We have observed space rocks for thousands of years. Iron meteorites have been prized throughout history: who can forget Meteoric Iron Dagger of King Tutankhamunor the Buddha carved from a meteorite who fell 15,000 years ago? Similarly, our history of comet sightings is long, many of which have had a significant impact on human history and the development of legends (or should that be an omen?). Halley’s Comet, of course, was immortalized in the famous Bayeux Tapestrymade in the eleventh century.
But what is the difference between an asteroid and a comet? Or a meteorite and a meteorite? The answers to these questions, along with an overview of the different types of space rock, can be found below.
If you’re looking for more stargazing tips, be sure to check out our beginners guide to astronomy and our UK full moon calendar to get the most out of the night sky. For a complete roundup of this year’s meteor showers, we’ve listed them all in our meteor shower calendar.
What is this space rock?
Asteroids: Small rocky objects, often irregular in shape, remnants of the formation of the solar system
Comets: Large icy bodies of frozen gas, dust, and rock, with a frozen core
Meteoroid: Fragments and debris of asteroids and comets
Meteor shower: Several meteoroids burn up in Earth’s atmosphere
Fireball : An exceptionally bright meteor that can be seen over a wide area
Meteorite: Meteoroids that survive the journey through Earth’s atmosphere and fall to the surface
Dwarf planet: A small spherical (or near-spherical) celestial body orbiting the Sun, which does not have enough mass to clear its vicinity of debris
Asteroids are small rocky objects that orbit the Sun. Most asteroids are irregular in shape, although a few are nearly spherical. There are over a million known asteroids, and most are found in the main asteroid belt between Mars and Jupiter. Asteroids are rocky remains left over from the early formation of the solar system.
When an asteroid shares its orbit with a planet, it is called a Trojan asteroid. There are two Trojan asteroids for Earth, however, both are difficult to see because they point Earth in our orbit around the Sun, and therefore appear on the horizon, near the Sun at sunrise. Not ideal viewing conditions.
Asteroid Bennu (shown in the video above by NASA’s Science Visualization Studio) made news in 2020 when NASA’s OSIRIS-REx mission successfully landed on the asteroid’s surface and collected over 400g of samples. It was an impressive achievement, and one that smashed the original 60g target.
Bennu is estimated to be around 4.5 billion years old, forming just after the formation of our solar system. It is most likely a piece of a much larger carbon-rich asteroid, which broke off about 700 million to 2 billion years ago and came closer to Earth every six years. .
But astronomers have to wait a little longer because the spacecraft is still on its way back, due to the passage on September 24, 2023 deliver the precious cargo. After that, the spacecraft will set off again, this time to study the near-Earth asteroid Apophis, under a new mission name. OSIRIS-APEX.
Comets are the snowballs of the cosmos. These icy bodies of frozen gas, rock, and dust orbit the Sun in highly elliptical paths, often falling as they go. When their orbit brings them closer to the Sun, they heat up. This causes solid ice to transform into gas, which is drawn into the distinctive comet tail. Perhaps the most famous comet is Halley’s Comet, which is due to return to our skies in July 2061.
Comets are so varied in size, orbit and composition that it has given rise to many classifications over the years. However, for the purposes of discussion, they can be classified into four broad categories:
- Non-periodic comets: comets that have passed through the solar system only once
- Short-period comets: comets with an orbit of less over 200 years
- Long-period comets: comets with an orbit of After over 200 years
- Lost comets: comets that “disappeared” and were not seen at their most recent perihelion (closest point to the Sun)
There are three main parts in a comet:
- Core: the solid core
- Coma: gases expelled by the nucleus
- Tail: the stream of gas and dust left in the comet’s wake
When comets or asteroids travel around the Sun, they leave a trail of debris in their wake. When Earth’s orbit intersects with this debris, the result is hundreds (or thousands) of light trails, appearing to radiate out from a point in the night sky. This is why meteor showers are often seen at the same time each year, and meteors are more numerous on certain nights.
