This illustration shows an imagined view of NASA’s Double Asteroid Redirection Test (DART) spacecraft approaching the asteroids Didymos and Dimorphos. The smaller spacecraft is LICIACube, built by the Italian Space Agency.
Credit: NASA/Johns Hopkins, APL/Steve Gribben

Scientists and engineers have designed spacecraft for many different purposes. Some bring people safely to the moon or the International Space Station. Others roam far into space to send pictures back to Earth for scientists to study. The National Aeronautics and Space Administration (NASA) built the Double Asteroid Redirection Test (DART) as a punching bag! DART was a spacecraft that intentionally collided with an asteroid.

Scientists planned the mission to find out whether a spacecraft collision could change an asteroid’s path through space. DART’s target asteroid was not a threat to Earth. But in the future, this method could be used to redirect a threatening asteroid away from Earth. Scientists are studying the effects of the DART impact to determine how the collision affected the asteroid’s path.

DART was a type of spacecraft called an impactor. An impactor smashes into the target it is studying, such as a planet, moon, or asteroid. Usually, scientists study the effects of the impact to learn about the physical characteristics of the target. Because DART was designed to move its target, it is considered a kinetic impactor.

DART traveled to Didymos, an asteroid that is about ½ mile (780 meters) wide. Didymos has a moonlet—a second, tiny asteroid in orbit around it—called Dimorphos. Dimorphos is sometimes nicknamed “Didymoon.” Dimorphos is 525 feet (160 meters) wide. Pairs of asteroids such as this one are known as binary systems. Scientists think about 15 percent of the asteroids closest to Earth are part of a binary system. Didymos and Dimorphos were chosen for the DART mission because their position is practical for a spacecraft to reach and because changes to Dimorphos’s orbit can be measured from Earth.

On Sep. 26, 2022, DART smashed into Dimorphos at a speed of 4.1 miles (6.6 kilometers) per second. Telescopes on Earth observed a bright flash at the moment of impact. Before the impact, Dimorphos orbited Didymos once every 11 hours and 55 minutes. The impact was expected to shorten this period and to move Dimorphos slightly closer to Didymos.

DART carried only one instrument, a camera. The camera helped it to steer automatically toward its target. DART also carried a shoebox-sized spacecraft called LICIACube. DART released LICIACube before it impacts Dimorphos. LICIACube photographed the impact test and its aftermath.

The National Aeronautics and Space Administration (NASA) launched DART on Nov. 24, 2021. The mission was sponsored by NASA’s Planetary Defense Coordination Office. The Italian Space Agency (ASI) built LICIACube. It is Italy’s first deep space probe.

January 4, 2018

Astronomers at the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) observatory at the summit of the dormant volcano Haleakala in Hawaii recently detected a mysterious object speeding through our solar system. This by itself was not unusual. One of the main missions of the Pan-STARRS project is to detect near-Earth objects in space that could possibly collide with our planet. However, researchers quickly realized that this space rock did not move like the asteroids and comets they routinely encounter. Astronomers quickly learned that this object was a visitor from beyond our solar system—the first interstellar asteroid ever observed.

This artist’s impression shows `Oumuamua the interstellar asteroid. Credit: ESO/M. Kornmesser

Scientists at NASA’s Center for Near-Earth Object Studies (CNEOS) in Pasadena, California, determined the object was an asteroid, now officially designated as A/2017 UI. Its composition and its unusual path through our solar system showed it was from outer space. Researchers at Pan-STARRS named the asteroid `Oumuamua, which is a Hawaiian word that means messenger.

`Oumuamua appeared only as a faint spot on the Pan-STARRS telescope as it zipped through our solar system at up to 196,000 miles per hour (355,431 kilometers per hour). But later images from other observatories showed that `Oumuamua was 1,300 feet (400 meters) long and about 10 times as long as it was wide, and spinning rapidly. `Oumuamua’s elongated cigar shape is a rarity among space objects. Most asteroids are compact and lumpy after having been battered by countless random collisions with other objects on their long journeys through space. `Oumuamua is also dark red, a color created as its rocky surface was bombarded by high-energy cosmic rays for millions or perhaps billions of years. A/2017 UI traveled in a retrograde orbit (opposite that of the planets) around the sun on its way toward the constellation Pegasus.

