What Are Asteroids? (Ages 14-18)

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

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

There are over a million known asteroids. Many asteroids receive names. An organization called the international astronomical union is responsible for assigning names to objects such as asteroids and comets.

Although all these celestial bodies orbit the sun, they are not the same. Unlike asteroids, which are rocky, comets are a mixture of dust and ice. The meteors are small space rocks which are close enough to enter the atmosphere of the earth, where they burn like a shooting star or landing on the ground in meteorite.

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

Asteroids were formed almost at the same time and in the same way as the planets of our solar system. A massive and dense cloud of gas and dust collapsed in a rotation disc, and the gravity of the center of the disc attracted more and more material to it. Over time, these pieces have repeatedly collided with each other, which sometimes leads to smaller fragments and other times grouping together, resulting in much larger objects.

Objects with a lot of mass – like planets – have produced enough severity to get out of spheres, but many small objects have not done so. They ended up becoming comets, small moons and, yes, asteroids. Although some asteroids have a spherical shape, most of them have irregular forms – sometimes oblong, mute or shredded.

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

Trojan asteroids orbit the sun on the same orbital path as a planet. They are at two specific points on the planetary orbit called Lagrange Points. At these points, the gravitational attraction of the planet and the sun is in balance, which makes these points neutral and stable to gravity. Many planets have been noted to have Trojan asteroids, including earth.

The location of an asteroid can also be influenced by the gravity of the planets that it passes and will be pushed or pulled on a path which brings it closer to the earth. When asteroids or comets are on an orbital path which is less than 30 million miles from the orbit of the earth, we call them objects close to the earth.

Yes! Throughout history, asteroids or pieces of asteroids collided with the earth, our moon and the other planets. The effects of some of these impacts are still visible. For example, Chicxulub Crater was created 65 million years ago when a massive asteroid hit the Yucatan peninsula in Mexico. The result of dust and gas that results in the atmosphere of the earth blocked sunlight, leading to a mass extinction which included dinosaurs. More recently, in 2013, the inhabitants of Chelyabinsk, Russia, witnessed an asteroid almost as wide as a tennis court explode in the atmosphere above them. This event has produced a powerful shock wave that caused damaged injuries and structures.

This is why the NASA planetary defense coordination office keeps a vigilant eye on almost land objects. The planetary defense team is based on telescopes and observatories on earth and space to detect and monitor objects like these which could move too close to our planet.

The agency is working on planetary defense strategies to use if an asteroid is discovered to go to us. For example, the NASA DART mission (double asteroid redirection) in 2022 was a first test of its genre: a spacecraft not mixed with an autonomous targeting system intentionally stolen in asteroid dimorphos, successfully modifying its orbit.

NASA detects and follows asteroids using telescopes on the soil and in space, radar observations and computer modeling. The agency also 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 from 2029. Other missions have in fact established physical contact with asteroids. For example, the Dart mission mentioned above had an impact on an asteroid to modify its orbit, and the Osiris-Rex (origins, spectral interpretation, identification of resources and security-Régolith Explorer) collected a sample of material of the surface of the Bennu asteroid and delivered the sample to the earth in 2023 for scientists to study.

Do you want a career where you can study asteroids? Here are some NASA jobs that do that exactly:

• Astronomer: These scientists observe and study planets, stars and galaxies. Astronomers make discoveries that help us understand the functioning of the universe and how it changes. This work requires solid experience in science, mathematics and computer science education.
• Geologist: Asteroids are made of different types of rock, clay or metal materials. Geologists study the properties and composition of these materials to learn more about the processes that have shaped the earth and other celestial bodies, such as planets, moons and asteroids.

Asteroid facts
Gallery: What is this space rock?
Center for studies of objects close to the earth
Planetary defense at NASA
Watch asteroid: keep an eye on almost land objects