A comet fragment explodes in the dark skies of Spain and Portugal

Revelers in Spain and Portugal ventured out into the temperate spring evening on Saturday, hoping for a memorable night. No one expected a visitor from space to explode above their heads.

At 11.46pm in Portugal, a fireball streaked across the sky, leaving behind a smoldering trail of glowing graffiti. Footage shared on social media shows jaws agape as the dark night briefly turns to day, blazing in shades of snowy white, otherworldly green and arctic blue.

Rocky asteroids cause sky-high streaks as they self-destruct in Earth's atmosphere with some frequency. But over the weekend, the projectile hurtled toward Earth at a remarkable speed: about 100,000 miles per hour, more than double that expected from a typical asteroid. Experts say it had a strange trajectory, not matching that normally taken by nearby space rocks.

That's because the intruder wasn't an asteroid. It was a fragment of a comet – an icy object that may have formed in the early solar system – that lost its battle with our planet's atmosphere 37 miles above the Atlantic Ocean. None of the objects are likely to have made it to the ground, the European Space Agency said.

“It's an unexpected interplanetary fireworks display,” said Meg Schwamb, a planetary astronomer at Queen's University Belfast.

It is not uncommon for comets to create shooting stars. “We have notable meteor showers throughout the year, which are the result of the Earth passing through specific cometary debris clouds,” Dr. Schwamb said. For example, the Perseids, which occur every August, are the result of the destruction of our world in the debris left by comet Swift-Tuttle.

These meteor showers and the lone fragment over the weekend light up the sky in a similar way. The air in front of objects is compressed and heated, which cooks, erodes, cracks and clears debris. This destructive process releases light and, if the projectile is large enough, a powerful shock wave as it releases its immense kinetic energy into the sky.

The weekend piece “is probably a little bit larger than a good fraction of the meteors we see during meteor showers, so this just created a bigger light show,” Dr. Schwamb said.

In addition to its flashy performance, the breakup of the comet fragment served as a test for experts hoping to defend the planet from large, killer asteroids.

One of the principles of planetary defense is to find space rocks before they find us; that way, the protectors of the planet will be able to try to do something about it. But the fragment of Portugal and Spain was not discovered before its disappearance.

“It would have been great to detect the object before it collided with Earth,” said Juan Luis Cano, a member of the European Space Agency's Planetary Defense Office.

The concern is that an object just larger than Saturday's missile could again escape detection and explode with lethal effects on an unsuspecting and unwarned city. Even the small 55-foot-tall meteor that exploded over the Russian city of Chelyabinsk in 2013, for example, was not identified before its arrival – and its airburst, equivalent to nearly 500,000 tons of TNT, caused widespread damage, which injured at least 1,200 people.

But as technology improves on the ground and in space, the hope is that even smaller, harmless objects from across the solar system (like the weekend's icy visitor, which experts estimate was just a few meters across) can be spotted , providing practice for planetary defense. researchers search the skies for common but elusive rocks, the size of a football field, that could destroy a city.

Fortunately, a number of next-generation observatories will be operational in the coming years, including one named after an American astronomer, the Vera C. Rubin Observatory in Chile, which will spot millions of faint and previously unknown asteroids.

For now, the show in Spain and Portugal reminds us that Earth is participating in the solar system's endless game of planetary billiards, and that working to find as many killer space rocks as possible is a task of the utmost importance.

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