Some researchers suggested that `Oumuamua was more than just an interstellar oddity, and that it was perhaps a message from a distant alien civilization. They pointed out that “a cigar or needle shape is the most likely architecture for an interstellar spacecraft, since this would minimize friction and damage from interstellar gas and dust.” Several groups scanned `Oumuamua for alien radio transmissions, including researchers at the Green Bank Telescope in West Virginia, the William Herschel Telescope in Spain’s Canary Islands, and the European Southern Observatory’s Very Large Telescope in Chile. No alien messages were found, and the consensus was that `Oumuamua was odd but completely natural. Scientists suspect that the so-called “space cucumber” has an icy core—as do most comets—beneath its hard carbonic shell.

October 13, 2017

Yesterday, on Thursday, October 12, a house-sized asteroid buzzed Earth, passing within the orbit of the moon and uncomfortably close to hundreds of orbiting communications and weather satellites. Officials at the United States National Aeronautics and Space Administration (NASA) insisted there was never any danger of a collision. They first discovered the space rock, called 2012 TC4, five years ago and have been following it ever since. The close shave by TC4 posed no threat, but it did provide NASA with an opportunity to test systems that detect orbiting objects and to develop plans to respond to a potentially catastrophic impact.

This illustration depicts the safe flyby of asteroid 2012 TC4 as it passes by Earth on Oct. 12, 2017. Credit: NASA/JPL-Caltech

Technicians from NASA and the European Space Agency’s Near-Earth Object program measured TC4 to be about 50 to 100 feet (15 to 30 meters) in diameter. It is traveling through space at about 16,000 miles (25,800 kilometers) per hour. At its closest, it will pass by southern Australia a mere 27,300 miles (44,000 kilometers) above the surface, just beyond the 22,400-mile (36,000-kilometer) plane of Earth’s most remote geosynchronous (fixed-orbit) satellites.

This illustration shows the path of asteroid 2012 TC4 as it passes within the moon’s orbit of Earth on Oct. 12, 2017. Credit: NASA/JPL-Caltech

Technicians at the space agencies determined that TC4 makes an elongated orbit around the sun every 609 days. They calculate that TC4 will pass close to Earth again in 2050 and 2079. For now, the calculations show that there is no danger of TC4 striking Earth. However, should TC4’s orbit be deflected by even a small amount, the chances of an impact could increase. Officials say there is a 1-in-750 chance that TC4 could strike Earth sometime after 2050. Those are long odds, to be sure, but NASA scientists continue to track TC4 nonetheless, as well as 12 other asteroids with a greater risk of impact.

Scientists know that Earth has been struck by asteroids and meteors repeatedly over the past 4.5 billion years. NASA experts estimate that objects the size of TC4 fly past Earth about three times per year. They point out, however, that the chances of a serious asteroid impact anytime soon are remote. But even the experts are occasionally taken by surprise. The Chelyabinsk meteor, a chunk of rock similar in size to TC4, fell into Earth’s atmosphere and exploded over the city of Chelyabinsk in southeastern Russia on Feb. 15, 2013. The impact frightened thousands of people and took NASA and other space-observing agencies by surprise. They did not see that one coming.

May 20, 2016

Dinosaur lovers know that a huge asteroid played a significant role in the ancient animal group’s extinction. An asteroid 65 million years ago led to the extinction of all dinosaurs except birds. But many other asteroids have struck Earth, some of them even larger than the one that ended the dinosaurs’ reign at the end of the Cretaceous. A team of researchers led by Andrew Glikson of the Australian National University has discovered evidence of an impact billions of years ago that was so colossal that it reshaped the very face of the earth. They published their findings in the journal Precambrian Research.

