Infrasonic Detection of Meteorites - Video

Infrasound arrays at an infrasound station at Qaanaaq, Greenland, part of the Nuclear Test Ban Treaty monitoring system 

Credits: CTBT Organization

In the early morning hours of November 17, 1998, a bright fireball was observed over northern New Mexico, about 150km away from Los Alamos.

The bolide was part of the annual Leonid meteor shower. Although the event did not produce any sonic boom reports, it was detected by an infrared radiometer and by an intensified camera located in the state.

Los Alamos National Laboratory (LANL) investigated the sighting in its role as a part of the International Monitoring System (IMS) created following the Comprehensive Test Ban Treaty (CTBT).

LANL found the presence of an infrasonic signal detected by six infrasound arrays. The signal matched the time and the direction of the fireball seen in the sky.

The infrasound recording indicated that the explosion occurred at 93.5 kilometer, matching the measurements from the camera.

The velocity obtained for the bolide from the signal was between 920 and 1150km/s. The meteorite was calculated to have a source energy equivalent to about 1.14 tons of TNT, where source energy is the kinetic energy when the shock wave is produced because of the passage through the atmosphere or the fragmentation of the meteoroid itself.

To detect nuclear explosions, IMS employs infrasound stations using microbarographs (acoustic pressure sensors) to detect very low frequency sound waves.

The investigation conducted by Douglas O. Revelle and Rodney W. Withaker at LANL showed that these detectors could also have been used to detect and measure objects entering the atmosphere.

According to Revelle, an array of low frequency sensors horizontally separated by a few hundred meters to a few kilometers can be used to determine both the direction and the elevation angle of the signals.

The determination uniquely locates the infrasound sources in a three-dimensional space within the atmosphere within certain errors.

Such data are also useful to estimate the frequency of occurrence of certain types of meteoroids. Revelle estimates that an event with the energy level of 10Mt, such as Tunguska, is likely to happen once every 120 years.

However, data from infrasound measurements reported that 30 ±9 large bolides with an energy level of 0.1kt are likely to enter Earth’s atmosphere every year.

The data shows that the number of entering debris increases as the source energy decreases and vice versa.

Historically, the primary source of data collection for reentering objects has always been visual or optical observations.

However, due to the extensive deployment of ILS infrasound sensors, this mode has shown its relevance along with radar and optical observation for the study of meteor physics.

The Los Alamos investigation was one of the first times that infrasound detection has been used to study objects reentering the atmosphere.

Revelle’s pioneering theoretical work on interaction between meteors and atmosphere led the way for future studies.

CTBTO detected 26 asteroid explosions in Earth's atmosphere over last ten years - Video

The B612 Foundation, a California-based nonprofit dedicated to raising awareness about the risks of an asteroid collision, has released this video visualising the 26 asteroids that have hit the Earth's atmosphere between 2000 and 2013.

The 26 asteroids were identified by listening devices manned by the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), a global detection network created to keep an ear out for unauthorised nuclear weapons testing. The impact explosions ranged in strength from one to 600 kilotons.

"To put that in perspective, the atomic bomb that destroyed Hiroshima in 1945, exploded with an energy impact of 15 kilotons," the B612 Foundation pointed out in a press release.

Between 2000 and 2013, CTBTO's a network of sensors that monitors Earth around the clock listening for the infrasound signature of nuclear detonations detected 26 explosions on Earth ranging in energy from 1 to 600 kilotons.

These explosions were not by caused by nuclear explosions, but rather by asteroid impacts.

While most of these asteroids exploded too high in the atmosphere to do serious damage on the ground, the evidence is important in estimating the frequency of a potential “city-killer-size” asteroid.

A list of the impacts shown in the video can be found on the B612 website here.

Asteroid Impacts on Earth Obstructed by Red Tape

Artist's view of last year’s fireball explosion over Chelyabinsk, Russia – termed a "superbolide" event.

Credit: Don Davis

Red tape is making it tougher for researchers to study and characterize asteroid strikes on Earth, which are apparently more common than previously thought, experts say.

The bureaucratic snafu affects the use of U.S. government space assets that help scientists study "airbursts" like the meteor that exploded without warning over Russia last year.

At issue is the ability to combine space data with outputs from a global network of seismic, infrasound and hydroacoustic sensors that have been deployed worldwide to provide treaty verification for a nuclear test ban.

This network is the International Monitoring System (IMS) overseen by the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO).

In recent years, the IMS has detected shock waves from many airbursts, providing further evidence that asteroid impacts are more frequent than previously thought.

Ideally, matching observations from spacecraft with measurements from CTBTOs infrasound detectors would give scientists more of a heads-up on what's raining down on Earth, experts say.



Memorandum of agreement
Last year, the Air Force Space Command signed a memorandum of agreement with NASA's Science Mission Directorate.

That document, inked on Jan. 18, 2013, spelled out specifics for the public release of meteor data from sources such as high-flying, secretive U.S. government space sensors.

With that agreement in place, NASA's Near Earth Object (NEO) Observation Program started receiving information on fireball events based on analysis of data collected by U.S. government sensors.

Details of atmospheric meteor explosions, as recorded by U.S. military spacecraft sensors, were posted on a publicly accessible NASA website run by the Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

In fact, the military-civil cooperation was spurred by the details of the February 2013 fireball explosion over Chelyabinsk, Russia, termed a "superbolide" event.

The website postings are designed to assist the scientific community's investigation of bolides, or exceptionally bright fireballs.

However, multiple scientists noted that the JPL website had not been updated recently. That presumably meant that there was some sort of delay, as some fairly big events were detected by infrasound in the last year.

"Because of budget and personnel reductions on our military partner, they ran into workforce issues to accomplish this task," said Lindley Johnson, NEO program executive within the Planetary Science Division of NASA's Science Mission Directorate in Washington, D.C.

"We are already in discussions with them about what it will take to get it restarted," Johnson told reporters