What are asteroids?

Asteroids are rocky objects that orbit the Sun just like planets do. In fact, sometimes asteroids are called “minor planets.” These space rocks were left behind after our solar system formed about 4.6 billion years ago.

Asteroids are found in a wide range of sizes. For example, one small asteroid, 2015 TC25, has a diameter of about 6 feet – about the size of a small car – while the asteroid Vesta is nearly 330 miles in diameter, almost as wide as the U.S. state of Arizona. Some asteroids even have enough gravity to have one or two small moons of their own.

There are more than a million known asteroids. Many asteroids are given names. An organization called the International Astronomical Union is responsible for assigning names to objects like asteroids and comets.

What’s the difference between asteroids, meteors, and comets?

Although all of these celestial bodies orbit the Sun, they are not the same. Unlike asteroids, which are rocky, comets are a mix of dust and ice. Meteors are small space rocks that get pulled close enough to enter Earth’s atmosphere, where they either burn up as a shooting star or land on the ground as a meteorite.

What are asteroids made of?

Different types of asteroids are composed of different mixes of materials. Most of them are made of chondrites, which are combinations of materials such as rocks and clay. These are called “C-type” asteroids. Some, called “S-type,” are made of stony materials, while “M-type” asteroids are composed of metallic elements.

How did the asteroids form?

Asteroids formed around the same time and in the same way as the planets in our solar system. A massive, dense cloud of gas and dust collapsed into a spinning disk, and the gravity in the disk’s center pulled more and more material toward it. Over time, these pieces repeatedly collided with each other, sometimes resulting in smaller fragments and other times clumping together, resulting in much bigger objects.

Objects with a lot of mass – like planets – produced enough gravity to pull themselves into spheres, but many smaller objects didn’t. These ended up becoming comets, small moons, and, yes, asteroids. Although some asteroids have a spherical shape, most have irregular shapes – sometimes oblong, bumpy, or jagged.

Where are asteroids found?

Most of the asteroids we know about are located in an area called the main asteroid belt, which is found in the space between Mars and Jupiter. But asteroids are found in other parts of the solar system, too.

Trojan asteroids orbit the Sun on the same orbital path as a planet. They’re found at two specific points on the planetary orbit called Lagrange points. At these points, the gravitational pull of the planet and the Sun are in balance, making these points gravity-neutral and stable. Many planets have been found to have Trojan asteroids, including Earth.

An asteroid’s location can also be influenced by the gravity of planets it passes and end up pushed or pulled onto a path that brings it close to Earth. When asteroids or comets are on an orbital path that comes within 30 million miles of Earth’s orbit, we call them near-Earth objects.

Could an asteroid come close enough to hit Earth?

Yes! Throughout history, asteroids or pieces of asteroids have collided with Earth, our Moon, and the other planets, too. The effects of some of these impacts are still visible. For example, Chicxulub Crater was created 65 million years ago when a massive asteroid struck Mexico’s Yucatan Peninsula. The resulting cloud of dust and gas released into Earth’s atmosphere blocked sunlight, leading to a mass extinction that included the dinosaurs. More recently, in 2013, people in Chelyabinsk, Russia, witnessed an asteroid almost as wide as a tennis court explode in the atmosphere above them. That event produced a powerful shockwave that caused injuries and damaged structures.

This is why NASA’s Planetary Defense Coordination Office keeps a watchful eye on near-Earth objects. The Planetary Defense team relies on telescopes and observatories on Earth and in space to detect and monitor objects like these that could stray too close to our planet.

The agency is working on planetary defense strategies to use if an asteroid is discovered to be heading our way. For example, NASA’s DART (Double Asteroid Redirection Test) mission in 2022 was a first-of-its-kind test: an uncrewed spacecraft with an autonomous targeting system intentionally flew into the asteroid Dimorphos, successfully changing its orbit.

How does NASA study asteroids?