The impact from a huge asteroid that struck Earth some 3.5 billion years ago would have vaporized the space rock, leaving behind tiny glass spherules. (The spherules are the tiny spots to the left of blue markers giving sizes in microns.) Credit: © A. Glikson, Australian National University

An asteroid is a rocky or metallic object smaller than a planet that orbits a star. Scientists think that asteroids are made up of material left over from the formation of the solar system. Such processes as erosion and volcanic activity have altered the planets and moons. But much of the material in asteroids is almost unchanged. Sometimes, these objects cross paths with Earth. The atmosphere protects the Earth from most asteroid strikes. Air friction will disintegrate an asteroid smaller than about 160 feet (50 meters) in diameter before it can reach the surface. But larger objects can reach Earth, forming impact craters and causing other damage.

Glikson and his team studied sediments in Marble Bar, Western Australia. The region has some of the oldest sediment on Earth. They analyzed small glass beads called spherules and determined that they formed from material vaporized during a huge impact event. They dated the impact to have occurred about 3.5 billion years ago. Other metals consistent with those found in asteroids were also discovered in the sediment. The team estimated that the asteroid measured 12 to 18 miles (20 to 30 kilometers) across. An impact with such an object would have caused earthquakes greater than magnitude 10 on the Richter scale and sent tsunamis hundreds of feet (or meters) high crashing to shore.

This asteroid struck an Earth that was very different from the one we know today. Although such an impact would have wrecked entire ecosystems if it occurred today (or in the time of the dinosaurs), there was very little life to disrupt 3.5 billion years ago. Life probably formed around the same time as the impact and consisted of water-dwelling, single-celled organisms. There was no life on land. Earth also looked nothing like it does today. Since the impact, the actions of plate tectonics have continuously reshaped the planet and destroyed the crater left by the asteroid. Thus, scientists will probably never learn where it struck. But the research done by Glikson and his team will help us better understand the history of the early solar system and the conditions on Earth just as life got its start.

March 9, 2016

Next month, in April, a deep-ocean drilling project will begin off the Yucatán coast in the Gulf of Mexico. Most such oceanic drill projects are concerned with oil exploration. This isn’t your usual drill team, however. The drillers in this case come from the International Ocean Discovery Program, the National University of Mexico, and the University of Texas. And they will be drilling into Chicxulub Crater, an impact crater formed by a giant asteroid that helped kill the dinosaurs some 65 million years ago. They won’t be looking for oil; they’ll be looking for answers.

The Chicxulub Crater along the northern coast of Mexico’s Yucatán Peninsula formed when an asteroid hit the earth about 65 million years ago. Debris from the impact may have led to the extinction of the dinosaurs.
Credit: WORLD BOOK map

At the end of the Cretaceous Period, dinosaurs were the dominant land animals on the planet. Other reptiles filled the seas, and birds—descendants of dinosaurs—roamed the skies. Mammals existed, but they were far smaller and less common than today. Then, an asteroid at least 6 miles (10 kilometers) wide slammed into the Yucatán Peninsula, in present-day Mexico. The impact threw up large amounts of gas and dust into the atmosphere. This material would have blotted out the sun for many years. Plants that use sunlight to make their food would have died out, followed by the animals that ate them. With no more prey animals, large carnivores starved as well.

When the dust settled, about half of all species on Earth had gone extinct. All the dinosaurs—except birds—were dead. Only a few kinds of other reptiles survived. Mammals survived too and over time evolved to become the dominant large-bodied animals on land and in the sea.

The Yucatán asteroid formed a large impact crater, which is now partly on the peninsula and partly in the Gulf of Mexico. Tens of millions of years of plate tectonics and erosion have taken their toll on the crater, and it is barely visible in satellite images today. But the mark that it left in the rocks should still be clear.

The team plans to use a drilling ship to sample rock deep beneath the ocean floor. In drilling into the crater, scientists hope the presence (or lack) of microfossils (tiny preserved remains of ancient organisms) will teach them more about the nature of the asteroid impact and how quickly life returned to the area afterwards.

