Asteroids and Comets

Asteroids: the rocky leftovers

Asteroids are rocky, airless bodies left over from the formation of the solar system about 4.6 billion years ago. Most of them orbit the Sun in the main asteroid belt, the broad region between the orbits of Mars and Jupiter, where the young solar system's material never gathered into a planet. The belt is not a crowded traffic jam; asteroids are spread across a vast volume, and spacecraft cross the belt without difficulty.

Asteroids come in a wide range of sizes. The largest object in the belt is Ceres, a dwarf planet roughly 940 kilometers across that was the first asteroid discovered, in 1801, and later reclassified because it is so much larger and rounder than its rocky neighbors. The largest of the remaining asteroids is Vesta, about 530 kilometers in diameter, while many asteroids are smaller than 10 meters across. The combined mass of everything in the asteroid belt is less than the mass of Earth's Moon. Composition varies too: many asteroids are primitive carbon-rich bodies, others are stony, and a few are largely metallic, such as Psyche, the target of a NASA mission.

Comets: the icy travelers

Comets are the icy counterparts of asteroids: small bodies of frozen gases, rock, and dust, sometimes called dirty snowballs, left over from the same era of solar system formation. While far from the Sun a comet is a frozen object only a few kilometers across, but as its orbit carries it inward the Sun's heat turns its ices into gas. Around the solid nucleus, a vast atmosphere of gas and dust called the coma develops; it can grow hundreds of thousands of kilometers wide.

Sunlight and the stream of charged particles called the solar wind push material away from the Sun, producing two tails. The dust tail is broad and somewhat curved, made of particles spread along the comet's path, while the ion tail is a straighter stream of ionized gas. Because both are pushed by the Sun, a comet's tails always point away from the Sun, not back along its orbit, and they can stretch for millions of kilometers.

Comets differ in how often they return. Short-period comets take less than about 200 years per orbit, Halley's comet being the most famous example, and most of them come from the Kuiper Belt, a doughnut-shaped region of icy bodies beyond Neptune that also holds Pluto and other dwarf planets. Long-period comets, which can take thousands or even millions of years per orbit, come from the Oort Cloud, a giant spherical shell of icy objects that surrounds the solar system far beyond the Kuiper Belt, roughly 100,000 times Earth's distance from the Sun.

Meteoroids, meteors, and meteorites: one object, three names

The same kind of debris is called different names depending on where it is. A meteoroid is a small rocky body still in space, ranging from a dust grain up to the size of a small asteroid. When a meteoroid enters Earth's atmosphere at high speed and burns up, the bright streak of light it produces is a meteor, the familiar shooting star. If a fragment survives the fiery passage and reaches the ground, that surviving piece is a meteorite.

This is not a rare process. NASA estimates that about 48.5 tons, or 44,000 kilograms, of meteoritic material falls to Earth every day, though nearly all of it vaporizes in the atmosphere. Meteor showers occur when Earth passes through the trail of dusty debris left behind by a comet; the Perseids of August, for example, are debris from comet Swift-Tuttle. Meteorites are valuable to science because many of them are ancient samples of asteroids or other bodies, delivered to laboratories at no launch cost.

Near-Earth objects and planetary defense

Most asteroids stay in the main belt, but some have orbits that bring them close to or across Earth's orbit. Together with comets that approach the inner solar system, these are called near-Earth objects, or NEOs. Their existence is an ordinary part of solar system geography, not a cause for alarm; what matters is knowing where they are.

That is the job of planetary defense programs. NASA established its Planetary Defense Coordination Office in 2016 to find, track, and better understand asteroids and comets that could pose an impact hazard, supported by the Center for Near-Earth Object Studies, which computes precise orbits. Survey telescopes, including the planned NEO Surveyor infrared space telescope, search for objects that come within about 30 million miles of Earth. Deflection has been demonstrated as well: NASA's DART mission in 2022 deliberately crashed a spacecraft into Dimorphos, the small moon of the asteroid Didymos, and measurably changed its orbit. Historical events such as the 1908 Tunguska airburst and the 2013 Chelyabinsk meteor show why cataloging these objects matters, but the goal of monitoring is prediction and preparedness, not fear.