Comet Fragment Explodes in Dark Skies Over Spain and Portugal

On Saturday, revelers across Spain and Portugal ventured into the temperate springtime evening, hoping for a memorable night. None were expecting a visitor from outer space exploding above their heads.

At 11:46 p.m. in Portugal, a fireball streaked across the sky, leaving a smoldering trail of incandescent graffiti in its wake. Footage shared on social media shows jaws dropping as the dark night briefly turns into 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 was plunging toward Earth at a remarkable speed — around 100,000 miles per hour, more than twice that expected by a typical asteroid. Experts say it had a strange trajectory, not matching the sort normally taken by nearby space rocks.

That’s because the interloper wasn’t an asteroid. It was a fragment of a comet — an icy object that may have formed at the dawn of the solar system — that lost its battle with our planet’s atmosphere 37 miles above the Atlantic Ocean. None of the object is likely to have made it to the ground, the European Space Agency said.

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

It is not rare for comets to create shooting stars. “We have notable meteor showers throughout the year, which are the result of the Earth crossing debris clouds of specific comets,” Dr. Schwamb said. For example, the Perseids, which occur every August, are the result of our world’s sweeping through litter left behind by Comet Swift-Tuttle.

These meteor showers, and the lone shard over the weekend, light up the sky in a similar manner. Air in front of the objects is compressed and heats up, which cooks, erodes, cracks open and obliterates the debris. That destructive process releases light — and, if the projectile is big enough, a powerful shock wave when it surrenders its immense kinetic energy into the sky.

The weekend’s “chunk is likely a bit bigger than a good fraction of the meteors we see during meteor showers, so this just made a bigger light show,” Dr. Schwamb said.

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

One tenet of planetary defense is to find space rocks before they find us; that way, the planet’s protectors can try to do something about them. But the shard over Portugal and Spain was not spied before its demise.

“It would have been great to detect the object prior to colliding with the Earth,” said Juan Luis Cano, a member of the Planetary Defence Office at the European Space Agency.

The worry is that an object just a little larger than Saturday’s missile could again escape detection and explode with lethal effect over an unaware, unwarned city. The meager, 55-foot meteor that exploded above the Russian city of Chelyabinsk in 2013, for example, wasn’t identified before its arrival, either — and its airborne blast, equivalent to nearly 500,000 tons of TNT, caused widespread damage, which injured at least 1,200 people.

But with improved technology on the ground and in space, the hope is that even tiny, harmless objects from around the solar system (like the weekend’s icy visitor, which experts estimate was a few feet across) can be spotted, providing practice for planetary defense researchers searching the heavens for the common but elusive football-field-size rocks that could destroy a city.

Fortunately, a series of next-generation observatories are set to come online in the next few years — including one named after an American astronomer, the Vera C. Rubin Observatory in Chile, which will spot millions of faint and previously undiscovered asteroids .

For now, the spectacle in Spain and Portugal reminds us that Earth is a participant in the solar system’s never-ending game of planetary billiards, and that working to find as many killer space rocks as possible is a task of the utmost importance.