• Why is there an asteroid belt?
  
  Orbital resonances with Jupiter disrupted the orbits of planetesimals, preventing them from accreting into a planet. Those that were not ejected from this region make up the asteroid belt today. Most asteroids in other regions of the inner solar system accreted into one of the planets.
• How are meteorites related to asteroids?
  Most meteorites are pieces of asteroids. Primitive meteorites are essentially unchanged since the birth of the solar system. Processed meteorites are fragments of larger asteroids that underwent differentiation.

• How do comets get their tails?
  
  
  The vast majority of comets do not have tails, as they reside so far from the Sun that they are perpetually frozen. Only those few comets that enter the solar system grow tails. As the comet approaches the Sun its nucleus—all the comet consists of when it is far away and frozen—heats up. Some of the comet’s ice sublimates into gas, and the escaping gases carry along some dust. The gas and dust form a coma and two tails: a plasma tail of ionized gas and a dust tail. Larger particles can also escape, becoming the particles that cause meteors and meteor showers on Earth.
• Where do comets come from?
  
  Comets that enter the solar system come from one of two reservoirs in the outer solar system: the Kuiper belt and the Oort cloud. The Kuiper belt comets still reside in the region beyond Neptune in which they formed during the birth of the solar system. The Oort cloud comets are thought to have formed in the region of the jovian planets, and were kicked out to the great distance of the Oort cloud by gravitational encounters with the planets.

• What is Pluto like?
  
  Pluto is much smaller than any other planet, with an orbit more elliptical and more inclined to the ecliptic plane than that of any other planet. It is made mostly of ices and has a very thin atmosphere of gases that are expected to freeze onto the surface as Pluto moves farther from the Sun in its 248-year orbit. It has a moon, Charon, with a slightly lower density than Pluto, suggesting that Charon may have been formed in a giant impact.
• Is Pluto a planet or a Kuiper belt comet?
  Whether Pluto should be called a “planet” is a matter of opinion, but its properties suggest that it is a Kuiper belt comet. Its composition and orbital properties match those of other Kuiper belt comets and do not fit in with the other planets. It is the largest known Kuiper belt comet today, but there may be larger ones still awaiting discovery.

• Have we ever witnessed a major impact?
  
  In 1994, we observed the impacts of comet Shoemaker–Levy 9 on Jupiter. The comet had fragmented into a string of individual nuclei, so there was a string of impacts that left Jupiter’s atmosphere scarred for months.
• Did an impact kill the dinosaurs?
  
  We are not certain whether an impact was the sole cause, but a major impact clearly coincided with the mass extinction in which the dinosaurs died out, about 65 million years ago. Sediments from the time show clear evidence of an impact, and an impact crater of the right age has been found near the coast of Mexico.
• Is the impact threat a real danger or just media hype?
  
  Impacts certainly pose a threat, though the probability of a major impact in our lifetimes is fairly low.
• How do other planets affect impact rates and life on Earth?
  
  Impacts of asteroids and comets are always linked in at least some way to the gravitational influences of Jupiter and the other jovian planets. These gravitational influences have shaped the asteroid belt, the Kuiper belt, and the Oort cloud, and sometimes still help determine when an object is flung our way.

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