asteroids and comets in space

Small Doesn’t Mean Insignificant: Asteroids, Comets, and Meteoroids

Last Updated: August 29, 2023

We tend to prioritize the bigger parts of the solar system like the Sun, planets, and moons, but the solar system contains many smaller bodies that orbit the sun. These smaller objects are asteroids, comets, and meteoroids, and they can be found in a few different locations in the solar system. 

In this article, we will discuss what these small bodies are, how they are different from each other, how we track them, and why they are still important, even if they are small.

Table of Contents

The Mighty Space Rocks: An Introduction to Asteroids

Asteroids are rocky or metallic objects, composed of metals, silicates, and carbon compounds and usually shaped like potatoes ranging in size from small pebbles to 329 miles (530 kilometers) in diameter for the biggest, Vesta. Most asteroids are found in the asteroid belt between Mars and Jupiter and in the icy Kuiper Belt beyond Neptune. 

However, some have broken out of their orbits and entered the inner solar system, sometimes colliding with Earth and Mars. You can see some asteroids with binoculars or telescopes in the night sky, but Vesta can be seen with the naked eye. Asteroids are sometimes referred to as minor planets or “small Solar System bodies” (SSSBs) which are small celestial bodies that orbit the sun but are not large enough to be considered a planet.

Research has shown that asteroids may have played an important role in the formation of the solar system. As remnants of the building blocks of early solar system formation, asteroids contain natural resources such as water, organics, and metals. Some asteroids may be the remnants of a planet that was destroyed early in the history of the solar system. Others may have been ejected from the main asteroid belt during collisions with other asteroids. Therefore studying them helps us understand the early stages of our solar system and many different missions have been dedicated to doing exactly that.

NASA’s NEAR Shoemaker mission was the first spacecraft to orbit an asteroid on Valentine’s Day 2000 and despite not being designed to do so, it was the first to land on one in February 2001, transmitting data for about 2 weeks. NEAR’s landing on Eros marked the first time a U.S. spacecraft was the first to land on a celestial body.

The Japan Aerospace Exploration Agency (JAXA) Hayabusa mission, supported by NASA JPL for communications and navigation, visited the asteroid Itokawa in 2005 and returned a sample of its surface material to Earth in 2010, the first mission to do so and the second to land on an asteroid. The Japanese spacecraft carried a mini-lander MINERVA.

NASA’s Dawn mission was the first mission to orbit two destinations: the asteroid Vesta in 2011 and the dwarf planet Ceres (the largest object in the asteroid belt which also hits the qualifications for a dwarf planet) in 2015. Dawn revealed an immense amount of information about the two large objects in the asteroid belt including 

  • Ceres was an ocean world, showing that dwarf planets could have harbored oceans in the past and some may even continue to do so today 
  • Ceres today harbors organics, opening the question of life
  • Confirmed that the HED (howardites, eucrites, and diogenites) meteorites came from Vesta, specifically the large south polar basin due to the Rheasilvia impact
  • Discovered hydrated and carbon-rich material on Vesta’s surface from impacts

JAXA’s Hayabusa2 spacecraft launched in 2014 to survey asteroid Ryugu and collect a surface sample for return to Earth. The samples returned to Earth in 2020 and will allow scientists to accurately date when the asteroid’s materials formed, and when collisions with other asteroids may have caused Ryugu’s materials to heat up. Because Hayabusa2 is still functioning, its mission has now been extended to visit two more asteroids, 2001 CC21 in 2026 and asteroid 1998 KY26 in 2031.

what are asteroids made of

NASA’s Double Asteroid Redirection Test, DART, is the first space mission to test a planetary defense mechanism in deflecting an asteroid from its path. Launched in November 2021 and arriving at near-Earth asteroid Didymos in September 2022, DART intentionally crashed into the asteroid’s small moon, Dimorphos on September 26. Analysis of the data on Didymos’s orbit in the next year and beyond with determine how much DART impacted the asteroid.

NASA’s recent OSIRIS-REx mission (Origins Spectral Interpretation Resource Identification Security – Regolith Explorer ) is the first U.S. mission to collect a sample of an asteroid and return it to Earth for study. Launching on September 8, 2016, OSIRIS-REx reached its asteroid target, Bennu, 1.2 billion miles (2 billion kilometers) away in 2018, first taking 2 years to orbit and study the asteroid especially since their reconnaissance indicated unexpected rocky boulders as opposed to the expected almost sandy surface of previous long-range data which greatly affected their plans for their Touch And Go (TAG) event. 

