By Donna Vanzetti

Evidence of craters caused by cataclysmic impacts can be seen across the terrestrial planets and moons within our Solar System, including right here on Earth.  The question is, are there more rogue asteroids lurking out in space on an Earth-bound trajectory?

NASA’s Planetary Defence Coordination Office is on the job, searching the skies for near-earth objects such as comets, asteroids, meteors, and meteorites.

So, what is the difference between these space rocks?  They all start out as chunks of rock and if they come from space and burn up from friction in our atmosphere, they ‘become’ meteors. However there are some details that make them all distinctive.


A comet is a rocky object left over from the formation of our Solar System. Comets form a long way out from the Sun, so they’re also made up of frozen gases and dust which makes them celestial snowballs!  A comet’s orbit brings it in closer to the Sun, it heats up which creates a long tail of dust and gas, often spectacular to see in the night sky.

Comet Neowise over Nanaimo, Canada. Image Credit: Shlomo Shalev


An asteroid is also made up of residual rock following the formation of the solar system.  However, being formed toward the inner solar system, asteroids are only rock or metal with no frozen gases or dust.

Both comets and asteroids travel through space, passing by Earth. If we’re lucky!

Asteroid 1998 OR2
An artists depiction of Asteroid 1998 OR2. Image: NASA


Meteoroids are the same as asteroids except they don’t just pass by Earth. Instead, they hurtle towards us and through our atmosphere.  During their hot journey down, due to their speed of approximately 73kms/second and friction, they usually completely vaporise before making it to the surface.  Most meteoroids are as small as a grain of sand burning up as faint meteors (or “shooting stars”) in our atmosphere.  Larger meteoroids create spectacular fireballs flying across the night sky.  Even bigger meteoroids can explode in a brilliant colourful flash such as the Russian meteor of February 2013, that exploded approximately 20kms above the Earth’s surface.

If a meteoroid makes it through Earth’s atmosphere without completely burning up and lands on Earth, we call it a meteorite.  On average less than 5 percent of the original object makes it to the ground and these are often the size of a small stone up to a fist.

However, there are many scars across the globe revealing impact craters made by much larger space rocks.  Currently there are 175 known impact structures with Vredefort Crater in South Africa the world’s largest at more than 300kms across.  Another prominent impact is the event that led to the extinction of the dinosaurs, off the coast of the Yucatan Peninsula in Mexico, around 65 million years ago.

Currently NASA’s Planetary Defence Coordination Office takes data from various Near-Earth Object Observation Agencies and uses the information to characterize and track potentially hazardous objects (PHO’s).  Near-Earth Objects (NEO’s) are classified as any object entering the Earth’s vicinity or crossing Earth’s orbit and no bigger than 140m across.  PHO’s are those objects crossing Earth’s orbit and are more than 140m across.  If a PHO were to hit Earth, it would cause regional devastation on a scale not seen in our living history.

There are more than 20,000 discovered NEO’s and currently 2,093 known PHO’s, of which 157 are estimated to be larger than 1km across.  However, there are no known asteroids currently on a collision course with Earth. Well, not tor the next few hundred years, anyway!

Perseid Meteor Shower
Perseid Meteor Shower

Bennu here we come!

To shed more light on these potentially dangerous bodies, NASA in July 2016, launched the space craft Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-Rex) out into the solar system to chase down an asteroid named Bennu.  The aim is to collect samples from the surface through one of three touch-and-go manoeuvres, earmarked to begin on October 20th this year and return the samples to Earth in September 2023.

Asteroid Bennu is 510m across and is listed as an NEO.  During its orbit around the Sun, it comes close to Earth and has a high likelihood of hitting Earth in the late 22nd century.

The sample return mission will provide critical information about the solar system formation processes, integral to better understanding our own place in space.  Plus, we can learn whether asteroids like Bennu contain natural resources such as water or precious metals.  Importantly, it will also allow scientists to study Bennu’s physical and chemical properties, crucial for planning an impact defence strategy.

Space rocks are a ‘clear and present danger’ to us here on Earth, but it is good to know that Planetary Defence Agencies around the world have a close eye on the sky.

Asteroid Bennu
Asteroid Bennu. Credit: NASA

report your fireball findings!

The Fireballs in the Sky team are based at Curtin University. They are on the lookout for new Citizen Scientists to report bright meteors.

We look forward to welcoming you to our friendly community of Stargazers & Astronomy Lovers where we thrive on making learning about the galaxy easy & fun!

Carol Redford - Founder Stargazers Club WA



Leave a Reply