The composition of comets earned them the nickname 'dirty snowballs'

Comets serve as time capsules of the early solar system

The frozen objects preserve the molecular composition of the solar system's formation, dating back about 4.6 billion years. Spectral analysis of the gases they release when heated upon approaching the sun in eccentric orbits reveals this composition and helps assess solar system formation models.

Shooting stars, or meteors, are fragments of asteroids and comets that burn up as they enter Earth's atmosphere, creating streaks in the sky called meteor showers.

Visualize the anatomy of a comet

The solid nucleus holds the frozen molecules, which are heated into a gaseous state once the comet is within three times Earth's distance from the sun. The surrounding coma, from which comets get their name, is a spherical cloud that can grow larger than Jupiter, from which tails develop.

The Oort cloud, our solar system's theoretical icy shell

Recurring comets lead astronomers to believe they originate from an extremely distant, icy shell encapsulating the solar system. They call it the "Oort cloud," and this giant swarm of icy objects is as fascinating as it is is massive. Home of comets and a tremendous number of planetesimals, this NASA site introduces you to the Oort.

The Oort Cloud is a shell of icy objects surrounding the solar system, from which many comets originate

Proposed in 1950 by Jan Oort, the theoretical structure is believed to be the origin of comets with periods of more than 200 years. Leftover planetary building blocks from the early solar system formed the Oort Cloud after being flung out by the gravity of planets like Jupiter.

Dark comets appear to be rocky asteroids but move like comets

Cometary orbits are known to exhibit accelerations independent of gravity that result from forces related to the release of gases. Dark comets exhibit this behavior but lack visible clouds of frozen material that have turned into a gas—the coma the comets display—indicating minor quantities of escaping subsurface ice.

The names of comets can change based on subsequent observations

The International Astronomical Union's coded naming system uses the first letter to indicate a comet's category, which can change over time. "D" represents comets that have been lost or broken up, while "A" applies to former comets that are now asteroids or dwarf planets.

As seen in the dark region of the Horsehead Nebula, clouds of space dust can block some forms of light from reaching telescopes, impairing observations and data collection.

A comet's tails point away from the sun, rather than trailing its motion

The ion tail, composed of gases ionized by solar radiation and pushed by the solar wind, points away from the sun. In contrast, the dust tail is pushed away by radiative pressure and curves away from the comet's motion. As the comet approaches the sun, both tails get longer.

Scientists determine the smells of objects in outer space

Using spectrographic techniques, scientists determine the chemical composition of planetary atmospheres, comets, and other objects in space. Combining these chemicals in labs on Earth has revealed locations in space that smell like nail polish remover, rotten fish, and urine.

Comets can be categorized based on their orbital period

Short-period comets orbit around the sun once every 200 years or less and are made up of Jupiter-family (less than 20 years) and Halley-type (between 20 and 200 years) comet subgroups. Long-period comets have orbits greater than 200 years, while exocomets originate in solar systems outside our own.

Despite being called 'ice giants,' Uranus and Neptune do not contain plentiful frozen water—the term refers to their abundance of elements heavier than hydrogen and helium.

NASA's Stardust mission collected and returned samples from a comet

Launched in 1999, the spacecraft traveled to comet Wild 2 and captured more than 10,000 dust particles from its coma using a tennis racket-like mesh covered in aerogel. It also took pictures of the comet before returning to Earth and releasing the collection canister during its 2006 flyby.

Hypotheses on the origins of Earth's water, from comets and asteroids to magma oceans

Before Earth's atmosphere formed, any liquid water would have boiled off, preventing the accumulation of water early on to create today's oceans. Planetary scientists continue to debate whether this water was delivered later from comet or asteroid collisions or via reactions between iron oxide in Earth's molten surface and hydrogen gas from the planetary nebula.

Explore an interactive solar system with 3D visualizations of meteor showers—streams of dust-like debris left behind from comets or asteroids that burn up in Earth's atmosphere.

There may be over one trillion comets in the Kuiper Belt

Named after Gerard Kuiper, who theorized the existence of a band of icy objects beyond Neptune in 1951, direct evidence for its existence did not arrive until 1992, when the first trans-Neptunian object, besides Pluto and its moon Charon, was observed. Comets with periods of less than 200 years originate from this belt.