 NASA detects and tracks asteroids using telescopes on the ground and in space, radar observations, and computer modeling. The agency also has launched several robotic explorers to learn more about asteroids. Some missions study asteroids from above, such as the Psyche mission, launched in 2023 to study the asteroid Psyche beginning in 2029. Other missions have actually made physical contact with asteroids. For example, the DART mission mentioned above impacted an asteroid to change its orbit, and the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification and Security – Regolith Explorer) spacecraft collected a sample of material from the surface of asteroid Bennu and delivered the sample to Earth in 2023 for scientists to study.

Career Corner

Want a career where you get to study asteroids? Here are some jobs at NASA that do just that:

• Astronomer: These scientists observe and study planets, stars, and galaxies. Astronomers make discoveries that help us understand how the universe works and how it is changing. This job requires a strong educational background in science, math, and computer science.
• Geologist: Asteroids are made of different types of rock, clay, or metallic materials. Geologists study the properties and composition of these materials to learn about the processes that have shaped Earth and other celestial bodies, like planets, moons, and asteroids.

More About Asteroids

Asteroid Facts
Gallery: What’s That Space Rock?
Center for Near Earth Object Studies
Planetary Defense at NASA
Asteroid Watch: Keeping an Eye on Near-Earth Objects

During his work on several historic missions, Casani rose through a series of technical and management positions, making an indelible mark on the nation’s space program.  

John R. Casani, a visionary engineer who served a central role in many of NASA’s historic deep space missions, died on Thursday, June 19, 2025, at the age of 92. He was preceded in death by his wife of 39 years, Lynn Casani, in 2008 and is survived by five sons and their families.

Casani started at the Jet Propulsion Laboratory in Southern California in 1956 and went on to work as an electronics engineer on some of the nation’s earliest spacecraft after NASA’s formation in 1958. Along with leading the design teams for both the Ranger and Mariner series of spacecraft, he held senior project positions on many of the Mariner missions to Mars and Venus, and was project manager for three trailblazing space missions: Voyager, Galileo, and Cassini.

His work helped advance NASA spacecraft in areas including mechanical technology, system design and integration, software, and deep space communications. No less demanding were the management challenges of these multifaceted missions, which led to innovations still in use today.

“John had a major influence on the development of spacecraft that visited almost every planet in our solar system, as well as the people who helped build them,” said JPL director Dave Gallagher. “He played an essential role in America’s first attempts to reach space and then the Moon, and he was just as crucial to the Voyager spacecraft that marked humanity’s first foray into interplanetary — and later, interstellar — space. That Voyager is still exploring after nearly 50 years is a testament to John’s remarkable engineering talent and his leadership that enabled others to push the boundaries of possibility.”

Born in Philadelphia in 1932, Casani studied electrical engineering at the University of Pennsylvania. After a short stint at an Air Force research lab, he moved to California in 1956 and was hired to work at JPL, a division of Caltech, on the guidance system for the U.S. Army Ballistic Missile Agency’s Jupiter-C and Sergeant missile programs.

In 1957, the Soviet Union launched Sputnik 1, the first human-made Earth satellite, alarming America and changing the trajectory of both JPL and Casani’s career. With the 1958 launch of Explorer 1, America’s first satellite, the lab transitioned to concentrating on robotic space explorers, and Casani segued from missiles to spacecraft.

One of his jobs as payload engineer on Pioneer 3 and 4, NASA’s first missions to the Moon, was to carry each of the 20-inch-long (51-cm-long) probes in a suitcase from JPL to the launch site at Cape Canaveral, Florida, where he installed them in the rocket’s nose cone.

At the dawn of the 1960s, Casani served as spacecraft systems engineer for the agency’s first two Ranger missions to the Moon, then joined the Mariner project in 1965, earning a reputation for being meticulous. Four years later, he was Mariner project manager.

Asked to share some of his wisdom in a 2009 NASA presentation, Casani said, “The thing that makes any of this work … is toughness. Toughness because this is a tough business, and it’s a very unforgiving business. You can do 1,000 things right, but if you don’t do everything right, it’ll come back and bite you.”

Casani’s next role: project manager for NASA’s high-profile flagship mission to the outer planets and beyond — Voyager. He not only led the mission from clean room to space, he was first to envision attaching a message representing humanity to any alien civilization that might encounter humanity’s first interstellar emissaries. 

