Asteroids are rocky worlds revolving around the sun that are too small to be called planets. They are also known as planetoids or minor planets. There are millions of asteroids, ranging in size from hundreds of miles to several feet across. In total, the mass of all the asteroids is less than that of Earth's moon.
Despite their size, asteroids can be dangerous. Many have hit Earth in the past, and more will crash into our planet in the future. That's one reason scientists study asteroids and are eager to learn more about their numbers, orbits and physical characteristics. If an asteroid is headed our way, we want to know about it.
Where are asteroids found?
Scientists have identified more than 1 million asteroids to date, according to NASA.
Asteroids lie primarily within three regions of the solar system. Most asteroids lie in a vast ring between the orbits of Mars and Jupiter. This main asteroid belt holds more than 200 asteroids larger than 60 miles (100 km) in diameter. Scientists estimate the asteroid belt also contains between 1.1 million and 1.9 million asteroids larger than 1 km (3,281 feet) in diameter and millions of smaller ones, according to NASA.
Not everything in the main belt is an asteroid — Ceres, once thought of only as an asteroid, is now also considered a dwarf planet. In the past decade, scientists have also identified a class of objects known as "main belt comets," small rocky objects with tails. While some of the tails form when objects crash into an asteroid, or by disintegrating asteroids, others may be comets in disguise.
Many asteroids lie outside the main belt.
For example, Trojan asteroids orbit the sun along the same path as a larger planet in two special places about 60 degrees ahead of and behind the planet. At these locations, known as Lagrange points, the gravitational pull of the sun and the planet are balanced. Jupiter has the most Trojans with more than 10,000 such objects, according to the International Astronomical Union's database. Other planets have a few Trojans: Neptune has 30, Mars has nine and Earth and Uranus each have one that scientists have identified to date.
Scientists also suspect that many of the solar system's moons were once asteroids, until they were captured by a planet's gravity and became satellites. Likely candidates include Mars' moons, Phobos and Deimos, and most of the outer moons of Jupiter, Saturn, Uranus and Neptune.
Near-Earth asteroids (NEAs) circle the sun at about the same distance as Earth does. These objects are split into sub-categories based on how the asteroid's orbit compares to Earth's, according to NASA.
For example, Amor asteroids have orbits that approach Earth's path but remain exclusively between Earth and Mars. Apollo asteroids have Earth-crossing orbits but spend most of their time outside the planet's path. Aten asteroids also cross Earth's orbit but spend most of their time inside Earth's orbit. Atira asteroids are near-Earth asteroids whose orbits are contained within Earth's orbit.
Astronomers also classify certain near-Earth asteroids as "Potentially Hazardous Asteroids" or PHAs. These rocks come within about 4.65 million miles (7.48 million kilometers) of Earth's orbit and are larger than about 500 feet (140 meters) across, according to NASA's Center for Near-Earth Object Studies (CNEOS). However, the classification does not imply that the asteroid poses a certain threat to Earth.
As of October 2021, scientists have discovered more than 27,000 near-Earth asteroids, according to CNEOS. Of these, just under 10,000 have diameters larger than 500 feet.
How are asteroids found?
In 1801, while making a star map, Italian priest and astronomer Giuseppe Piazzi accidentally discovered the first and largest asteroid, Ceres, orbiting between Mars and Jupiter. Although Ceres is classified today as a dwarf planet, it accounts for a quarter of all the mass of all the known asteroids in or near the main asteroid belt.
Since about 2000, NASA has spearheaded a campaign to identify and track near-Earth asteroids. Programs like the Catalina Sky Survey in Arizona and the Pan-STARRS telescopes in Hawaii specialize in identifying these objects and have each discovered thousands of asteroids, according to CNEOS.
How did asteroids form?
Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.
Understanding of how the solar system evolved is constantly expanding. Two fairly recent theories, the Nice model and the Grand Tack, suggest that the gas giants moved around before settling into their modern orbits. This movement could have sent asteroids from the main belt raining down on the terrestrial planets, emptying and refilling the original belt.
What are asteroids like?
Nearly all asteroids are irregularly shaped, although a few of the largest are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter. The surfaces of most asteroids are thought to be covered in dust.
As asteroids revolve around the sun in their elliptical orbits, they also rotate, sometimes tumbling quite erratically. More than 150 asteroids are also known to have a small companion moon, according to NASA, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, as do triple asteroid systems.