A 101 on the science and history of meteor showers

Bits of rock, ice, and dust flying through space called meteors burn up in a spectacular display when they hit our atmosphere 60 miles up at an average of 30,000 mph. Some of these are isolated objects, but others are recurring phenomena caused by...

Asteroids are artifacts of water's presence in the solar system

Asteroids contributed to planet formation and carried water from the solar system's outer reaches toward the center. They were also early providers of the necessary elements and conditions for life to evolve on Earth.

Extraterrestrial volcanoes spew material present during their formation

Unlike Earth's volcanoes that release lava, cryovolcanoes on distant moons erupt cold jets of water, ammonia, and other light gases present during the formation of their host planet. The movement of internal heat produced during formation is thought to drive icy plume movement.

Ice found in another star system for the first time

NASA's James Webb Space Telescope has discovered frozen water in a young star system. The ice is similar to that found in Kuiper Belt objects in our solar system, and provides insights into planetary formation processes.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

Snowflakes form when water vapor turns directly into ice

Every water molecule has a slightly bent shape due to electron repulsion and regularly forms bonds with hydrogen molecules. When they freeze, they form a six-sided (or hexagonal) lattice, resulting in the six-sided symmetry of snowflakes. As the snowflake falls, more water molecules freeze onto its six sides, forming the unique, beautiful geometric designs.

Ceres—the largest object in the asteroid belt—may have once harbored a muddy ocean

NASA's Dawn mission revealed Ceres' surface composition, which contains salts, water ice, and organic material. Ceres' ocean likely froze over entirely in the past, when the presence of water may have allowed life to exist.

View the composition of the universe, dominated by dark energy and dark matter

About 95% of the observable universe is made of substances whose composition is unknown, but most of what remains (4%) is in the form of clouds of hydrogen and helium, followed by gases and plasma within stars (0.5%) and neutrinos created in nuclear reactions (0.3%).

Shooting stars are streaks of light produced by meteoroids

As these rocks enter Earth's atmosphere at high speeds, friction causes them to burn up, briefly leaving streaks of light. The color of these fireballs can be used to determine the composition of the meteoroid, if it does not reach the ground and become a meteorite for analysis.

There may be 74,963 kinds of ice, resulting from water molecules rearranging themselves

The theoretical limit greatly expands the known types of ice, with type Iₕ being the most common on Earth and forming hexagonal crystals. The extreme environments of planets like Uranus and Neptune can create a variety of ice with unique structures and properties, as these conditions can reshape these crystals into new geometries.

The faint triangle of light occasionally seen above the horizon, known as zodiacal light, is due to sunlight reflecting off ice and dust scattered throughout the inner solar system.

Explore a guide to observing comets

Binoculars or telescopes can be used to see vivid tails, glowing comas, and brief bursts of brightness from these celestial objects. For photography, a DSLR or mirrorless camera with a wide-aperture lens, long exposure settings, and accessories such as tripods and star trackers is recommended.

Critical organic molecules have been found on an asteroid

NASA's OSIRIS-REx mission returned samples containing amino acids and nucleobases from asteroid Bennu. Without Bennu having been destroyed—as is common for asteroids that enter Earth's atmosphere—its samples serve as a time capsule of the solar system's early chemistry.

Read about the dangerous comet behind the Perseids meteor shower

Every summer, Earth passes through the debris trail of the Swift-Tuttle comet, producing the spectacle of the Perseids meteor shower. The phenomenon is a glorious opportunity for amateur stargazers, but astronomers have long noted the comet's orbit...

Cloud formation requires water vapor and condensation nuclei to build from

The nuclei—particles like dust, salt, and smoke—act like jumpstarts that water molecules can start to accumulate onto to form clouds. Condensation nuclei are about 1/100th the size of cloud droplets, which are mostly water.

Our solar system is currently traveling through a 'dark matter hurricane'

The phenomenon refers to a stream of cosmic debris, including stars and dark matter, leftover from a small galaxy that passed too close to the Milky Way and was torn apart. There are over 30 such streams known in our galaxy, and the stream intersecting the solar system is predicted to continue to do so for millions of years.

Earth's first oceans may have been as much as 230 degrees Celsius (446 degrees Fahrenheit), with atmospheric pressure preventing them from boiling.