On October 20, 2020, the spacecraft hovered over Nightingale crater and the sampling arm made contact with the surface of Bennu for about six seconds, releasing a burst of nitrogen gas which stirred up rocks and surface soil and then captured them in the sampler head. The spacecraft had enough nitrogen for three attempts, to collect between 2 and 70 ounces (60–2,000 grams), but the first attempt indicated that the sampler head had collected the minimum sample. In fact, footage revealed that it was so packed that the lid had not closed and was leaking material. NASA opted to forgo the planned post operational maneuvers including weighing the sample to reduce the amount of lost material and store it in the capsule. Estimates based on the 17.1% of the sampler head visible in footage indicates that the mission likely recovered more than 4.4 lbs. (2 kg) of material, a huge success in comparison to the minimum 60 gram sample they weren’t even sure they would be able to achieve based on the unexpected terrain. It is currently on its return trip back, planning to return the sample to Earth via a parachute drop on September 24th, 2023.

NASA’s New Horizons mission may have finished its most well-known mission of studying Pluto and its moons in 2015, but it is still flying through the Kuiper Belt working on its secondary mission of studying Kuiper Belt objects. The Kuiper Belt contains many different types of objects including asteroids, comets, and confirmed and potential dwarf planets.

Some recently launched and upcoming asteroid missions include:

  • NASA’s Lucy mission: launched in October 2021, Lucy will visit seven Trojan asteroids between 2027 and 2033, plus a bonus main-belt asteroid in 2025 and will be the first mission to visit these Trojans
  • NASA’s NEA Scout mission: launched in November 2022, NEA Scout will utilize a solar sail to demonstrate a low-cost method of asteroid reconnaissance for future human and robotic missions
  • NASA’s Psyche mission: set to launch October 2023, Psyche will travel to the metallic asteroid of the same name in the asteroid which is theorized to be a planetesimal, the first building block of a planet, that never met the other characteristics to become a planet to better understand “the crashes, smashes, mergers, and acquisitions in the early days of our solar system’s formation.”
asteroid cluster in space artist rendition

Comets: The Cosmic Snowballs

Comets are icy SSSBs composed of dust, rocks, and ices. When close enough to the Sun, comets heat up and shed gas and dust, forming a visible coma (a thin atmosphere) and sometimes also a tail typically between 1 and 10 million kilometers (about 600,000 to 6 million miles) long which distinguishes them from asteroids.

The orbital periods of comets range from a few years to hundreds of thousands of years. Short-period comets (ones with orbital periods of less than 200 years) originate in the Kuiper Belt, while long-period comets are thought to originate in the Oort cloud (a huge cloud of gas dust, and ice at the outer reaches of the solar system). The most well-known is Halley’s Comet, a short-period comet visible from Earth every 75–76 years. 

Comets are also from the early formation of our solar system like the planets and are about 4.6 billion years old. According to NASA research, comets “may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.” 

Below are some of the recent comet missions that have furthered this research.

The Stardust mission flew through the tail of Comet Wild 2 in 2004 and collected samples of comet dust, making it the first spacecraft to return a comet sample and extraterrestrial material (2 years later) from outside the orbit of the moon to Earth. Since it was still operational after launching the specimen capsule to Earth, the mission was extended (Stardust-NExT) to fly by comet Tempel 1 on Valentine’s Day 2011.

NASA’s Deep Impact mission crashed a spacecraft into the comet Tempel 1 in 2005 to probe beneath the surface to study its composition. It is the mission to travel the furthest to study a comet, over 4 AU. The mission included the flyby spacecraft and the impactor which was deployed on Tempel 1’s orbital path so that the comet crashed into it. After completing its primary mission of 2 flybys and intercepting the Tempel 1 with the impactor, NASA extended Deep Impact’s mission in 2007 since it had plenty of fuel left. The new supplemental mission was known as EPOXI, from the combination of the two components of this new mission: Extrasolar Planet Observations (EPOCh) and Deep Impact Extended Investigation (DIXI). 

The spacecraft performed three Earth flybys to launch it toward Comet 103P/Hartley (or Hartley 2), during which it made observations of exoplanets around 8 separate stars in 2008. In 2010, it encountered Hartley 2, producing fantastic data including the fact that the two different sections of the comet had different compositions of materials. With limited fuel left, a third trajectory was charted for Near Earth Asteroid 2002 GT in 2020 and performed distant observations of C/200P1 (Garradd) in early 2012 and C/2012 S1 (ISON) in early 2013. Unfortunately, NASA lost contact with Deep Imact later that year, ending the mission.

