Saw the fireball over Singapore? Here's what to know about 'brighter than usual' meteors and their rarity

SINGAPORE: A bright streak was seen in the skies over Singapore last Friday (May 24) night, with people in multiple locations around the country recording footage of the phenomenon.

The footage of what was described as a "fireball" or a "bright meteor" falling towards Earth was widely shared across social media platforms, causing excitement and spurring discussions on what exactly had happened.

So what caused the light show, how big was it and how often do such incidents occur?

CNA spoke to two experts to find out.

WHAT WAS IT?

The experts that CNA spoke to said that the fireball was likely an object from space entering and burning up in Earth's atmosphere, however, they added that it is difficult to pinpoint what exactly the object was.

"This might be a comet or asteroid fragment and without any remnant object, it is not easy to tell what exactly it was," said Dr Abel Yang a senior lecturer from the Department of Physics at the National University of Singapore (NUS).

"In general, there is material in the solar system left over from planet formation, and this material – in the form of small asteroids and comets – occasionally collides with the Earth. When this happens we get a fireball as the object burns up in the Earth's atmosphere."

Dr Cindy Ng, who is also a senior lecturer in the NUS Physics Department, concurred.

"In general, the likely causes of fireballs are meteoroids entering the atmosphere of Earth," she said.

"Meteoroids entering Earth's atmosphere can cause meteors while a fireball is a brighter-than-usual meteor."

According to the US space agency NASA, meteoroids refer to small rocks – between the size of a dust grain and a small asteroid – in space.

When a meteoroid enters Earth's atmosphere and burns up, it is referred to as a meteor and sometimes a fireball – or, more popularly, as a shooting star.

Should part of the rock survive its journey through the planet's atmosphere and hit the ground, it will become known as a meteorite.

Some social media users speculated that the object could have been a piece of space debris or space junk like part of a disused satellite.

However, Dr Yang and Dr Ng said that the re-entry of such man-made objects into Earth's atmosphere is quite different from what was witnessed last week.

"The fireball caused by man-made space debris usually moves slower, and we would have had advance warning since the larger space debris that can cause a large fireball is usually monitored by radar," said Dr Yang.

"In addition, most large space debris are usually intentionally deorbited over uninhabited parts of the ocean to reduce the risk of debris hitting a populated area."

Dr Ng said that the re-entry trajectories of pieces of space debris are different from the trajectories of comets entering the atmosphere, and scientists can, in fact, determine what an object was by analysing these trajectories.

"Scientists are tracking the larger pieces. Hence, if the trajectory of the object that caused a fireball matches that of a piece of space debris being tracked by scientists, then scientists can conclude that the cause of the fireball was that piece of space debris," she said.

The object that lit up the skies over Spain and Portugal on May 18 was identified by the European Space Agency (ESA) through analysis of its trajectory, Dr Ng added.

"In the case of the fireball that occurred in Spain, the fireball was captured by ESA's fireball camera, so scientists could analyse the trajectory and reveal that the object was a comet," she said.

HOW LARGE WAS THE OBJECT?

Dr Yang said that the fireball was probably a small object – probably less than a few metres in length and likely less than 1m in size – "given that there are no reports of injuries or property damage".

"Larger objects may cause a shockwave or fireball when they enter the Earth's atmosphere and larger impactors are known to have caused property damage," he said, highlighting the Chelyabinsk meteor, which caused injuries and widespread damage when it exploded over Russia's Ural region in 2013.  

Dr Ng gave a similar estimate of the object's size.

"In general, the largest meteoroids are estimated to be 1m in size," she said.

She said this was about the size of the object seen over Spain and Portugal, citing an estimate from ESA.

"The object that caused the fireball in Spain was estimated by ESA to be roughly 1m in size. ESA estimated the size of the object by analysing the data on the amount of energy released by the fireball," she said, adding that this data was also available because the fireball was caught on camera by the agency.

WHAT WAS THE ALTITUDE OF THE FIREBALL?

Given the object's size and considering the fact that it did not cause a shockwave, the fireball was probably seen "at a rather high altitude, possibly around 60km to 100km", said Dr Yang. 

Dr Ng shared a similar view.

"In general, meteors typically occur at altitudes from 76km to 100km. They usually disintegrate at altitudes of 50km to 95km. The object that caused the fireball in Spain burnt up at an altitude of approximately 60km," she said.

HOW RARE ARE FIREBALLS?

While the meteor was undoubtedly a memorable treat for all who saw it, fireballs "are actually quite common, with small impactors being much more common", said Dr Yang.

Dr Ng, meanwhile, said: "There are probably more than 500,000 fireballs a year all over Earth."

There is a reason why they are not seen more often, however.

"Most of the hits occur over the ocean since the ocean covers about 70 per cent of the Earth's surface area," said Dr Yang.