Asteroids are the rocky remains of the solar system's formation

These celestial bodies were not integrated into the precursors of planets about 4.6 billion years ago and instead continue to orbit the sun, primarily within a band between Mars and Jupiter—the asteroid belt.

'Watermelon snow'—patches of pink, orange, and red in snow—is the result of extremophilic green algae that thrive in extremely frigid conditions and take the shape of red spheres in winter.

A look at Uranus, our distant icy cousin

NASA provides a profile of the seventh planet from the sun and the third largest—Uranus. More than just a punchline, the planet is cold, windy, and utterly inhospitable. It's also a lovely blue-green, due to the large amounts of methane it contains, with faint rings and two dozen moons. Learn more about this not-so-gentle gas giant.

View a gallery of notable comets

The collection features 1973's Kohoutek, which had been dubbed the "comet of the century," 1965's Ikeya-Seki—one of the brightest comets of the millennium—and 2002's NEAT, a comet that survived a coronal mass ejection as it passed near the sun.

The ancient Greeks and Romans viewed comets and meteors as divine omens

Both considered the appearance of a comet or its fragments as a precursor to significant events, such as the birth of a notable figure or the death of a ruler. One observed in 44 BCE was believed to be a sign of the deification of Julius Caesar after his assassination.

As of 2025, scientific evidence strongly supports 3I/ATLAS being a comet

Claims of alien technology would require the celestial object to transmit measurable technological signals, which were not detected by the world's largest fully steerable radio telescope. Previous telescope observations identified that 3I/ATLAS's chemical composition, structure, and orbital characteristics are consistent with those of an interstellar comet.

The importance of the mesosphere, the coldest part of Earth's atmosphere

Atmospheric gases are dense enough in this region to begin burning up debris traveling toward Earth's surface, producing meteor showers, or shooting stars. Although we normally associate decreasing temperatures with increasing altitude, temperatures increase beyond the mesosphere layer due to the absorption of high-energy ultraviolet radiation and X-rays.

Find which comets are currently visible in the night sky

Incorporating sightings reported in the Comet Observation database and information from NASA's Jet Propulsion Laboratory, this list provides a finder chart, star map, tracker, simulated orbit, and key properties for each comet, all viewable based on a user's input position on Earth.

One of the hottest known exoplanets seems to snow an active ingredient of sunscreen, titanium dioxide—but only on the dark side of the planet.

Mars once had oceans and a thick atmosphere

It formed over millions of years as small clumps of matter orbiting the sun smashed together, solidified, and cooled. Geological features indicate the presence of past surface water, which evaporated away due to the planet's weak gravitational field.

In 1994, fragments of comet Shoemaker-Levy 9 crashed into Jupiter

Hubble captured the collisions, which created massive black impact sites visible from space. It represented the first time scientists could prepare for, and observe, an interplanetary collision in real time, allowing them to gather valuable information about Jupiter's atmosphere.

Solar wind may have contributed to Mars losing its atmosphere

Scientists believe that over hundreds of millions of years, energetic particles from the sun colliding with atmospheric particles, flinging them away from Mars' weak gravitational field like colliding billiard balls. As atmospheric pressure decline, water also evaporated away from the surface.

The sun emits a continuous stream of plasma known as the solar wind

Composed of charged particles—electrons and ions—the solar wind is deflected around the Earth by the planet's magnetic field. More powerful solar storms, known as coronal mass ejections, are short-lived but produce huge clouds of plasma that disrupt technology upon reaching Earth.

Diamond rain is thought to form within Uranus and Neptune, whose interiors can compress carbon atoms from methane into these falling crystals.

Watch a visualization of the formation of the planets

The nebular hypothesis proposes that a swirling cloud of gas and dust coalesced into a dense center, forming a star amid nuclear fusion. Lighter elements were blown far from the star while heavier elements remained nearby, serving as primary materials for terrestrial and jovian planets, respectively.

Deep Impact was the first mission to probe beneath a comet's surface

The NASA mission sent a probe to deliberately crash into comet Tempel 1, producing an explosion equivalent to 4.7 tons of TNT. The payload included a compact disk with more than 625,000 names of people who participated in the "Send Your Name to a Comet!" campaign.

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.