“I approached Carl Sagan,” he said in a 2007 radio interview, “and asked him if he could come up with something that would be appropriate that we could put on our spacecraft in a way of sending a message to whoever might receive it.” Sagan took up the challenge, and what resulted was the Golden Record, a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth.

Once Voyager 1 and 2 and their Golden Records launched in 1977, JPL wasted no time in pointing their “engineer’s engineer” toward Galileo, which would become the first mission to orbit a gas giant planet. As the mission’s initial project manager, Casani led the effort from inception to assembly. Along the way, he had to navigate several congressional attempts to end the project, necessitating multiple visits to Washington. The 1986 loss of Space Shuttle Challenger, from which Galileo was to launch atop a Centaur upper-stage booster, led to mission redesign efforts before its 1989 launch.

After 11 years leading Galileo, Casani became deputy assistant laboratory director for flight projects in 1988, received a promotion just over a year later and then, from 1990 to 1991, served as project manager of Cassini, NASA’s first flagship mission to orbit Saturn.

Casani became JPL’s first chief engineer in 1994, retiring in 1999 and serving on several nationally prominent committees, including leading the investigation boards of both the Mars Climate Orbiter and the Mars Polar Lander failures, and also leading the James Webb Space Telescope Independent Comprehensive Review Panel.

In early 2003, Casani returned to JPL to serve as project manager for NASA’s Project Prometheus, which would have been the nation’s first nuclear-powered, electric-propulsion spacecraft. In 2005, he became manager of the Institutional Special Projects Office at JPL, a position he held until retiring again in 2012.

“Throughout his career, John reflected the true spirit of JPL: bold, innovative, visionary, and welcoming,” said Charles Elachi, JPL’s director from 2001 to 2016. “He was an undisputed leader with an upbeat, fun attitude and left an indelible mark on the laboratory and NASA. I am proud to have called him a friend.”

Casani received many awards over his lifetime, including NASA’s Exceptional Achievement Medal, the Management Improvement Award from the President of the United States for the Mariner Venus Mercury mission, and the Air and Space Museum Trophy for Lifetime Achievement.

News Media Contacts

Matthew Segal / Veronica McGregor
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-8307 / 818-354-9452
matthew.j.segal@jpl.nasa.gov / veronica.c.mcgregor@jpl.nasa.gov

NASA, Axiom Space, and SpaceX are targeting 2:31 a.m. EDT, Wednesday, June 25, for launch of the fourth private astronaut mission to the International Space Station, Axiom Mission 4.

The mission will lift off from Launch Complex 39A at NASA’s Kennedy Space Center in Florida. The crew will travel to the orbiting laboratory on a new SpaceX Dragon spacecraft after launching on the company’s Falcon 9 rocket. The targeted docking time is approximately 7 a.m. Thursday, June 26.

This launch opportunity comes after NASA and Roscosmos officials discussed the status of the recent repair work in the transfer tunnel at the aft (back) most segment of the orbital laboratory’s Zvezda service module. Based on the evaluations, NASA and Roscosmos agreed to further lower the pressure in the transfer tunnel to 100 millimeters of mercury, and teams will continue to evaluate going forward. Safety remains a top priority for NASA and Roscosmos.

“NASA and Roscosmos have a long history of cooperation and collaboration on the International Space Station. This professional working relationship has allowed the agencies to arrive at a shared technical approach and now Axiom Mission 4 launch and docking will proceed,” said acting NASA Administrator Janet Petro. “We look forward to the launch with Axiom Space and SpaceX for this commercial international mission.”

For this mission, NASA is responsible for integrated operations, which begins during the spacecraft’s approach to the space station, continues during the crew’s stay aboard the orbiting laboratory conducting science, education, and commercial activities, and concludes once the spacecraft departs the station.

Live coverage of launch and arrival activities will stream on NASA+. Learn how to watch NASA content through a variety of platforms, including social media.

Peggy Whitson, former NASA astronaut and director of human spaceflight at Axiom Space, will command the commercial mission, while ISRO (Indian Space Research Organisation) astronaut Shubhanshu Shukla will serve as pilot. The two mission specialists are ESA (European Space Agency) project astronaut Sławosz Uznański-Wiśniewski of Poland, and HUNOR (Hungarian to Orbit) astronaut Tibor Kapu of Hungary.

