar system between the orbits of Mars and Jupiter, where many asteroids are found

Primitive

those that have changed little chemically since the beginning of the solar system

Differentiated

to become distinct or specialized