When they are not detected - The Meteor blast in Indonesia

On 8 October 2009, a rock crashed into the atmosphere above South Sulawesi, Indonesia and raised concerns that we should not forget, what if it was bigger and came undetected as this one?

The asteroid, estimated to have been around 10 metres (30ft) across, hit the atmosphere at an estimated 45,000mph that day and the explosion was huge.

In a press release from the Near Earth Object (NEO) program, the explosion was detected by many International Monitoring System (IMS) infrasound stations, five of them 10,000 km (6200 miles) away, and one 18,000 km (11,100 miles) from the blast.

These stations monitor seismic waves, infrasound (low frequency soundwaves), hydroacoustic, and radionuclide emissions as part of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). They are well equipped to monitor explosions of nuclear weapons, but also detect other events such as meteorite impacts and asteroid explosions, tsunamis and earthquakes.

When analyzed, the amount and intensity of low frequency sound waves created by the explosion allowed researchers Elizabeth Silber and Peter Brown of the Meteor Infrasound Group at the Univ. of Western Ontario to determine that the explosion caused by the asteroid was on the scale of 30 – 50 kilotons of TNT.
Luckily, due to the height of the explosion – estimated at between 15 and 20 km (nine to 12 miles) above sea level – no damage was caused on the ground.
However, if the object had been slightly larger – 20 to 30 metres (60 to 90ft) across – it could easily have caused extensive damage and loss of life, say researchers.

Very few objects smaller than 100 meters (300ft) across have been spotted and catalogued by astronomers.
Tim Spahr, director of the Minor Planet Center in Cambridge, Massachusetts, warned that it was inevitable that minor asteroids would go unnoticed. He said: "If you want to find the smallest objects you have to build more, larger telescopes.

"A survey that finds all of the 20-metre objects will cost probably multiple billions of dollars."
The fireball was spotted by locals in Indonesia, and a YouTube video taken that day "appears to show a large dust cloud consistent with a bright, daylight fireball", according to the Ontario researchers.

If this asteroid were made of metal instead of rock, as suposed, it would likely have impacted the ground causing a lot of damage. Fortunately for the residents of Bone and the surrounding area, the rock broke up in a large fireball instead. 

Of course, events like this always raise the question of why the object wasn’t detected before it even entered the atmosphere. Scientists are concerned that it was not spotted by any telescopes, and that had it been larger it could have caused a disaster.

An asteroid or comet fragment around 60 meters across is believed to have been behind the Tunguska Event, a powerful explosion that took place over Russia in 1908. The blast has been estimated at equivalent to 10-15 million tons of TNT – enough to destroy a large city.

Asteroids of this size are predicted to impact the Earth about every 2-12 years, and the last one of this magnitude was a bolide over the Marshall Islands on February 1, 1994. That impactor was estimated to be between 4.4 and 13.5 meters. A full analysis of that event is available on the SAO/NASA Astrophysics Data System.

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