The drilling evidence may also better explain how responsible the impact was for the mass extinction at the end of the Cretaceous Period. Most scientists agree that the asteroid did most of the damage, but others argue that other causes, such as massive volcanic activity in present-day India, were more to blame. Such extinction hypotheses are not mutually exclusive, however: it may have taken more than one disaster to knock out the dinosaurs. In fact, they may even be linked. Recent studies have suggested that the Yucatán impact caused the spike in volcanism on the other side of the globe in India. Whatever the case, the drilling team will help get to the bottom of this and other stories, as well as to the bottom of the impact crater itself!

January 21, 2015

The photos are fuzzy, but astronomers eagerly studying the first images of the dwarf planet Ceres taken by NASA’s Dawn spacecraft think they may be seeing craters. Astronomers wouldn’t be surprised to learn that Ceres, like many planets and moons in the solar system, has been whacked by space rocks during its billion-year history. Still, the images are the clearest yet of Ceres–three times better than the previous best, which were taken by the Hubble Space Telescope. The new images were taken when Dawn was about 238,000 miles (383,000 kilometers) away from Ceres, about the distance between Earth and the moon. Over the next six weeks, as Dawn speeds closer and closer, astronomers will be able to determine if the surface of Ceres really is dotted by craters. And if all goes well, that finding will be only the first of many discoveries about this largely mysterious object. Astronomers expect to learn much more, especially after Dawn enters orbit around Ceres on March 6. Launched in 2007, Dawn was designed to study both Ceres and Vesta, the third-largest asteroid in the solar system.

The dwarf planet Ceres appears in both visible and infrared light in some of the first images of Ceres taken by the Dawn spacecraft. The craft was 238,000 miles (383,000 kilometers) away at the time. In the infrared image, which is really a temperature map of Ceres, white is warmer and red is colder. (NASA/JPL-Caltech/UCLA/ASI/INAF)

Ceres, which is shaped like a slightly squashed sphere, is about 590 miles (950 kilometers) in diameter. It is so large that it accounts for more than one-fourth of the total mass (amount of matter) of all of the millions of asteroids in the Main Belt, a region between the orbits of Mars and Jupiter. In fact, Ceres has enough mass to be considered a dwarf planet.

On entering obit around Ceres, Dawn will become the first spacecraft to circle two deep-space objects. The craft reached Vesta for a one-year study in 2011. While on this mission, Dawn revealed that Vesta has three layers–an inner core, middle layer called the mantle, and an outer crust. Scientists think both Ceres and Vesta probably formed early in the solar system’s history from many smaller bodies that collided and stuck together, a process called accretion. However, the gravitational pull of Jupiter likely prevented the two from combining with other objects and growing to the size of a planet.

Additional World Book articles:

• Space exploration
• Exploring the Suburban Solar System (a special report)

April 22, 2014

The sky turned blood red. Heavy clouds of dust choked the atmosphere. Tsunamis thousands of feet high raced across Earth’s oceans, as the surface water boiled off. The entire planet shook for 30 minutes, triggering powerful earthquakes. The cause of this cataclysm was a massive asteroid about as wide as the state of Rhode Island that slammed into Earth 3.26 billion years ago. And in a remarkable piece of research, scientists from Stanford University in California have been able to reconstruct this event by studying tiny particles of rock about the size of the head of a pin.

The asteroid came down during an early period of Earth’s history called the Late Heavy Bombardment, which lasted from about 4.1 billion to 3.8 billion years ago. Earth, at this time, has been described as a “shooting gallery,” as large numbers of rocky bodies from the still-forming solar system pummeled the planet. Finding evidence from these impactors is difficult. Although the crater created by the asteroid in the study was some 300 miles (500 kilometers) across, it has long since vanished. It was erased by erosion and the movement of the tectonic plates that make up Earth’s outer crust. What remains from the impact are spherules, solid spheres created when rock vaporized and thrown into the atmosphere by the impact falls back to Earth.

