Asteroids, comets, and other small worlds


  • Asteroids, comets and other small worlds are leftovers from the disk of dust and gas that formed the planets in our Solar System.
  • Some of these worlds contain water and organics — the building blocks of life as we know it — and may have brought those materials to Earth long ago.
  • Because processes like weathering and internal heating have not altered most of these worlds’ surfaces, they give us a peek into our origins.

Why we study asteroids, comets and other small worlds

Four and a half billion years ago, our Solar System formed from a swirling disk of dust and gas orbiting the infant Sun. The Sun gobbled up most of the disk’s material, while cohesion and gravity pulled together clumps of rock, metal, and ice that eventually grew into planets. Generally speaking, asteroids, comets, and other small worlds are leftovers from that planet-building process.

On Earth, processes like weather and internal heating eliminate traces of the past. But physical changes in small worlds occur mostly due to external forces like solar radiation and impacts from other space rocks, making them time capsules that preserve information about the early Solar System.

Mercury, Venus, Earth, and Mars were too hot in their early days for water and organic compounds necessary for life as we know it to stay on their surfaces. The materials that kickstarted life likely came from asteroids and comets that formed farther away from the Sun. Saturn and Jupiter probably formed closer to the Sun before jostling for position and migrating out to their current positions four billion years ago. In the process, the giant planets’ gravities scattered asteroids and comets all over the Solar System. Some slammed into Earth and likely brought water and organics here, providing the precursors for life. By studying asteroids, comets and other small worlds, we learn about where we come from and how the planets formed.

We also study asteroids to prevent Earth from being hit by one. Asteroids are susceptible to small-scale forces like heat emission that change their orbits in ways we can’t always predict. Small worlds also come in a variety of densities, from loosely-bound clumps of boulders to tightly packed objects with planet-like cores. The strategy to deflect these different types of asteroids would be very different.

What are asteroids?

Asteroids are irregularly shaped objects ranging in size from a few meters to hundreds of kilometers across. Most are located in the asteroid belt between Mars and Jupiter, but some stray closer, becoming near-Earth asteroids if they get within 1.3 astronomical units (AU) of the Sun (the Earth’s average distance to the Sun is 1 AU). If a near-Earth asteroid crosses Earth’s orbit and is bigger than 140 meters, it is considered a potentially hazardous object. There are also two clumps of asteroids that share Jupiter’s orbit called the Trojan asteroids.

What are comets?

Comets are asteroid-like worlds that haven't spent enough time in the inner Solar System for the Sun to blast away their icy materials. Commonly referred to as dirty snowballs, they shed ice, dust, and gas as they approach the Sun, forming long, beautiful tails. Comets come from the Oort Cloud, a sphere of objects 2,000 AU away that surrounds our Solar System.

Some comets like Hyakutake, which passed Earth in 1996, have orbits that take tens of thousands of years to complete. Others have shorter periods, like Halley, which returns to the inner Solar System every 75 years.

Comets provide double the fun for skywatchers

Comets leave a trail of dust behind as they pass through the inner Solar System. If Earth passes through one of these trails, the particles slam into our atmosphere, creating a meteor shower. The annual Perseid meteor shower, which happens every August, is caused by debris from comet Swift-Tuttle, which last passed through the inner Solar System in 1992 and won't return for 133 years.

Kuiper Belt Objects (KBOs)

The Kuiper Belt, named after planetary scientist Gerard Kuiper, is a region of small worlds beyond Neptune’s orbit between 30 and 50 AU. Some KBOs like Eris and Pluto are large enough to be classified as dwarf planets by the International Astronomical Union. Such worlds are geologically rich, and larger than even the biggest asteroid belt object, Ceres, which itself is massive enough to be spherical, geologically active, and very much planet-like.

Other KBOs are more asteroid-like, such as Arrokoth, a lumpy, snowman-esque world visited by NASA’s New Horizons spacecraft in 2019. Unlike comets with elliptical orbits that take them close to the Sun repeatedly, such KBOs have circular orbits which haven’t been altered since their birth 4.5 billion years ago. Further, their great distance means solar heat and radiation have minimal effects on their surface. KBOs are also spared of the frequent impacts faced by objects in the denser, inner Solar System. These facts make KBOs among the pristine targets of interest in the Solar System.

