two rocky asteroids in black space

NASA's Galileo spacecraft captured this view in August 1993, minutes before its closest approach to asteroid 243 Ida and its tiny moon, Dactyl. Image credit: NASA/JPL

10 Things: What's That Space Rock?

Feature | October 8, 2018


    The path through the solar system is a rocky road. Asteroids, comets, Kuiper Belt Objects—all kinds of small bodies of rock, metal and ice are in constant motion as they orbit the Sun. But what’s the difference between them, anyway? And why do these miniature worlds fascinate space explorers so much? The answer is profound: they may hold the keys to better understanding where we all come from.

    rocky asteroid in dark space
    This picture of Eros, the first of an asteroid taken from an orbiting spacecraft, came from NASA's NEAR mission in February 2000. Image credit: NASA/JPL

    1. Asteroids

    Asteroids are rocky, airless worlds that orbit our Sun. They are remnants left over from the formation of our solar system, ranging in size from the length of a car to about as wide as a large city. Asteroids are diverse in composition; some are metallic while others are rich in carbon, giving them a coal-black color. They can be “rubble piles,” loosely held together by their own gravity, or they can be solid rocks.

    Most of the asteroids in our solar system reside in a region called the main asteroid belt. This vast, doughnut-shaped ring between the orbits of Mars and Jupiter contains hundreds of thousands of asteroids, maybe millions. But despite what you see in the movies, there is still a great deal of space between each asteroid. With all due respect to C3PO, the odds of flying through the asteroid belt without colliding with one are actually pretty good.

    Other asteroids (and comets) follow different orbits, including some that enter Earth’s neighborhood. These are called near-Earth objects, or NEOs. We can actually keep track of the ones we have discovered and predict where they are headed. The Minor Planet Center (MPC) and Jet Propulsion Laboratory’s Center for Near Earth Object Studies (CNEOS) do that very thing. Telescopes around the world and in space are used to spot new asteroids and comets, and the MPC and CNEOS, along with International colleagues, calculate where those asteroids and comets are going and determine whether they might pose any impact threat to Earth.

    For scientists, asteroids play the role of time capsules from the early solar system, having been preserved in the vacuum of space for billions of years. What’s more, the main asteroid belt may have been a source of water—and organic compounds critical to life—for the inner planets like Earth.

    rocky comet nucleus venting a jet of gas
    The nucleus of Comet 67P/Churyumov-Gerasimenko, as seen in January 2015 by the European Space Agency’s Rosetta spacecraft. Image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

    2. Comets


    On July 4, 2005, NASA crashed a washing machine-sized probe into a comet the size of Manhattan. The probe was released from NASA’s Deep Impact spacecraft, which captured images of comet Tempel 1 before, during and after the collision. Image Credit: NASA/JPL/UMD

    Comets also orbit the Sun, but they are more like snowballs than space rocks. Each comet has a center called a nucleus that contains icy chunks of frozen gases, along with bits of rock and dust. When a comet’s orbit brings it close to the Sun, the comet heats up and spews dust and gases, forming a giant, glowing ball called a coma around its nucleus, along with two tails – one made of dust and the other of excited gas (ions). Driven by a constant flow of particles from the Sun called the solar wind, the tails point away from the Sun, sometimes stretching for millions of miles.

    While there are likely billions of comets in the solar system, the current confirmed number is 3,535. Like asteroids, comets are leftover material from the formation of our solar system around 4.6 billion years ago, and they preserve secrets from the earliest days of the Sun’s family. Some of Earth’s water and other chemical constituents could have been delivered by comet impacts.

    shattered boulders in space
    An artist re-creation of a collision in deep space. Image credit: NASA/JPL-Caltech

    3. Meteoroids

    Meteoroids are fragments and debris in space resulting from collisions among asteroids, comets, moons and planets. They are among the smallest “space rocks.” However, we can actually see them when they streak through our atmosphere in the form of meteors and meteor showers.

    bright streak above curved earth seen from above
    This photograph, taken by an astronaut aboard the International Space Station, provides the unusual perspective of looking down on a meteor as it passes through the atmosphere. The image was taken on Aug. 13, 2011, during the Perseid Meteor Shower that occurs every August. Image credit: NASA

    4. Meteors

    Meteors are meteoroids that fall through Earth’s atmosphere at extremely high speeds. The pressure and heat they generate as they push through the air causes them to glow and create a streak of light in the sky. Most burn up completely before touching the ground. We often refer to them as “shooting stars.” Meteors may be made mostly of rock, metal or a combination of the two.

    Scientists estimate that about 48.5 tons (44,000 kilograms) of meteoritic material falls on Earth each day.

    shooting stars in starry sky with shiloutted pine trees
    The constellation Orion is framed by two meteors during the Perseid shower on Aug. 12, 2018 in Cedar Breaks National Monument, Utah. Image credit: NASA/Bill Dunford

    5. Meteor Showers

    Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically—these events are termed meteor showers. They occur when Earth passes through trails of particles left by comets. When the particles enter Earth’s atmosphere, they burn up, creating hundreds or even thousands of bright streaks in the sky. We can easily plan when to watch meteor showers because numerous showers happen annually as Earth’s orbit takes it through the same patches of comet debris. This year’s Orionid meteor shower peaks on Oct. 21.

    man and meteorite rock on ground
    An SUV-sized asteroid, 2008TC#, impacted on Oct. 7, 2008, in the Nubian Desert, Northern Sudan. Dr. Peter Jenniskens, NASA/SETI, joined Muawia Shaddas of the University of Khartoum in leading an expedition on a search for samples. Image credit: NASA/SETI/P. Jenniskens

    6. Meteorites

    Meteorites are asteroid, comet, moon and planet fragments (meteoroids) that survive the heated journey through Earth’s atmosphere all the way to the ground. Most meteorites found on Earth are pebble to fist size, but some are larger than a building.

