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Dozens of near-Earth asteroids formed from the breakup of a comet 20,000 years ago, study finds

A swarm of 88 near-Earth asteroids hidden in the debris that produces the Taurid Meteor Shower comes from the breakup of a single comet just 20,000 years ago, a new study has revealed. 

Studying the various objects within the ‘Taurid complex’ allowed astronomers from the University of Antioquia in Medellín, Colombia, to further understand their origin.

The larger asteroids were spotted in the stream in the 1980s, prompting astronomers William Napier and Victor Clube to suggest they shared a ‘parent’ with Comet Encke – a periodic comet that completes an orbit of the Sun once every three years. 

Some of these asteroids are more than a mile wide, making them too large to have been produced from Comet Encke itself, according to study authors Ignacio Ferrín and Vincenzo Orofino.

Their new study involved a review of dozens of research papers published since the 1980s, and a measurement of reflected light from the larger space rocks.

This allowed them to present ‘further evidence’ that Comet Encke, and the larger asteroids, all came from the breakup of a 62-mile-wide ice ball 20,000 years ago.

The team say the asteroids in the stream could pose a threat to the Earth, and that other objects that came from the ancient comet may have hit the planet in the past. 

A swarm of 88 near-Earth asteroids hidden in the debris that produces the Taurid Meteor Shower come from the breakup of a single comet just 20,000 years ago, a new study has revealed 

What is the Taurid complex?

Each year, from the end of October, the skies play host to the meteor shower, dubbed ‘nature’s fireworks’.

The Taurids display is created by debris left behind by Encke’s comet, named after the astronomer who discovered it’s annual trajectory in 1819.

Researchers from the Astronomical Institute of the Czech Academy of Science have been keeping track of these fragments.

They have found two asteroids, called 2015 TX24 and 2005 UR, which are part of a previously undiscovered branch of the Taurids’ debris.

The space rocks measure 650 feet to 900 feet (200 to 300 meters) across and have been registered on the International Astronomical Union’s list of ‘potentially hazardous’ asteroids.

But the Czech team is concerned that the hidden debris field may contain even larger objects.

Earth passes through part of the stream every year, appearing as shooting stars in the sky every October in the southern hemisphere, and November in the north. 

Comet Encke was first spotted in 1786, and, like other comets entering the inner solar system, left a stream of debris in its wake as it came closer to the sun. 

Having such a large, well-populated and unpredictable complex of rocks, debris and dust getting regularly close to the Earth made them the subject of a lot of academic study over the past few decades, some focusing on the larger asteroids.

Experts have previously linked Taurid impacts to the death of prehistoric cultures, and global climate cooling during a glacial period, known as the Younger Dryas.

It is also thought the Tunguska event, which saw a small asteroid explode five miles above an inhabited part of Russia in 1908, was linked to the Taurid stream.

Millions of trees were destroyed covering a 1m200 square mile region of the country, and happened when Comet Encke was at its minimum distance to the Earth.

The Chelyabinsk meteor, which injured over 1,500 people when it broke up in 2013, also likely came from the Taurid complex, the team behind this study said.

And, in 2005, NASA astronomer Rob Suggs spotted a brief flash of light from a lunar impact event while testing a new 250mm telescope and video camera, later confirming it was part of the Taurid meteor shower.

The Colombian team, with astronomers from the University of Salento in Italy, re-analysed dozens of papers published on impacts, allowing them to confirm that the complex was made of up to 88 larger objects.

They then used a technique called secular light curves, finding changes in the brightness of each member of the complex, and found 67 per cent of them had evidence of ‘cometary activity’. 

This provided a ‘smoking gun’ for the shared origin theory, according to the team. 

Napier welcomed the findings of this new research, suggesting that having asteroids in orbits resembling that of Comet Encke indicates either some unknown dynamical process, or that they are degassed fragments of a progenitor comet. 

