Meteors & Meteor Showers:
Meteors, or "shooting stars," are bits of dust and rocky material falling through Earth's atmosphere. These objects are heated to incandescence by friction with our atmosphere and streak across the sky, creating short-lived, glowing trails. Most meteors are no bigger than a pea or small pebble.
Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically - these events are called meteor showers. Some showers occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet. Meteor showers are usually named after a star or constellation that is close to the place in the sky from which the meteors appear to radiate. Perhaps the most famous are the Perseids, which peak in August every year. Every Perseid meteor is a tiny piece of the comet Swift-Tuttle, which swings by the Sun every 135 years. Other meteor showers and their associated comets are the Leonids (Tempel-Tuttle), the Aquarids and Orionids (Halley), and the Taurids (Encke). Most comet dust in meteor showers burns up in the atmosphere before reaching the ground; some dust is captured by high-altitude aircraft and analyzed in NASA laboratories.
Chunks of rock and metal from asteroids and other planetary bodies that survive their journey through the atmosphere and fall to the ground are called meteorites. Most meteorites found on Earth are the size of a pebble or fist, but some are considerably larger.
Early Earth experienced many large meteorite impacts that heated and probably sterilized the planet's crust. More recently, about 65 million years ago, a very large meteorite impact created the 300-kilometer-wide (180-mile-wide) Chicxulub crater on the Yucatan Peninsula. This event is thought to have contributed to the extinction of about 75 percent of marine and land animals on Earth at the time, including the dinosaurs.
Meteorites, as well as asteroids, are not pieces of a fully-formed planet that broke apart; rather, they represent some of the original, diverse materials from which the planets formed. The study of meteorites tells us much about the conditions and processes during the formation and earliest history of the solar system.
Read more about meteors & meteorites >
What is a meteoroid?
"Meteoroid" is a generic term for a small piece of space debris, usually meant to include objects as small as a grain of sand and as large as a meter in size. In the inner solar system, meteoroids are generally made of rocky material, while in the outer solar system they can also consist of ices. Micrometeoroids are the smaller members of this family of objects, typically having no more mass than a paperclip.
Meteoroids are a hazard for spacecraft exploring the solar system. Although the chances of striking any debris in deep space that could do significant damage to a spacecraft are low, they are not zero. The protective blankets that cover spacecraft not only help to regulate their temperature, but also serve as a protective barrier against impacts by micrometeoroids.
Trans-Neptunian Objects (TNOs), or Kuiper Belt Objects (KBOs):
The Kuiper Belt (also called the Edgeworth-Kuiper Belt) is a region of the solar system that extends from about 30 AU (the orbit of Neptune) to 55 AU and is populated with icy, comet-like bodies. There are likely tens of thousands of Kuiper Belt objects, or KBOs, larger than 100 km (62 miles) across and an estimated trillion or more comets. (An astronomical unit [ 1AU] is the distance from Earth to the Sun.) Because many have orbits that cross that of Neptune, these bodies are often called trans-Neptunian objects, or TNOs.
In 1992, astronomers detected a faint speck of light from an object about 42 AU from the Sun -- the first time a Kuiper Belt object had been sighted. Since that time more than many hundreds of KBOs have been identified. In fact it was the discovery of this population of objects that led astronomers to understand that Pluto was merely one member of a much larger family.
Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how reflective the object's surface is. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.
One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun. The parent body, Haumea, apparently collided with another object that was roughly half its size. The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours. It spins so fast that it has pulled itself into the shape of a squashed American football. Haumea and two small moons -- Hi'iaka and Namaka -- make up the family.
In March 2004, a team of astronomers announced the discovery of a planet-like trans-Neptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system. The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU -- instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.
In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto.
The discovery of Eris -- orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) -- forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category.
Missions to the Kuiper Belt:
While no spacecraft has yet reached the Kuiper Belt, NASA's New Horizons spacecraft is scheduled to arrive at Pluto in 2015. Following its encounter with the small worlds of the Pluto system, the New Horizons mission team hopes to study one or more KBOs after its Pluto mission is complete.
More about the Kuiper Belt >
Centaurs are a class of small bodies that orbit the sun between Jupiter and Neptune. Their true identity is one of the enduring mysteries of astronomy. Because centaurs share some characteristics of both asteroids and comets, astronomers generally have not been certain whether centaurs are asteroids flung out from the inner solar system or comets traveling in toward the sun from afar. Because of their dual nature, they take their name from the creature in Greek mythology whose head and torso are human and legs are those of a horse.
- 4.55 billion years ago - Formation age of most meteorites, taken to be the age of the solar system.
- 65 million years ago - Chicxulub impact that leads to the death of 75 percent of the animals on Earth, including the dinosaurs.
- 50,000 years - Age of Barringer Meteorite Crater in Arizona.
- 1478 B.C. - First recorded observation of meteors.
- 1794 A.D. - Ernst Friedrick Chladni publishes the first book on meteorites.
- 1908 (Tunguska), 1947 (Sikote Alin), 1969 (Allende and Murchison), 1976 (Jilin) - Important 20th-century meteorite falls.
- 1943: Astronomer Kenneth Edgeworth suggests that a reservoir of comets and larger bodies resides beyond the planets.
- 1950: Astronomer Jan Oort theorizes that a vast population of comets may exist in a huge cloud on the distant edges of our solar system.
- 1951: Astronomer Gerard Kuiper predicts the existence of a belt of icy objects just beyond the orbit of Neptune.
- 1969: Discovery of meteorites in a small area of Antarctica leads to annual expeditions by U.S. and Japanese teams.
- 1982-1983: Meteorites from the Moon and Mars are identified in Antarctic collections.
- 1992: After five years of searching, astronomers David Jewitt and Jane Luu discover the first KBO, 1992QB1.
- 1996: A team of NASA scientists suggests that Martian meteorite ALH84001 may contain evidence of microfossils from Mars, a still-controversial claim.
- 2002: Scientists using the 48-inch Oschin telescope at Palomar Observatory find Quaoar, the first large KBO hundreds of kilometers in diameter. This object was photographed in 1980, but was not noticed in those images.
- 2004: Astronomers using the 48-inch Oschin telescope announce the discovery of Sedna (2003VB12).
- 2005:Astronomers announce the discovery of 2003UB313. This object, later named Eris, is slightly larger than Pluto.
- 2005: NASA's Mars Exploration Rover Opportunity finds a basketball-size iron-nickel meteorite on Mars.
- 2008: The Kuiper Belt object provisionally known as 2005FY9 ("Easterbunny") is recognized in July as a dwarf planet and named Makemake (pronounced MAHkeh-MAHkeh) after the Polynesian (Rapa Nui) creation god. In September, 2003EL61 ("Santa") was designated a dwarf planet and given the name Haumea after the Hawaiian goddess of fertility and childbirth.
- 2009: Opportunity finds another, much larger and heavier, iron-nickel meteorite, estimated to be 10 times as massive as the first meteorite the rover discovered.