True Colors of Pluto
True Colors of Pluto This is the most accurate natural color image of Pluto taken by NASA's New Horizons spacecraft in 2015. The diversity of geologic landforms on Pluto’s surface rivals that of Mars.Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

How we study asteroids, comets, and small worlds

Because small worlds are scattered all over the Solar System, we use different approaches to explore them. Asteroids — in particular, near-Earth asteroids — are relatively easy to reach with a spacecraft, but their low gravities make orbiting them a challenge. Many asteroids are close enough for us to image with Earth-based radar, which helps us figure out their shapes.

Comets fly through the inner Solar System at a variety of angles. Halley’s comet, for instance, enters the inner Solar System clockwise, from beneath our orbital plane. This means a spacecraft trying to rendezvous with the comet must cancel out Earth's counter-clockwise momentum, pick up speed in the opposite direction, and tilt the plane of its orbit significantly. This requires significant amounts of fuel, which is why solar sail missions have been proposed for reaching certain comets.

The Oort Cloud is too distant to visit for now, but the Kuiper Belt is within reach — providing you have some extra time on your hands. New Horizons was the fastest object to ever leave Earth at the time of its launch, and still took 9 years to reach Pluto.

Exploring asteroids

NASA’s Jupiter-bound Galileo spacecraft made the first asteroid flyby in 1991, coming within 1600 kilometers of Gaspra in the main asteroid belt. The pictures revealed an irregularly shaped world formed by debris from a collision between other objects. Two years later in 1993, Galileo gave us a close look at another main belt asteroid, Ida, which turned out to have a tiny moon now named Dactyl.

NASA’s NEAR Shoemaker spacecraft entered orbit around near-Earth asteroid Eros in 2000 and later touched down on its surface—both firsts for any spacecraft. The agency in 2007 launched the Dawn spacecraft to orbit Vesta and then Ceres, the two largest worlds in the main asteroid belt. Under the IAU’s planet classification system, Vesta is an asteroid and Ceres is a dwarf planet.

Dawn found water-bearing minerals on Vesta’s surface and abundant water ice and organics on Ceres. Because these materials evaporate close to the Sun, Vesta and Ceres either formed farther away or were struck by objects carrying water and organics that formed farther away. Ceres has hundreds of bright spots formed by briny (salty) water bubbling up from a subsurface reservoir. While icy moons like Enceladus and Europa have subsurface oceans that are warmed by the pull of nearby giant planets, Ceres appears to have enough internal heat to keep its subsurface water from freezing.

In 2010, Japan’s Hayabusa spacecraft returned samples from Itokawa, a peanut-shaped near-Earth asteroid a half-kilometer long. Water found in the samples was remarkably similar to that of Earth’s oceans, indicating much of our water could have come from similar asteroids. Follow-up mission Hayabusa2 returned samples of asteroid Ryugu to Earth in December 2020, and NASA’s OSIRIS-REx spacecraft safely delivered samples from asteroid Bennu to Earth in September 2024.

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Why do we need sample return?

It may seem like a lot of trouble to fly all the way to another world just to bring a few handfuls of rock and soil back to Earth. But certain questions can only be answered by tools that are too large, heavy, and power-hungry to fly on spacecraft. Sample return is particularly important for:

Precision: Space-based instruments can’t tell us the date of a sample as accurately as Earth-based instruments can.

Reproducibility: On Earth, we can double-check our results with multiple instruments. That’s particularly important when those results could be something as astonishing as detecting life on another object.

Duration: Technology improves over time, so storing samples on Earth allows future generations to run newer, better experiments.

A NASA mission called Lucy launched in October 2021 to visit 7 of Jupiter’s Trojan asteroids for the very first time. Jupiter's journey in the early Solar System to its present location must be imprinted in the diverse Trojans as they moved along with Jupiter, giving us a chance to figure out what really happened. The agency’s Psyche spacecraft will launch in 2023 and arrive at the same-named, main-belt asteroid Psyche in 2026. Scientists think 220-kilometer-wide Psyche is an exposed iron-nickel core of a small world that formed early in our Solar System's history but never reached planetary size. Studying it will give us unique insights about how the planets formed.

In 2021, NASA launched DART, the Double Asteroid Redirection Test. DART deliberately crashed itself into asteroid Didymos’ small moon, Dimorphos, successfully changing the moon’s orbital speed. This proved that such a technique could be used to deflect an asteroid predicted to hit our planet.

