Near-Earth Objects and Comets

Near-Earth Objects

• Near-Earth objects (NEOs) are any small Solar System bodies orbiting the Sun whose closest approach to the Sun is less than 1.3 times the Earth-Sun distance.
• This definition relates to the object's orbit, not its current position. Objects are still considered NEOs even when far from Earth.
• Scientists recognized the threat of impacts from NEOs in the 1980s.
• There is mounting evidence that the Cretaceous-Paleogene extinction event was caused by a large asteroid impact.

Comets

• The first near-Earth objects observed by humans were comets.
• The word "comet" comes from the Greek word kōmḗ, meaning "hair of the head," and was used to describe the tail of a comet.
• Comets are icy, small Solar System bodies.
• The solid core structure of a comet is called the nucleus.
• Cometary nuclei are composed of an amalgamation of rock, water ice, and frozen carbon dioxide, methane, and ammonia.
• Most comets have elongated elliptical orbits that take them close to the Sun for part of their orbit, then into the further reaches of the Solar System.
• When a comet warms as it passes close to the Sun, a process called outgassing occurs. Gases are released, forming a huge, extremely thin atmosphere around the comet called the coma.
• The force from the Sun's radiation pressure and solar wind causes a tail to form, pointing away from the Sun.
• Comets have a wide range of orbital periods; the longer the period, the more elongated the elliptical orbit.
• Short-period comets originate in the Kuiper belt or related scattered disc, beyond Neptune's orbit.
• Long-period comets originate in the Oort cloud, much farther from the Sun than the Kuiper belt (thousands of times farther).
• The appearance of a comet is called an apparition. If close enough and bright, a comet can be seen from Earth without a telescope.

Asteroids

• An asteroid is an object orbiting within the inner Solar System.
• Asteroids are typically rocky, metallic, or icy bodies.
• Most known asteroids are located between the orbits of Mars and Jupiter, in a region called the asteroid belt.
• Near-Earth asteroids are asteroids in a near-Earth orbit.
• Eros was the first near-Earth asteroid to be discovered in 1898.
• Near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.

Cretaceous-Paleogene Extinction Event

• The Cretaceous-Paleogene (K-Pg) extinction event was a mass extinction that occurred approximately 66 million years ago.
• This event led to the extinction of all non-avian dinosaurs.
• The event is marked in the geologic record by a thin layer of sediment called the K-Pg boundary.
• This layer contains unusually high levels of iridium, a siderophile (iron-loving) element that is extremely rare in Earth's crust.
• Iridium is more common in comets and asteroids.
• The impact hypothesis, also known as the Alvarez hypothesis, suggests that the K-Pg extinction was caused by the impact of a massive asteroid.
• The impact is theorized to have created a lingering impact winter, halting photosynthesis in plants and plankton, devastating the global environment.
• The Chicxulub crater, discovered in the early 1990s, supports this theory.

Oort Cloud

• The Oort Cloud is a vast cloud of icy planetesimals thought to surround the Sun.
• The concept of the Oort cloud was proposed by Jan Oort and named in his honor.
• Telescopes have not yet detected objects in the Oort cloud.

Additional notes

• Comets have been visited by uncrewed probes.
• Newton proved comets follow parabolic orbits.
• Halley applied Newton's methods to various cometary appearances.
• Halley confidently predicted the reappearance of a particular comet, later known as Halley's Comet.