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The main difference between an asteroid and a comet is that a comet shows a coma (tail) due to sublimation of its near-surface ices by solar radiation. A few objects were first classified as minor planets but later showed evidence of cometary activity.
The asteroid and comet belts orbit the Sun from the inner rocky planets into outer parts of the Solar System, interstellar space. [16] [17] [18] An astronomical unit, or AU, is the distance from Earth to the Sun, which is approximately 150 billion meters (93 million miles). [19] Small Solar System objects are classified by their orbits: [20] [21]
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing.This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma.
A meteor or shooting star [8] is the visible passage of a meteoroid, comet, or asteroid entering Earth's atmosphere. At a speed typically in excess of 20 km/s (72,000 km/h; 45,000 mph), aerodynamic heating of that object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake.
This incident involved an explosion that was probably caused by the airburst of an asteroid or comet 5 to 10 km (3.1 to 6.2 mi) above the Earth's surface, felling an estimated 80 million trees over 2,150 km 2 (830 sq mi). [83] In February 1947, another large bolide impacted the Earth in the Sikhote-Alin Mountains, Primorye, Soviet Union.
Asteroid 101955 Bennu seen ejecting particles on January 6, 2019, in images taken by the OSIRIS-REx spacecraft. Castalia is a proposed mission concept for a robotic spacecraft to explore 133P/Elst–Pizarro and make the first in situ measurements of water in the asteroid belt, and thus, help solve the mystery of the origin of Earth's water. [64]
Project Icarus received wide media coverage, and inspired the 1979 disaster movie Meteor, in which the US and the USSR join forces to blow up an Earth-bound fragment of an asteroid hit by a comet. [97] The first astronomical program dedicated to the discovery of near-Earth asteroids was the Palomar Planet-Crossing Asteroid Survey.
2016 WF 9 is unusually dark for a near-Earth asteroid. It is possibly an extinct comet, but without the comet-like dust and gas cloud. [4] It was first observed on 27 November 2016 by NEOWISE, the asteroid-and-comet-hunting portion of the Wide-Field Infrared Survey Explorer mission. [4]