Exploring comets

In 1985, NASA’s International Cometary Explorer, ICE, became the first spacecraft to fly past a comet, coming within 7,900 kilometers of comet Giacobini-Zinner. Although ICE had no cameras, it detected water and carbon monoxide, confirming the theory that comets are similar to dirty snowballs.

A year later, a fleet of international spacecraft flew past Halley’s comet, including Europe’s Giotto spacecraft, which captured images of a comet’s nucleus for the very first time. The pictures revealed a potato-shaped world 16 kilometers long tumbling through space, spewing jets of dust, gas, and ice.

NASA’s Stardust probe flew past comet Wild 2 in 2004, collecting samples of dust and returning them to Earth for the very first time. In the samples scientists found amino acids, organic chemicals that are part of the building blocks of life. In 2005, NASA's Deep Impact spacecraft shot a 364-kilogram (800-pound) impactor into comet Tempel 1, kicking up fluffy dust that helped scientists understand what lies beneath cometary surfaces.

The European Space Agency's Rosetta mission, launched in 2004, orbited and dropped a lander on comet 67P/Churyumov-Gerasimenko as the comet flew through the inner Solar System, giving us an unprecedented look at how comets change over time. Rosetta found comet 67P’s water to be different from Earth’s and even to that of many other comets and asteroids, indicating diverse origins of water in our Solar System.

All previous comet missions have studied comets with relatively short orbital periods. ESA’s Comet Interceptor mission, scheduled to launch in 2028, hopes to give us our first close look at a very long-period comet, a pristine object that has entered the inner Solar System only a few times. Comet Interceptor will lie in wait at a stable spot in space called L2 where the Sun and Earth’s gravity balance out. When astronomers discover an interesting long-period comet heading through the inner Solar System, Comet Interceptor will travel to it, splitting into three separate spacecraft during close approach for multi-point observations.

Exploring the Kuiper Belt

NASA’s New Horizons is the only spacecraft to explore the Kuiper Belt. It flew past Pluto in 2015 and revealed a planet-like world with its mountains, glaciers, and a frozen “heart” of solid nitrogen ice — all indicators of recent geological activity. Scientists also saw hints that Pluto may harbor a subsurface liquid water ocean. Even Pluto’s moon Charon turned out to have remarkably varied landforms.

With New Horizons still in great shape, scientists using the Hubble Space Telescope found another KBO in New Horizons’ neighborhood to visit that we now call Arrokoth. The spacecraft flew past Arrokoth in 2019 and found a 35-kilometer-long world likely among the most primitive, unaltered objects in our Solar System. Arrokoth’s 2 lobes look like they formed due to a slow-speed collision, contradicting the notion that small worlds are prone to chaotic, high-speed smashups.

New Horizons is still flying through the Kuiper belt on its way out of our Solar System, while scientists look for another potential flyby target.

Making New Horizons happen

Were it not for the efforts of The Planetary Society, its members and its supporters, the New Horizons mission might never have gotten off the drawing board. From 1990 to 2000, NASA considered but ultimately rejected 4 separate Pluto missions before approving New Horizons in 2001. “The Planetary Society gave continued, strong support for a whole variety of different Pluto missions that never made it off the drawing board,” said New Horizons principal investigator Alan Stern in 2015.

Hayabusa2 Second Touchdown (short clip)
Hayabusa2 Second Touchdown (short clip) This short clip shows Hayabusa 2's second touchdown, which occurred on 11 July 2019 at 01:06 UTC, as seen from the spacecraft's small monitor camera (CAM-H). The sequence plays at roughly 10 times normal speed. See the full video here.Image: JAXA

The Planetary Society and asteroids, comets, and other small worlds

Space missions to asteroids, comets, and small worlds don’t just happen—they require years of smart policy and stable funding. Visit our space policy and advocacy program page to learn about our current legislative priorities and how we educate, train, and mobilize our members to become effective space advocates. You can also get educated on how the U.S. science community sets priorities for space missions through the decadal survey process.

The Planetary Society not only advocates for the exploration of small worlds, we also work to prevent Earth from being struck by one. You can learn how we help observers find, track, and characterize near-Earth asteroids, and take our free online course to get equipped with the basics of planetary defense.

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Acknowledgements: This page was initially written by Jatan Mehta in 2020.