June 26, 2019 at 10:44 pm #3001
Networks of cameras are making it easier to track meteors and also to find the bits that reach the ground
Every day between 100 and 600 tonnes of rock hurtles into Earth’s atmosphere. The reason so little of this bombardment makes it to the planet’s surface is that much of it is burnt up by atmospheric friction, which creates the fireball that is the visible sign of a meteor’s arrival. As for the bits that do get through, once landed, they are known as meteorites.
Roughly 60,000 objects of meteoritic origin have been picked up and catalogued. Most are fragments from a much smaller number of individual falls. Of these falls, only 36 were observed as they arrived with enough fidelity to calculate the orbit of the original meteor before it entered the atmosphere. If more such data were available it could, by showing where the rocks came from, cast more light on the composition of the solar system. It might also help in moving orbiting spacecraft out of danger.
The tracking of meteors is carried out by arrays of cameras on Earth. The oldest of these is the European Fireball Network (efn), which dates back to 1951 and is operated by the Astronomical Institute of the Czech Academy of Sciences. When it launched its equipment was primitive—two groups of eight cameras capturing images on glass photographic plates using all-night-long exposures. Each camera group covered half the sky. Now, the network deploys 24 state-of-the-art digital cameras equipped with fish-eye lenses in 18 stations scattered across Austria, the Czech Republic and Slovakia. Two more stations, in Germany, are planned for later this year.
The digital cameras take back-to-back photographs, with 35 second exposures, from dusk to dawn. Fish-eye lenses allow a single exposure to cover the whole sky immediately above each camera. If more than one camera sees the same fireball—which is usually the case—that meteor’s course can be triangulated, with a precision of about ten metres, by comparing the images. This yields two valuable pieces of information. Plotting the path backwards reveals the rock’s orbit before it slammed into Earth’s atmosphere. Projecting it forward suggests a potential landing site.
The efn’s cameras also contain radiometers that measure changes in a fireball’s luminosity 5,000 times a second. This reveals the rock’s entry speed, its probable mineral make-up, the amount of fragmentation and deceleration rate. If the data indicate anything is likely to have reached the ground, an alert is automatically emailed to the network’s operators.
To calculate an impact’s location, researchers take into account how wind affects the trajectory during 20km or so of “dark flight”, after a fireball has burned out. A decade ago, half of meteorites found as a result of the efn’s data were within 500 metres of the predicted spot. That figure has now shrunk to 100 metres. Pavel Spurný, the network’s co-ordinator, usually keeps the impact zone secret until his team, of trusted helpers, can search for it. Meteorites have commercial as well as scientific value. Giving the game away too early risks losing finds to professional collectors.
The efn’s hardware was not hugely expensive. The network’s cameras cost about $30,000 a piece. Operating the system adds $114,000 a year, according to Dr Spurný. But it has improved the success rate enormously. Between 1951 and 2014, when the new cameras started to be rolled out, rocks from five falls were recovered. Since then, that total has doubled. Even so, cloudy skies can foil the instruments. And meteorites, many of which are small and dark, are not always easy to find in the vegetation and darkish soils of central Europe.
For all these reasons, Phil Bland, a British meteorite expert, reckoned the pickings are better on the flat, brushless, lightly coloured deserts of Western Australia—a place where, as a bonus, the skies are mostly clear. Dr Bland, who works at Curtin University, in Perth, has therefore set up what he calls the Desert Fireball Network (dfn). This now sports 52 camera observatories, though the cameras themselves are, at $10,000 a pop, cheaper and less snazzy than the efn’s. These cameras keep a persistent eye on the western third of Australia’s night sky.
The dfn has been a success. It has produced, Dr Bland says, a big data set “of gorgeous orbits” for incoming rocks. The number of meteorites believed to have landed has overwhelmed the team’s resources. They have recovered stones from four falls, but are in need of adventuresome volunteers to mount expeditions into the outback to gather the remains of more than 30 others.
In America, meanwhile, the NASA All-sky Fireball Network, run by America’s space agency, operates 18 cameras across the United States. Its goal is not to find meteorites, but to protect spacecraft from collisions. By studying fireballs, the agency’s Meteoroid Environment Office in Huntsville, Alabama, which operates this particular network, improves estimates of the number, size, speed and trajectory of space rocks in areas where satellites operate. The forecasts of Earth’s periodic peak bombardment by objects from a cloud of cometary debris called the Draconids, for example, has improved from an accuracy of about two hours in 2012 to just 30 minutes today, says Bill Cooke, who runs the project.
Dr Cooke’s team use the data the network collects to calculate the risks faced by individual spacecraft. NASA publishes these numbers so that insurance underwriters can take them into account, as can mission operators. In areas with higher collision risks, controllers may temporarily shut down high-voltage subsystems that, if struck, might fry the spacecraft they are part of, or reorient a craft so that the narrow edges of its solar panels face any onrushing space rocks, minimising the risk of impact.
Spacecraft engineers also use Dr Cooke’s data to design better “bumper shields”. These consist of layers of Kevlar and other materials spaced so that they gradually break apart an incoming meteor, depriving it of energy. To keep launch weights down, not all sides of a spacecraft are shielded equally, usually the rear is the most heavily armoured part.
To gauge a projectile’s destructive power, one must know its speed. A team at the University of Western Ontario, in Canada, clocks meteors smaller than grains of sand. Using high-frequency radar, the team fires pulses into the sky 500 times a second, day and night. These detect not meteors themselves, but rather the trails of ions, generated by friction within the air, that they leave behind. The radar sees this as a “giant wire in the sky”, says Peter Brown, the team leader. An array of microphones sensitive enough to measure shock waves from meteors a centimetre or more across provides additional data. Dr Brown puts the average speed of such shooting stars at about 20km a second—significantly faster than many had thought.
That is bad news for satellites. But if the various meteor-monitoring networks around the world can help improve the forecasting of peak meteoric activity, then the number of spacecraft suddenly found to be in peril will be reduced.
From: The Economist – Science and technology section of the print edition, titled “Skyfalls” 28/3/19
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