Geologist Donald Loew discovered the spherules about a decade ago in South Africa at a site known as the Barberton greenstone belt. There, the spherules cover some of the oldest rocks on Earth. To reconstruct the size of the asteroid, Loew and his co-author, physicist Norman Sleep, used information about the thickness and chemical composition of the spherule layer. Then they created a computer model to reconstruct the impact, the first model created in such detail for an event older than 3 billion years. The scientists were surprised by the results. “We knew it [the impact] was big, but we didn’t know how big,” Loew said.

The asteroid thought to have crashed into Earth 3.26 billion years ago dwarfed the asteroid linked to the extinction of the dinosaurs 65 milion years ago. (American Geophysical Union)

The scientists found that the asteroid was from 22 to 37 miles (37 to 59 kilometers) in diameter. That’s three times as wide as the asteroid linked to the extinction of the last of the dinosaurs 65 million years ago. When the asteroid hit Earth, it was traveling at a speed of 44,000 miles (71,000 kilometers) per hour. That initial impact created seismic waves as strong as those of a magnitude 10.8 earthquake, larger than any earthquake ever recorded.

The impact, the scientists said, may have had significant effects. It could have wiped out many of the microscopic, single-celled organisms that comprised life on Earth at the time, thus altering the course of evolution. The impact may also have broken up or shifted the direction of some of Earth’s tectonic plates, triggering the beginning of the current system of plate movement.

Additional World Book articles:

• The Early Earth (a Special Report)
• Stones from Space (a Special Report)
• When Worlds and Comets Collide (a Special Report)

May 31, 2013

A large asteroid that is swinging past Earth today is bringing along a surprise companion–a moon. Asteroid 1998 QE2 will make its closest approach to Earth at a comfortable distance of 3.6 million miles (5.8 million kilometers) at 4:59 p.m. Eastern Daylight Savings Time. That distance is about 15 times as far as the orbit of Earth’s moon, so there’s no chance that the asteroid could crash into us.

Scientists have been tracking the asteroid since its discovery on August 19, 1998, by the Massachusetts Institute of Technology Lincoln Near Earth Asteroid Research program in Socorro, New Mexico. Radar images of the asteroid suggest that it is about 1.7 miles (2.7 kilometers) in diameter. The moon is an estimated 2,000 feet (600 meters) across, more than 15 times the length of a football field. Only about 16 percent of asteroids larger than 655 feet (200 meters) in diameter belong to systems with two or more objects.

A series of radar images show asteroid 1998 QE2 as it appeared when it was about 3.75 million miles (6 million kilometers) from Earth. The small white dot at lower right in the images is the asteroid's moon. (NASA/JPL-Caltech/GSSR)

Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, began observing the asteroid and its moon on May 29 using two radar telescopes—the Deep Space Network antenna at Goldstone, California, and the Arecibo Observatory in Puerto Rico. The radar observations, which will continue until June 9, will allow the scientists to determine the size and shape of the asteroid and its moon and to study the objects’ surface features. The first radar images of the asteroid revealed dark areas that may be impact craters or another kind of depression in the surface.

NASA’s Near-Earth Object Program identifies and tracks comets and asteroids that move around the sun near Earth’s orbit. These comets and asteroids are called near-Earth objects or NEO’s. The major role of the program is to determine if any NEO’s could strike Earth. By the end of 2008, the program had discovered and tracked more than 90 percent of the NEO’s larger than 0.62 mile (1 kilometer). The program is now focusing on finding 90 percent of NEO’s larger than 450 feet (140 meters). As of mid-2013, the program had found 9,858 NEO’s. At least 860 of them have a diameter of approximately 0.62 mile.

