Chapter 13 Astronomy

Compare asteroids of the asteroid belt with Earth-approaching asteroids. What is the main difference between the two groups?

While compositionally similar, the asteroid belt objects have fairly stable orbits. Earth-approaching asteroids have rapidly changing orbits and could potentially collide with our planet. Such a collision could produce relatively minor to catastrophic damage.

Describe the main differences between C-type and S-type asteroids.

S-types are stony or silicate-based bodies with few carbon compounds and are more reflective. The C-types are rich in carbon compounds and are less reflective. The C-types are considered primitive and are little changed since their formation.

Why are asteroids and comets important to our understanding of solar system history

Since comets and most asteroids are small, they have not undergone chemical differentiation as have large planet-sized bodies. As such, they retain the structure and composition they acquired upon formation. These objects provide evidence for the conditions that existed in the solar nebula and early solar system.

Briefly describe NASA's Spaceguard Survey. How many objects have been found in this survey?

Begun in 1998, the Spaceguard Survey is designed to discover all near-Earth asteroids (NEAs) that are greater than 1 km in diameter. It is objects of this size that could cause globally significant damage, should one strike Earth. To date, close to 1000 objects of this size have been found.

Describe the nucleus of a typical comet and compare it with an asteroid of similar size.

Cometary nuclei are quite small, on the order of a few kilometers in diameter, and are composed of ices, volatile organic compounds, silicate grains, and dust. Asteroids of similar size are denser and contain more stony and/or metallic materials. Comets react strongly to solar heating, releasing gases and dust, sometimes forming long tails that point away from the Sun.

Describe the origin and eventual fate of the comets we see from Earth.

Comets that "fall" into the inner solar system were once located either in the Oort cloud about 50,000 AU from the Sun or the Kuiper belt. The Oort cloud is far enough away that the gravitational influence of passing stars can perturb a comet's orbit. Some perturbations can send a comet out into interstellar space never to return. But others can send the comet nucleus inward toward the Sun; it is these comets that occasionally dazzle us here on our planet. Kuiper belt chunks can be perturbed by interactions with Neptune. A comet headed toward the inner solar system could hit the Sun or impact a planet. Or it could be "caught" by an interaction with one of the giant planets to become a really short-period comet. A comet trapped in the inner solar system will have a lifespan of just a few thousand orbits before it collides with a planet or all the volatiles escape, making it a "dead" comet.

Describe the two types of comet tails and how each are formed.

Comets typically have both a dust tail and an ion tail. The dust tail is larger and brighter than the ion tail and is formed by the action of sunlight imparting energy to the dust particles and pushing them away gently from the nucleus. The ion tail is composed of charged particles and are pushed away by the streams of charged particles emanating from the Sun (the solar wind). The dust tail tends to curve as the particles go into orbit around the Sun, whereas the ion tail tends to be straighter as the charged particles are pushed in the direction that is away from the Sun.

Who first calculated the orbits of comets based on historical records dating back to antiquity?

Edmund Halley presented the calculations for 24 cometary orbits in 1705. One of these he predicted to return to the vicinity of Earth in 1758. That comet, which did indeed return as predicted, is now known as Halley's Comet.

What evidence do we have for the existence of the Kuiper belt? What kind of objects are found there?

Over a thousand members of the Kuiper belt have been found directly in recent years, including dwarf planets Pluto, Eris, and Makemake. The size and numbers of these icy objects suggest that many additional smaller bodies must also exist in the same region, in a disk extending out to about 50 AU from the sun.

Give brief descriptions of both the Kuiper belt and the Oort cloud.

The Kuiper belt is a disk-shaped region of space beyond the orbit of Neptune that is dynamically stable. It is the source of short-period comets. The Oort cloud is much farther out than the Kuiper belt. It is a spherical region surrounding the Sun out to near 50,000 AU. This is the source of newly discovered long-period comets.

In addition to the ones mentioned in Exercise 3, what is the third, rarer class of asteroids?

The M-types, or metallic asteroids.

Give a brief description of the asteroid belt.

The asteroid belt is a region of space between the orbits of Mars and Jupiter containing a few large minor planets and thousands of smaller bodies. These all have orbital periods ranging from 3.3 to 6 years.

What classification is given to objects such as Pluto and Eris, which are large enough to be round, and whose orbits lie beyond that of Neptune?

These objects are called dwarf planets, and given their location, are also called trans-Neptunian objects (TNOs).

Vesta is unusual as it contains what mineral on its surface? What does the presence of this material indicate?

Vesta's surface is partially covered with basalt. Basalt indicates volcanic activity at some point in Vesta's distant past, and thus that Vesta must be a differentiated world.