Thus, a meteor could more accurately be described as a an event, rather than an object: when this tiny particle (called a meteoroid) enters the upper parts of the atmosphere and heats the air around it to incandescence. It is the glow that we see as a meteor.
The Eta Aquariid and Orionid meteor shower are created by Earth’s orbit passing through the trail left by Halley’s Comet, while the impressive Leonids are remnants of Comet Tempel-Tuttle.
Meteoroids are your classic space rocks. As fragments and debris of asteroids and comets, they are among the smallest bodies in the solar system. These particles continue to orbit the Sun in approximately the same orbit as the parent body from which they originated, and over time they move away from the parent as the orbit becomes littered with these particles.
When a meteoroid enters the Earth’s atmosphere and burns up, it leaves a bright trail across the sky and is known as a meteor – or more popularly, a shooting star. Meteoroids can range in size from as small as a speck of dust to small asteroids.
If the collision of a meteoroid with the atmosphere is frontal, it can reach a speed of up to 45 miles per second, while if it hits the atmosphere directly behind the Earth, it can arrive at about 7.5 miles per second. Usually the speed is somewhere in between.
If a meteor entering the atmosphere exceeds Venus in brightness, it is a fireball. A large fireball will be visible for 5-10 seconds, and if you see a fireball that appears to be close, be sure to listen for a rumble or “bang” type sound! A fireball that lasts more than 10 seconds is however more likely to be a satellite or an airplane shape fall back to Earth.
Sometimes a fireball can explode, as was the case with the Tagish Lake Meteorite which fell in 2000. It exploded into the sky with a force about a quarter of that of the Hiroshima bomb, shattering into about 500 pieces that rained down around Tagish Lake in British Columbia, Canada.
It is the meteoroids that survive the journey to the Earth’s surface. So far, over 69,000 meteorites have been found (and named) on Earth.
If you see a meteor streak across the sky with brightness similar to a quarter moon, chances are it will survive the trip and land on Earth. Meteorites fall every day, but finding one is incredibly rare.
There are different types of meteorites, and most can be classified as an iron meteorite, a stony meteorite, or a stony iron meteorite. These categories are broadly defined by the amount of iron-nickel metal contained in the meteorite:
- Iron Meteorite: those that are almost entirely made of metal
- Stony Meteorite: those that consist almost entirely of silicate crystals
- Stony Meteorite: those with similar amounts of metal and silicate crystals
These broad categories are subdivided based on the structure, chemistry, and minerals contained in the meteorite. For example, pallasites are a beautiful type of stony iron meteorite with large translucent olivine green crystals, fully embedded in metal.
Pallasites are rare, and only 61 are known to date. Their exact origin is still hotly debated., but some scientists believe they originate from the core-mantle boundary of the ancient worlds, while other research argues that they originated higher in the mantle. But whatever form they take, pallasites are awesome.
Dwarf planets are massive enough to be affected by gravity and can take on a round or nearly round shape. However, unlike planets, they are unable to clear their orbital path. Each of the known dwarf planets in the solar system is smaller than our Moon. There are five officially recognized dwarf planets in our solar system: Pluto, Ceres, Makemake, Haumea, and Eris.
Although we all know that Pluto should be recognized as a planet…
As indicated by the International Astronomical Unionthe criteria for a dwarf planet are:
- It is in orbit around the Sun
- Round (or nearly round) in shape, having been drawn into that shape by its own gravity
- Not a satellite from another planet
- Is not massive enough to clear its neighborhood of debris
Another potential dwarf planet, RR 2015245, was discovered by the Investigation of the origins of the outer solar system at the Mauna Kea observatories on the Big Island of Hawaii in 2015, and was photographed by Hubble in 2020. It measures approximately 600 km in diameter and is currently under investigation to determine if it has a satellite ( moon) and what the orbit of that satellite might be.
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