The average temperature of the surface of a typical asteroid is minus 100 degrees Fahrenheit (minus 73 degrees Celsius). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system.
Asteroids come in a variety of shapes and sizes. Some are solid bodies, while others are smaller piles of rubble bound together by gravity. One, which orbits the sun between Neptune and Uranus, comes with its own set of rings. Another has not one but six tails. Many asteroids also sport moons.
What types of asteroids are there?
Most asteroids fall into one of three classes based on their composition:
The C-type or carbonaceous asteroids are grayish in color and are the most common, including more than 75% of known asteroids. They probably consist of clay and stony silicate rocks, and inhabit the main belt's outer regions.
The S-type or silicaceous asteroids are greenish to reddish in color, account for about 17% of known asteroids, and dominate the inner asteroid belt. They appear to be made of silicate materials and nickel-iron.
The M-type or metallic asteroids are reddish in color, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickel-iron.
There are many other rare types based on composition as well — for instance, V-type asteroids typified by Vesta have a basaltic, volcanic crust.
Do asteroids hit Earth?
Ever since Earth formed about 4.5 billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA.
An asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a "nuclear winter," severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say.
Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis. According to NASA, space rocks smaller than 82 feet (25 m) will most likely burn up as they enter Earth's atmosphere.
On Feb. 15, 2013, an asteroid slammed into the atmosphere over the Russian city of Chelyabinsk, creating a shock wave that injured 1,200 people. The space rock is thought to have measured about 65 feet (20 m) wide when it entered Earth's atmosphere.
What is a meteorite?
When an asteroid, or a part of it, crashes into Earth, it's called a meteorite. Here are typical compositions:
- Iron: 91%
- Nickel: 8.5%
- Cobalt: 0.6%
- Oxygen: 6%
- Iron: 26%
- Silicon: 18%
- Magnesium: 14%
- Aluminum: 1.5%
- Nickel: 1.4%
- Calcium: 1.3%
Can we protect Earth from asteroids?
Dozens of asteroids have been classified as "potentially hazardous" by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet. Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react. Though the technology would have to be developed, possibilities include exploding the object or diverting it.
Image gallery: Potentially dangerous asteroids
For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening.
When asteroids do close flybys of Earth, one of the most effective ways to observe them is by using radar, such as the system at NASA's Goldstone Deep Space Communications Complex in California. In September 2017, the near-Earth asteroid 3122 Florence cruised by Earth at 4.4 million miles (7 million km), or 18 times the distance to the moon. The flyby confirmed its size (2.8 miles or 4.5 km) and rotation period (2.4 hours). Radar also revealed new information such as its shape, the presence of at least one big crater, and two moons.
In a NASA broadcast from earlier in 2017, Marina Brozovic, a physicist at NASA's Jet Propulsion Laboratory, said radar can reveal details such as its size, its shape, and whether the asteroid is actually two objects (a binary system, where a smaller object orbits a larger object.) "Radar is a little bit like a Swiss army knife," she said. "It reveals so much about asteroids all at once."
In the unlikely event that the asteroid is deemed a threat, NASA has a Planetary Defense Coordination Office that has scenarios for defusing the situation. In the same broadcast, PDCO planetary defense officer Lindley Johnson said the agency has two technologies at the least that could be used: a kinetic impactor (meaning, a spacecraft that slams into the asteroid to move its orbit) or a gravity tractor (meaning, a spacecraft that remains near an asteroid for a long period of time, using its own gravity to gradually alter the asteroid's path.)
NASA's Double Asteroid Redirection Test (DART) mission, scheduled to launch in November 2021, will test the kinetic impactor approach on the small moon of a near Earth asteroid called Didymos. DART will slam into the moonlet as astronomers on Earth watch to see how much its orbital period around Didymos changes.
If an asteroid did threaten Earth, PDCO would also consult with the White House and the Federal Emergency Management Agency (FEMA) and likely international space agencies to determine what to do. However, there is no known asteroid (or comet) threat to Earth and NASA carefully tracks all known objects through a network of partner telescopes.
Did asteroids bring Earth water?
Ironically, the collisions that could mean death for humans may be the reason we are alive today. When Earth formed, it was dry and barren. Asteroid and comet collisions may have delivered the water-ice and other carbon-based molecules to the planet that allowed life to evolve. At the same time, the frequent collisions kept life from surviving until the solar system calmed down. Later collisions shaped which species evolved and which were wiped out.
