Catching Fireballs

At 02:06 in the morning of November 29, a white light suddenly appeared in the sky over many places in southern Henan, accompanied by violent explosions and ground shaking. From the large number of security video surveillance and dashcam videos posted on the Internet, it can be seen that at the moment of the explosion, the night was illuminated like day, and shadows of buildings and trees on the ground were also cast.

This is a typical fireball event. The Center for NEO Studies (CNEOS) of the United States gave the airburst (peak brightness) at 32.6 degrees north latitude, 113.5 degrees east longitude, 38.4 kilometers in altitude, carrying kinetic energy of about 130 tons of TNT equivalent.

The Jiangsu Fireball Monitoring Network being built by the Purple Mountain Observatory also recorded this fireball event. Through analysis, the meteor trajectory is located in the Nanyang-Zhumadian-Xinyang area of ​​Henan Province. The fireball appeared at an altitude of only about 5 degrees, which has reached the limit of monitoring conditions.

The fireball event on November 29th photographed by Jiangsu Fireball Monitoring Network ￨ Source: Purple Mountain Observatory

The scientific significance of the fireball monitoring network

Meteors are astronomical phenomena caused by the entry of near-Earth small celestial bodies or interplanetary matter into the Earth's atmosphere. Meteors brighter than magnitude -4 (about the brightness of Venus) are usually called fireballs. Fireball events are common astronomical events. The diameter of the parent body before entering the Earth's atmosphere usually ranges from ten centimeters to several meters. Some larger fireballs do not burn up after passing through the atmosphere, and they bring precious meteorites to mankind.

There are a large number of small solar system objects around the Earth, which pose a real threat to the Earth and the human living environment. Near-Earth objects with larger diameters can be identified and monitored through astronomical telescope surveys, but near-Earth objects with smaller diameters (such as objects with a diameter of less than 10 meters, which number in the billions) are difficult to be discovered by telescopes outside the atmosphere, and it is impossible to understand their orbital characteristics, landing sites, impact hazards and other key information. We can only get a glimpse of them by monitoring the fireball events caused by them entering the Earth's atmosphere.

The fireball event on November 9th photographed by Jiangsu Fireball Monitoring Network ￨ Source: Purple Mountain Observatory

However, since fireballs move fast and appear at low altitudes, they are only visible in local areas on the earth (about 100 kilometers). Therefore, it is of great practical significance to deploy a fireball monitoring network to carry out regional fireball monitoring.

Current status of fireball monitoring at home and abroad

Several regional fireball monitoring networks have been established around the world, including: Australia's Desert Fireball Network (DFN); NASA's Meteorite Tracking and Recovery Network, Canada's Meteor Observation and Recovery Project 2.0 (MORP 2.0) and the Southern Ontario Meteor Network (SOMN) in North America; and the European Meteor Network (EN) (mainly composed of Czech and German sites), Slovakia's Meteor Video Monitoring Network (SVMN), Spain's Meteor Network (SPMN), Finland's Fireball Network (FFN), the United Kingdom's Fireball Network (UKFN), and France's Fireball Observation and Meteorite Recovery Network (FRIPON) in Europe. In addition, Europe has also established the Meteor Video Monitoring Database (EDMOND), which aggregates data from various European meteor monitoring networks.

Distribution of fireball monitoring networks around the world ￨ Source: Author

In the early days, astronomy enthusiasts in Beijing, Jiangsu, Shandong, Xinjiang and other places spontaneously established some scattered meteor video monitoring sites and small-scale networks. It was not until recent years that my country took some actions on the meteor video monitoring network. In 2017, the Qingdao Aishan Observatory initiated and established the first meteor video monitoring network in cooperation with the National Astronomical Observatory, and more than 40 sites have been laid across the country.

Jiangsu Fireball Monitoring Network

In 2021, Purple Mountain Observatory started to build a fireball monitoring system integrating software and hardware. The first phase will realize the monitoring of fireball events over the whole Jiangsu Province and surrounding areas, and gradually promote the application to the deployment of regional/national fireball monitoring networks.

Distribution of Jiangsu fireball monitoring sites￨ Source: Author

〇 Monitoring website requirements

Fireballs usually appear at an altitude of 80 to 100 kilometers. When deploying stations, it is necessary to ensure that the meteors appear at a suitable elevation angle. Secondly, avoid too small an angle between the fireball trajectory planes (the plane formed by the station and the meteor trajectory) of each station, which will affect the positioning results. Taking all factors into consideration, it is generally recommended that the interval between stations is 100 kilometers, and multiple stations are distributed in a triangle.

For single-station monitoring equipment, if the monitoring points are arranged at intervals of 100 kilometers, 10 arc minutes corresponds to a range of about 300 meters at an altitude of 100 kilometers. Taking into account errors such as target measurement and positioning calculation, the resolution of the monitoring equipment must be better than 10 arc minutes per pixel. When a meteor is within 20 kilometers from the ground, its flight speed decreases and it no longer continues to burn and glow. At this time, the meteor is in the dark flight stage. In order for two stations 100 kilometers apart to monitor a fireball at the same time, the single-station monitoring elevation angle must be greater than 11 degrees, that is, at least 160 degrees of wide sky area must be covered. Therefore, according to the needs of fireball monitoring and identification, the constraints of the monitoring equipment include but are not limited to the following: 1) frame rate higher than 10fps, 2) field of view greater than 160 degrees, 3) resolution better than 10 arc minutes/pixel.

〇Several fireballs have been successfully detected during the testing phase

According to the requirements of the layout interval of fireball monitoring stations, monitoring equipment has been deployed in Xianlin Park of Purple Mountain Observatory, Xuyi Observatory of Purple Mountain Observatory and Jiufengshan Observatory of Nanjing Astronomy Enthusiasts Association in Xingdian. The monitoring equipment uses QHY5III485C astronomical camera with 2.5mm/f1.6 fisheye lens to achieve 180-degree full night sky coverage, and transmits the observation data back to the data center of Xianlin Park through the control terminal.

On September 4, 2021, the Xingdian Jiufengshan Observatory monitoring point successfully captured the first fireball of the monitoring network during the testing phase.

The Xingdian Jiufengshan Observatory successfully captured the first fireball in the monitoring network. Image source: Purple Mountain Observatory

After initial debugging and testing, the system can observe stars with a limiting magnitude of +3 and fireballs with a limiting magnitude of -1.0. At this stage, the three stations have achieved multi-station joint measurement and have connected with astronomy enthusiasts in Shanghai, Zhejiang, Anhui and other places to monitor several fireballs.

Fireball Collection￨ Source: Purple Mountain Observatory

○Later planning

In the future, a fireball monitoring network covering a wider area will be built, and more efficient data processing methods will be used to obtain high-precision positions, orbit determinations, and landing point predictions of fireballs, providing data support for better quantitative assessments of the threat of near-Earth objects colliding with the Earth and for effectively establishing dynamic models of near-Earth objects entering the Earth's atmosphere.

Astronomy enthusiasts from all over the country who are interested in meteor monitoring are especially welcome to join us and work together. We will provide training and guidance on networking experience and data processing technology.

About the Author

Xu Zhijian

China Three Gorges University and Purple Mountain Observatory jointly train 2020 graduate students, majoring in celestial mechanics and astrometry.

Rotating Editor-in-Chief: Ji Jianghui

Editor: Wang Kechao