Additional World Book articles:

• Dawn
• Eros
• Vesta
• When Worlds and Comets Collide (a special report)
• What Has Caused Mass Extinctions (a special report)

March 22, 2013

Scientists at the 44th Lunar and Planertary Science Conference (LPSC) held this week announced that the object that hit Earth 65 million years ago, leading to massive extinctions of species across the planet, may have been smaller than previously believed. Scientists have long thought that the object that impacted Earth was a huge, relatively slow-moving asteroid. This belief was based in part upon the stratum (layer) of Earth discovered by the American physicist Luis Alvarez and and his son, geologist Walter Alvarez. This layer, called the Cretaceous-Paleogene Boundary (at one time called the Cretaceous-Tertiary, or K-T boundary), contains a large amount of the chemical element iridium (Ir). The element is very rare in Earth’s crust but is common in such space bodies as asteroids and meteorites.

Instead of a large asteroid, as shown below, some scientists now believe a much smaller comet, like the one shown above, led to the extinction event that occurred at the end of the Cretaceous Period. (Lick Observatory)

(NASA/JPL-Caltech)

The iridium layer was not the only evidence that led scientists to think a large asteroid had struck Earth. There is also evidence of the impact. The Chicxulub crater in Mexico’s Yucatan Peninsula, is more than 112 miles (180 kilometers) in diameter and dates to the same geologic time period as the iridium layer; most scientists think the object that caused this crater is the object that spread a layer of iridium over the Earth and led to the extinction of the dinosaurs and many other animals and plants. Based on the level of iridium deposited and the size of the crater, scientists thought that only a large asteroid could have caused such an event.

Professor Jason Moore, a paleoecologist at Dartmouth College, presented findings at the 2013 conference showing that the geochemistry of the Chicxulub crater shows lower iridium levels than originally reported and lower levels than he believes could be created by a body as large as an asteroid. Dr. Moore believes that a body that could leave as little iridium and leave such a huge crater could not be a large asteroid. Moore believes the body that created the crater in Mexico was a small, fast-moving comet.

Comet or asteroid, the effect on the dinosaurs was devastating. Except, of course, for a group evolved from feathered dinosaurs that still does very well on Earth–the vertebrate class Aves, or birds.

Additional World Book articles:

• Cretaceous
• Dinosaur
• Extinction
• Geology (Back In Time)

February 14, 2013

An asteroid barreling through space will pass Earth tomorrow by only about 17,200 miles (27,700 kilometers), the closest encounter with such a large space rock that researchers have ever known about in advance. Earth’s nearest neighbor in space, the moon, is 225,000 miles (362,000 kilometers) from Earth, and even some artificial satellites are farther away from Earth. Traveling at the rate of 68,800 miles (110,700 kilometers) per hour, the asteroid–named 2012 DA14–will miss Earth by only 15 minutes. Scientist and educator Bill Nye, “the science guy,” estimates that the diameter of 2012 DA14 is from 130 to 400 feet (40 to 90 meters). Asteroids of similar size created Arizona’s Meteor Crater and caused the 1908 Tunguska explosion in Siberia.

Scientists believe that asteroids are chunks of material left over from the formation of the solar system. Most asteroids orbit the sun between the orbits of the planets Mars and Jupiter, in a region known as the Main Belt. The gravitational pull of Jupiter probably prevented these pieces from coming together to form a full-sized planet. Some asteroids may represent the nuclei (cores) of comets that are no longer active.

The asteroid known as 2012 DA14 will pass closer to Earth than the orbit of many satellites. (NASA/JPL Near-Earth Object Program)

The atmosphere protects Earth from most asteroid strikes. Air friction causes an asteroid smaller than about 160 feet (50 meters) in diameter to disintegrate before it can reach the surface and cause damage. Larger asteroids can impact the environment of the entire globe. The impact of an asteroid with a diameter of around 0.6 mile (1 kilometer) would kick large amounts of dust into the atmosphere. The dust would block sunlight, which would cool the air for many months. One such strike occurred 65 million years ago on what is now Mexico’s Yucatan Peninsula. Many scientists think environmental damage caused by the collision led to a mass extinction. The extinction eliminated huge numbers of species, including the dinosaurs.

Additional World Book articles:

• Dawn
• Eros
• Vesta
• When Worlds and Comets Collide (a special report)
• What Has Caused Mass Extinctions (a special report)