According to NASA's Center for Near Earth Object Studies (CNEOS), "It seems possible that the origin of life on the Earth's surface could have been first prevented by an enormous flux of impacting comets and asteroids, then a much less intense rain of comets may have deposited the very materials that allowed life to form some 3.5 - 3.8 billion years ago."
How are asteroids named?
Over the first half of the 19th century, several asteroids were discovered and classified as planets. William Herschel coined the phrase "asteroid" in 1802, but other scientists referred to the newfound objects as minor planets. By 1851, there were 15 new asteroids, and the naming process shifted to include numbers, with Ceres being designated as (1) Ceres. Today, Ceres shares dual designation as both an asteroid and a dwarf planet, while the rest remain asteroids.
Since the International Astronomical Union is less strict on how asteroids are named when compared to other bodies, there are asteroids named after Mr. Spock of "Star Trek" and rock musician Frank Zappa, as well as more solemn tributes, such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Naming asteroids after pets is no longer allowed.
Asteroids are also given numbers — for example, 99942 Apophis.
The first spacecraft to take close-up images of asteroids was NASA's Galileo in 1991, which also discovered the first moon to orbit an asteroid in 1994.
In 2001, after NASA's NEAR spacecraft intensely studied the near-earth asteroid Eros for more than a year from orbit, mission controllers decided to try and land the spacecraft. Although it wasn't designed for landing, NEAR successfully touched down, setting the record as the first to successfully land on an asteroid.
In 2006, Japan's Hayabusa mission became the first spacecraft to land on and take off from an asteroid when it visited the near-Earth asteroid Itokawa. Although the spacecraft encountered a series of technical glitches, it returned a small amount of asteroid material to Earth in June 2010.
NASA's Dawn mission launched in 2007 bound for the main asteroid belt and began exploring Vesta in 2011. After a year of work there, it left the asteroid for a trip to Ceres, arriving in 2015. Dawn was the first spacecraft to visit either Vesta and Ceres. The mission ended in 2018 when the spacecraft ran out of fuel, although it will continue orbiting Ceres for about 50 years.
Japan built on its Hayabusa experience to build a second asteroid sample-return mission, dubbed Hayabusa2. The spacecraft visited a near-Earth asteroid called Ryugu and studied the body for about 18 months. That work included deploying small hopping rovers and blasting the asteroid with an artificial crater. In December 2020, like its predecessor, Hayabusa2 delivered pieces of Ryugu to Earth for scientists to study with more advanced technology than they can send on spacecraft.
Nearly simultaneously, NASA also flew its own sample-return mission to a near-Earth asteroid. In September 2016, Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) launched to explore the asteroid Bennu and collect a sample. The spacecraft is now trekking back to Earth, with delivery scheduled for September 2023.
In 2021, NASA will attempt to launch the first-ever mission to the Trojan asteroids out in Jupiter's orbit. The mission, called Lucy, will fly past one main-belt asteroid and seven Trojans. Scientists hope that by snapping photos of a broad range of Trojans, they can begin to understand why these objects are so diverse, and how their story intersects with that of the solar system at large. Lucy will make its first flyby in 2025, will make its first Trojan flyby in 2027 and is currently scheduled to operate until 2033.
Also in 2021, NASA will launch its first-ever planetary defense mission to an asteroid. The DART spacecraft will slam into the small moon of the asteroid Didymos in order to test a technique scientists might be able to use on an asteroid threatening Earth. The impact will occur in late September 2022.
In 2022, NASA will launch the Psyche mission to study an asteroid of the same name. Scientists believe that Psyche, which is located in the main asteroid belt, contains much higher amounts of metal than most asteroids do. The oddity may mean that Psyche is the bare core of a planet that lost its rocky shell. Scientists also wonder whether metal-rich worlds like these once hosted volcanoes that spilled molten iron across the asteroid's surface. The Psyche spacecraft will arrive at its target in 2026.
Can we mine asteroids?
NASA, other space agencies and private companies are all intrigued by the possibility of extracting resources from asteroids. Water, which can be processed into rocket propellant to save spacecraft from needing to launch the weight of their return fuel, is one commonly proposed resource some are interested in extracting from asteroids, as well as from the moon.
Some people are also interested in mining metals from asteroids, arguing that there are huge amounts of money to be earned from the asteroid belt. Others say that this model is more difficult to make financially viable.
Additional reporting by Elizabeth Howell and Nola Taylor Redd, Space.com Contributors. Senior writer Meghan Bartels updated this page on Oct. 15, 2021.