Once docked, the private astronauts plan to spend about two weeks aboard the orbiting laboratory, conducting a mission comprised of science, outreach, and commercial activities.

As part of a collaboration between NASA and ISRO, Axiom Mission 4 delivers on a commitment highlighted by President Donald Trump and Indian Prime Minister Narendra Modi to send the first ISRO astronaut to the station. The space agencies are participating in five joint science investigations and two in-orbit STEM (science, technology, engineering, and mathematics) demonstrations. NASA and ISRO have a long-standing relationship built on a shared vision to advance scientific knowledge and expand space collaboration.

The private mission also carries the first astronauts from Poland and Hungary to stay aboard the International Space Station.

NASA’s mission coverage is as follows (all times Eastern and subject to change based on real-time operations):

Wednesday, June 25

12:30 a.m. – Axiom Space and SpaceX launch coverage begins.

1:40 a.m. – NASA joins the launch coverage on NASA+.

2:31 a.m. – Launch

NASA will end coverage following orbital insertion, which is approximately 15 minutes after launch. As it is a commercial launch, NASA will not provide a clean launch feed on its channels.

Thursday, June 26

5 a.m. – Arrival coverage begins on NASA+, Axiom Space, and SpaceX channels.

7 a.m. – Targeted docking to the space-facing port of the station’s Harmony module.

Arrival coverage will continue through hatch opening and welcome remarks.

All times are estimates and could be adjusted based on real-time operations after launch. Follow the space station blog for the most up-to-date operations information.

The International Space Station is a springboard for developing a low Earth economy. NASA’s goal is to achieve a strong economy off the Earth where the agency can purchase services as one of many customers to meet its science and research objectives in microgravity. NASA’s commercial strategy for low Earth orbit provides the government with reliable and safe services at a lower cost, enabling the agency to focus on Artemis missions to the Moon in preparation for Mars while also continuing to use low Earth orbit as a training and proving ground for those deep space missions.

Learn more about NASA’s commercial space strategy at:

https://www.nasa.gov/commercial-space

-end-

Joshua Finch
Headquarters, Washington
202-358-1100
joshua.a.finch@nasa.gov

Anna Schneider
Johnson Space Center, Houston
281-483-5111
anna.c.schneider@nasa.gov

NASA astronaut Zena Cardman inspects her spacesuit’s wrist mirror in this portrait taken at NASA’s Johnson Space Center in Houston on March 22, 2024. Cardman will launch to the International Space Station as part of NASA’s SpaceX Crew-11 mission. This will be her first spaceflight.

Cardman was selected by NASA as a member of the 2017 “Turtles” Astronaut Class. The Virginia native holds a bachelor’s degree in biology and a master’s degree in marine sciences from the University of North Carolina, Chapel Hill. Her research focused primarily on geobiology and geochemical cycling in subsurface environments, from caves to deep sea sediments. Cardman’s experience includes multiple Antarctic expeditions. Since completing initial training, Cardman has supported real-time station operations and lunar surface exploration planning.

This photo was one of the winners of NASA’s 2024 Photos of the Year.

Ozone high in the stratosphere protects us from the Sun’s ultraviolet light. But ozone near the ground is a pollutant that harms people and plants. The San Joaquin Valley has some of the most polluted air in the country, and NASA scientists with the new Ozone Where We Live (OWWL) project are working to measure ozone and other pollutants there. They need your help!  

Do you live or work in Bakersfield, CA? Sign up to host an ozone sensor! It’s like a big lunch box that you place in your yard, but it’s not packed with tuna and crackers. It’s filled with sensors that measure temperature and humidity and sniff out dangerous gases like methane, carbon monoxide, carbon dioxide, and of course, ozone. 

Can you fly a plane? Going to the San Joaquin Valley? Sign up to take an ozone sensor on your next flight! You can help measure ozone levels in layers of the atmosphere that are hard for satellites to investigate. Scientists will combine the data you take with data from NASA’s TEMPO satellite to improve air quality models and measurements within the region. Find out more here or email: Emma.l.yates@nasa.gov