The team say the asteroids in the steam could pose a threat to the Earth, and that other objects that came from the ancient comet may have hit the planet in the past

The team say the asteroids in the steam could pose a threat to the Earth, and that other objects that came from the ancient comet may have hit the planet in the past

WHAT WAS THE ANCESTOR COMET LIKE? 

The ancestor may have been a ‘rubble pile’ made of elementary, rocky or carbonaceous blocks, held together inside an icy matrix. 

The fragmentation of this pile, possibly due to tidal forces from the Sun or another body, would have resulted in multiple types of child objects.

The children would have been  relatively large, with the same rubble pile structure, and quite small, consisting of the original rocky blocks.

Oljato, one of the asteroids in the complex, would be an example of rubble pile, held together by an icy matrix still sufficient to produce cometary activity. 

Large inactive objects, such as Morpheus would be similar to Oljato, but their original icy component could be exhaust or sealed in the interior.

Smaller inactive objects, such as 2006 SO198, could be original rocky blocks.

‘The fact that an object shows a spectrum similar to that of main-belt asteroids does not preclude a cometary nature of that object,’ the team explained.

They predict it may have originated in the inner solar system, where rocky objects were born in its early years.

However, as Jupiter migrated further out, tidal forces would have pushed many of these objects to the outer solar system giving them an icy nature.

They then return to the inner solar system as comets, fuelled by the ice turning to gas as it heats up, and leaving a tail of dust and debris.

The ancestor may have been a ‘rubble pile’ made of elementary, rocky or carbonaceous blocks, held together by a massive icy matrix at its core and out through the rubble pieces.

The fragmentation of this pile, possibly due to tidal forces from the Sun or another body, would have resulted in multiple types of child objects. 

Oljato, one of the asteroids in the complex, is an example of rubble pile, held together by an icy matrix still sufficient to produce cometary activity, but much smaller than the parents.

Large inactive objects, such as Morpheus would be similar to Oljato, but their original icy component could be sealed in the interior. 

Meanwhile, smaller inactive objects, such as 2006 SO198, could be original rocky blocks.

‘The fact that an object shows a spectrum similar to that of main-belt asteroids does not preclude a cometary nature of that object,’ the team explained. 

Taurid meteors tend to be larger than normal meteors, which means they are bright and penetrate deeper into the Earth’s atmosphere, many being fireballs. 

While this usually is a harmless phenomenon, the discovery of larger asteroids within one ‘dangerous branch’ of the stream could pose a real risk. 

The Earth only passes through this potentially dangerous branch once every few years, causing greater numbers of shooting stars, and potentially resulting in large objects that could fall to the Earth, rather than break up in the atmosphere.

Future encounters with the branch are predicted for 2022, 2025, 2032 and 2039. 

David Asher, Armagh Observatory astronomer and not involved in this study, told Discover Magazine that this work helps to build a picture of the original Taurid.

He predicted that in 2032 and 2036 we are likely to pass through the centre of the Taurid complex, where there will be a ‘noticeable enhancement of fireballs.’

Ferrin said outgassing from comet-like objects within the complex could be hiding smaller, but still potentially dangerous, asteroids that might hit the Earth.

‘The Tunguska cosmic body was 60 to 90 meters in diameter,’ he told Discover Magazine, adding that we ‘now believe the complex may contain many more objects of that size. It is not the tame, simple and innocent complex we thought it was.’

Explained: The difference between an asteroid, meteorite and other space rocks

An asteroid is a large chunk of rock left over from collisions or the early solar system. Most are located between Mars and Jupiter in the Main Belt.

A comet is a rock covered in ice, methane and other compounds. Their orbits take them much further out of the solar system.

A meteor is what astronomers call a flash of light in the atmosphere when debris burns up.

This debris itself is known as a meteoroid. Most are so small they are vapourised in the atmosphere.

If any of this meteoroid makes it to Earth, it is called a meteorite.

Meteors, meteoroids and meteorites normally originate from asteroids and comets.

For example, if Earth passes through the tail of a comet, much of the debris burns up in the atmosphere, forming a meteor shower.

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