The European Space Agency’s Rosetta mission was designed to study the comet 67P/ Churyumov-Gerasimenko, making it the first robotic mission from Earth to rendezvous with a comet, the first to follow a comet in its orbit, and the first to deploy a lander to a comet’s surface. Launching in 2004, Rosetta performed multiple gravity assists around Earth and even Mars, first performing a flyby of Asteroid Steins in 2008 and Asteroid Lutetia in 2010 before rendezvousing with comet 67P/ Churyumov-Gerasimenko in August 2014 and the Philae lander making contact in November.

Further readings about comets: The Most Famous Comets Discovered By Astronomers

anatomy of a comet

NASA’s New Horizons spacecraft flew past Arrakoth or MU69 in 2019, revealing it to be a “bi-lobed baby comet” with 2 separate material sections fused together similar to 67P/Churyumov-Gerasimenko visited by Rosetta as well as the first comet we ever saw up close, 1P/Halley, and others. 

The upcoming ESA Comet Interceptor mission will be the first mission to study a pristine comet, a long-period comet that has not traversed into the inner solar system multiple times before. Being able to study a comet that has not gone through these journeys which cause which causes erosion of its surface will help us better understand these bodies’ original appearance and make-up. 

Comet Interceptor will launch to Lagrange Point 2 (L2) 1.5 million kilometers (1 million miles) away from Earth (the same orbit where the James Webb Space Telescope is) and search for a target. Once a likely target is discovered, Comet Interceptor will travel to intercept it and split into 3 spacecraft (two sub-satellites in addition to the main one) before the encounter for observations from multiple points. It is currently scheduled for launch in 2029 in cooperation with JAXA.

Meteoroids - The Shooting Stars of Space

Meteoroids are rocky or metallic SSSBs ranging in size from dust grains to small asteroids. They are the debris left behind by comets and asteroids, which can collide with one another. When meteoroids enter Earth’s atmosphere, they become meteors, (a.k.a. shooting stars), streaking through the sky and typically burning up in a few seconds, but if they are large enough that they don’t burn up completely, the remaining material that impacts Earth is called a meteorite.

Similar to comets and asteroids, they provide valuable evidence for research into the early formation of the solar system as they are often fragments from collisions between these early formation objects. The NEO Observations Program supports agencies and institutions in detecting and reporting meteors and fireballs, as well as searching for meteorites on the ground to help contribute to scientific research relating to these astronomical objects. One of the major projects of this program is the Antarctic Search for Meteorites (ANSMET).

finding meteorites with metal detectors

Cataloging and mapping these small bodies

Similar to space junk, asteroids, comets, and meteoroids cause concern for space travel as impacting anything at such high speeds can cause immense damage and potentially impact the spacecraft’s trajectory and speed. 

Because of this, cataloging and mapping these smaller bodies of the solar system has become of increasing importance as we continue to explore our neighborhood in space. There are a number of organizations that contribute to this work:

  • America’s Department of Defense’s Global Space Surveillance Network (SSN)
  • NASA’s Planetary Defense Coordination Office
  • Near-Earth Object (NEO) Observations Program
  • The International Astronomical Union’s Minor Planet Center (MPC)
  • Center for Near-Earth Object Studies (CNEOS)
  • NASA’s Planetary Data System Small Bodies Node

These organizations are supported by a variety of projects utilizing tools like powerful telescopes and radar including 

  • The NEOWISE Project which repurposes the Wide-field Infrared Survey Explorer space telescope
  • The Infrared Telescope Facility (IRTF)
  • JPL Planetary Radar
  • Goldstone Solar System Radar (GSSR)
  • Arecibo Planetary Radar
  • NASA Ames Asteroid Threat Assessment Project (ATAP)
  • Various sky survey projects utilizing telescopes from all around the world

Thanks to this calataloging and mapping, you can fly around the solar system discovering asteroids, comets, and NEOs in NASA’s Eyes on Asteroids visualization program.


While they often don’t receive as much attention as the planets, moons, and Sun, small Solar System bodies such as asteroids, comets, and meteoroids are just as crucial in understanding the formation of our solar system. Tracking these small bodies is also important when planning space missions as they can become obstacles that could jeopardize a spacecraft. Asteroids, comets, and meteoroids are valuable clues to the early formation of the solar system.

As we study our neighborhood in space in comparison to other star systems, we continually learn from new missions studying targets from our massive Sun and largest planet down to these small solar system bodies. In fact, these smaller bodies may hold the keys to understanding how our solar system formed and how water and life developed in our world.

Sarah H.

Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at and authors self-help and children’s books.

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