    Early Earth experienced many large meteorite impacts that caused extensive destruction. Well-documented stories of modern meteorite-caused injury or death are rare. In the first known case of an extraterrestrial object to have injured a human being in the U.S., Ann Hodges of Sylacauga, Alabama, was severely bruised by a 8-pound (3.6-kilogram) stony meteorite that crashed through her roof in November 1954.

    ceres in color
    The largest object in the asteroid belt is actually a dwarf planet, Ceres. This view comes from NASA’s Dawn mission. The color is approximately as it would appear to the eye. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

    7. Dwarf Planets

    Don’t let the name fool you; despite their small size, dwarf planets are worlds that are just as compelling as their larger siblings. Dwarf planets are defined by astronomers as bodies massive enough to be shaped by gravity into a round or nearly round shape, but they don’t have enough of their own gravitational muscle to clear their path of other objects as they orbit the Sun. In our solar system, dwarf planets are mostly found in the Kuiper Belt beyond Neptune; Pluto is the best-known example. But the largest object in the asteroid belt is the dwarf planet Ceres. Like Pluto, Ceres shows signs of active geology, including ice volcanoes.

    Have exactly one minute? Find out more about dwarf planets: Space Shorts: What Is a Dwarf Planet?

    illustration showing spacecraft and big space rock
    An artist's rendition of the New Horizons spacecraft passing by the Kuiper Belt Object MU69 in January 2019. Image credits: NASA/JHUAPL/SwRI

    8. Kuiper Belt Objects

    The Kuiper Belt is a disc-shaped region beyond Neptune that extends from about 30 to 55 astronomical units -- that is, 30 to 55 times the distance from the Earth to the Sun. There may be hundreds of thousands of icy bodies and a trillion or more comets in this distant region of our solar system.

    Besides Pluto, some of the mysterious worlds of the Kuiper Belt include Eris, Sedna, Quaoar, Makemake and Haumea. Like asteroids and comets, Kuiper Belt objects are time capsules, perhaps kept even more pristine in their icy realm.

    diagram showing different solar system zones
    This chart puts solar system distances in perspective. The scale bar is in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the Sun to the Earth, which is about 93 million miles or 150 million kilometers. Neptune, the most distant planet from the Sun, is about 30 AU. Image credit: NASA/JPL-Caltech

    9. Oort Cloud Objects

    The Oort Cloud is a group of icy bodies beginning roughly 186 billion miles (300 billion kilometers) away from the Sun. While the planets of our solar system orbit in a flat plane, the Oort Cloud is believed to be a giant spherical shell surrounding the Sun, planets and Kuiper Belt Objects. It is like a big, thick bubble around our solar system. The Oort Cloud’s icy bodies can be as large as mountains, and sometimes larger.

    This dark, cold expanse is by far the solar system’s largest and most distant region. It extends all the way to about 100,000 AU (100,000 times the distance between Earth and the Sun) – a good portion of the way to the next star system. Comets from the Oort Cloud can have orbital periods of thousands or even millions of years. Consider this: At its current speed of about a million miles a day, NASA’s Voyager 1 spacecraft won’t reach the Oort Cloud for more than 300 years. It will then take about 30,000 years for the spacecraft to traverse the Oort Cloud, and exit our solar system entirely.

    animation showing spacecraft lowering sample arm to surface
    This animation shows NASA’s OSIRIS-REx spacecraft collecting a sample of the asteroid Bennu, which it is expected to do in 2020. Image credit: NASA/Goddard Space Flight Center

    10. The Explorers

    Fortunately, even though the Oort Cloud is extremely distant, most of the small bodies we’ve been discussing are more within reach. In fact, NASA and other space agencies have a whole flotilla of robotic spacecraft that are exploring these small worlds up close. Our mechanical emissaries act as our eyes and hands in deep space, searching for whatever clues these time capsules hold.

    A partial roster of current or recent missions to small, rocky destinations includes, from NASA:

    • OSIRIS-REx – Now approaching the asteroid Bennu, where it will retrieve a sample in 2020 and return it to the Earth for close scrutiny.
    • New Horizons – Set to fly close to MU69 or “Ultima Thule,” an object a billion miles past Pluto in the Kuiper Belt on Jan. 1, 2019. When it does, MU69 will become the most distant object humans have ever seen up close.
    • Psyche – Planned for launch in 2022, the spacecraft will explore a metallic asteroid of the same name, which may be the ejected core of a baby planet that was destroyed long ago.
    • Lucy – Slated to investigate two separate groups of asteroids, called Trojans, that share the orbit of Jupiter – one group orbits ahead of the planet, while the other orbits behind. Lucy is planned to launch in 2021.
    • Dawn – Finishing up a successful seven-year mission orbiting planet-like worlds Ceres and Vesta in the asteroid belt.

    Plus these missions from other space agencies:

    • The Japan Aerospace Exploration Agency (JAXA)’s Hayabusa2 – Just landed a series of small probes on the surface of the asteroid Ryugu.
    • The European Space Agency (ESA)’s Rosetta – Orbited the comet 67P/Churyumov-Gerasimenko and dispatched a lander to its surface.