Asteroids and Friends

1994 PC1

On 18 January 2022, an asteroid measuring 0.6 miles across passed close to Earth. The catchily named (7482) 1994 PC1 was discovered in 1994. The asteroid was 1.2 million miles from Earth as it passed, which is just over 5 times the distance between Earth and the Moon.

The asteroid was discovered on 9 August 1994, by Scottish-Australian astronomer Robert McNaught at an Australian observatory. As it passed Earth last month, it travelled at an eye watering 12.15 miles per second. It is not predicted to come as close to Earth again until 18 January 2105.

Impression of 1994 PC1 travelling through the constellation Cetus. Screenshot from my Star Walk 2 app.

Asteroid 1994 PC1 is termed a hazardous near Earth object by NASA scientists. This is due to its size, and the fact that it passed at less than half the distance from the Earth to the Sun. While a collision was always extremely unlikely, a small deviation in its orbit could have set 1994 PC1 on course for Earth.

The coverage of 1994 PC1 made me reflect on the myriad different objects in space, travelling with us within the solar system. Today’s blog post considers some of those most commonly observed.

Asteroids

I’ll start with asteroids, objects which orbit the Sun. Most are made of rock, but some are metal such as nickel and iron. They vary in size; some asteroids are the size of small moons. (4) Vesta, the largest, is almost 330 miles in diameter and orbits within the asteroid belt between Mars and Jupiter.

A number of asteroids have their own orbiting moons. At present, around 300 asteroids are known to have moons. And some asteroids themselves become moons. Some theories propose that Earth’s moon was originally an asteroid, which was caught by Earth’s gravity as it passed.

The Moon. Our only natural satellite, and the fifth largest moon in the solar system.

Although asteroids orbit the Sun, they are not planets. A planet has to meet a set of defined criteria. It must:

  • Orbit a star (within our solar system, this is the Sun)
  • Be large enough that gravity has shaped it into an ellipsoid
  • Have cleared its orbital path of other large objects

Meteoroids

Meteoroids are lumps of rock or iron, and are much smaller than asteroids. Ranging in size from a grain of sand to a pebble, they orbit the Sun in the same way that planets do. Meteoroids are spread throughout the solar system. From the rocky planets to the asteroid belt, on through the gas giants, and beyond to the Kuiper Belt and Oort Cloud.

Meteoroids are formed in a number of ways. Some are remains of the tail of comets. Others arise when collisions occur between asteroids. Or when there are impacts on rocky bodies such as Mars, or the Moon.

Meteors

Meteors are often referred to as shooting stars. These are meteoroids which travel towards Earth, burning up as they approach. Having been travelling more or less unrestricted through space, the meteoroid suddenly encounters the air within our atmosphere. The speed of the meteoroid causes the air in front of it to compress rapidly, leading to a sudden increase in temperature. This causes the meteoroid to burn, creating a glowing meteor. The meteors usually disintegrate entirely, in the temperatures of up to 1600 °C.

The changing life cycle of a meteoroid

Meteorite

If a meteor survives the journey through Earth’s atmosphere, and reaches the ground, it is known as a meteorite. Around seventeen meteorites are estimated to reach Earth every day. Most land in uninhabited areas and aren’t large enough to cause damage.

The largest meteorite known to have hit the planet landed an estimated 2 billion years ago in what is now Free State, South Africa. Known as the Vredefort Crater, the impact area was declared a UNESCO World Heritage Site in 2005. With an estimated diameter of 190 miles, the crater is the world’s largest known impact structure.

Comet

Finally, comets. Formed of frozen dust, rock, gases, and ice, the nucleus of a comet is made up of remains of the formation of the solar system. The nucleus is surrounded by a nebulous cloud of gases and dust. Named the coma, the cloud forms as parts of the nucleus fall away. Together, the nucleus and the coma are referred to as the head of a comet.

Ranging in size from a few miles wide to tens of miles wide, astronomers estimate there are about 100 million comets orbiting the Sun. A comet follows an elliptical orbit around the Sun. This means they can spend hundreds of years in the far reaches of the solar system, before travelling close to the Sun again.

The name, comet, comes from the Latin word cometa, meaning ‘long-haired’. This references the tail of a comet, which forms when the comet travels closer to the Sun. Comprising dust and gases which have melted from the head, the tail can be up to 93 million miles long.

Comet Neowise, photographed in 2020

Solar winds blow the tail of the comet away from the Sun, regardless of the direction of travel of the comet. 

There are many sources of information about space online. Some of my favourites are the NASA site, and that of the European Space Agency (ESA). There are a range of interesting resources on the BBC Bitesize website too. Finally, the British Astronomical Association has a useful calendar of celestial events for the year. You can find it here. One of my favourites is the Perseid meteor shower in August. The peak this year is 12/13 August. It’s usually worth staying up for!