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As frequent rocket launches make space more accessible, spent vehicles are plummeting back to Earth. In May, a 23-ton Chinese rocket splashed into the Indian Ocean near the Maldives, ending days of uncertainty over where it would land. And in March, the four-ton upper stage of a SpaceX Falcon 9 rocket caused a spectacle when it broke apart over the Pacific Northwest. 

On top of rockets, large projects like internet satellite constellations are just now ramping up, so the current drizzle of falling space debris is likely to keep intensifying. 

Now, a team of researchers believes they have the ideal prototype for tracking the brewing celestial storm: dozens of cameras that keep unwavering watch over Spain’s sky. Designed to spot natural fireballs, in February the Spanish Meteor and Fireball Network (SPMN) picked up a Falcon 9 rocket stage burning up over the Mediterranean Sea. The detection suggests that, together with similar “fireball networks” elsewhere, the SPMN could become a valuable tool for helping space organizations understand and minimize the terrestrial risks of space debris, keeping the odds of a rocket crashing through a roof vanishingly low. 

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“Fireball networks can be very useful for the aerospace community, as we have very precise information about the things that are going through space,” says Josep Trigo-Rodríguez, an astrophysicist from the Institute of Space Sciences (CSIC-IEEC) in Barcelona, and coordinator of the SPMN. 

The Spanish fireball network

The SPMN was never supposed to track falling rockets. For a quarter century, the 200 or so cameras, which are spread out across 37 sites on the Iberian Peninsula, have watched the night sky for the bright streaks left behind when meteors plow into Earth’s atmosphere. The scientists behind the network catalog hundreds of fireballs each year, which they use for two purposes. 

First, they look ahead to predict where the space rocks might have fallen. Using the network, researchers successfully recovered a meteorite from Northern Spain in 2004, which at the time was only the 10th meteorite found in this way. 

Second, they look backwards to estimate where in space the meteor came from. Calculating the original orbits of these objects has helped astronomers discover streams of smaller pieces coming from more threatening asteroids and comets. 

“We are trying to understand the sources of hazards to humans coming from space,” Trigo-Rodríguez says. 

An artificial fireball

Now the team is trying to get a handle on hazards originating closer to home. 

On Feb. 16, three of the network’s cameras picked up a fireball that, from their perspectives in southern and eastern Spain, appeared to be traveling across the crown-shaped constellation of Cassiopeia.

But this fireball moved completely unlike those the SPMN usually detects. When meteors arrive from deep space they come in hot, hurtling through the atmosphere at a steep angle and glowing for just seconds. This object took its time, hanging in the sky for minutes. SPMN researchers quickly realized it must be a piece of space debris, since objects in Earth’s orbit move more slowly and travel nearly parallel to the ground. 

By tweaking the software typically used to analyze the furious flashes of natural fireballs to fit the leisurely arc of the debris, the group calculated the object’s precise path through the atmosphere. The researchers then compared the trajectory with the orbits of debris listed in a US government catalog and found a match. Their fireball was the upper stage rocket of a SpaceX launch of 60 Starlink satellites from earlier that night. The group released their calculations on Sept. 2 in a pre-print, which has been accepted for publication in the Journal Astrodynamics

“To our knowledge it’s the first time that someone has done this using wide-field imagery,” Trigo-Rodríguez says, referring to the way fireball network cameras capture broad swaths of the sky. 

Tracking space debris

And it likely won’t be the last. Rocket launches are on the rise, and SpaceX is just one of a handful of companies in the process of assembling swarms of thousands of internet satellites. These satellites will operate for roughly five years before swan diving into the atmosphere. Trigo-Rodríguez expects that the refreshed software, which was written by his Ph.D. student Eloy Peña-Asensio, an aeronautic engineer at the Autonomous University of Barcelona and CSIC-IEEC, will flag many more instances of falling debris in the future. Doing so, he says, serves three main purposes. 

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To start, identifying space debris could help calm eyewitnesses who may be alarmed at unusual lights in the sky. The March incident in the Pacific Northwest, for instance, appeared dramatic enough to prompt a child of one observer to ask, “Mom, are we ok?” 

Second, studying the objects’ paths could lead to their recovery. Collecting satellite shards might have some limited scientific value (Trigo-Rodríguez’s past research has found that molten balls of metal can imitate natural meteorites, winning them the tongue-in-cheek-nickname, “meteorwrongs”). But more importantly, it could help researchers understand what can survive a fall from space and whether the debris could be dangerous. 

Next, public knowledge of where rocket and satellite pieces end up could put pressure on space organizations to act responsibly. Most rocket stages dive into oceans through a combination of luck and design (most of the Earth is water and launches generally aim for the middle of the Pacific), but countries are legally responsible for damages if anything goes wrong. 

In 1978, for example, a nuclear-powered Soviet satellite crashed in northern Canada, strewing radioactive material across a 600-mile-long strip of land. The Canadian government charged the Soviet Union $6 million Canadian dollars ( about $18 million in US dollars today), and eventually received half that much. 

Fireball networks capable of recognizing space debris, Trigo-Rodríguez suggests, could increase transparency. “All the space agencies around the world should take care to put all these rockets on the right trajectories to decay far from people,” he says. 

Driven by the scientific desire to collect meteorites, fireball networks are already going global. Networks in Australia, Canada, the US, the UK, Argentina, Morocco, and other countries have unified to form the “Global Fireball Observatory,” which continues to expand. Preparing them for artificial fireballs would require just a simple software upgrade. 

“We can establish closer cooperation with [the] aerospace [community] in order to use all the infrastructure we have already built,” Trigo-Rodríguez says. 

Correction Sept. 13, 2021: This post has been updated to correctly name the body of water over which the Falcon 9 rocket stage was burning. It was the Mediterranean Sea; there is no Mediterranean Ocean.

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