On Saturday, revelers from across Spain and Portugal ventured out into the warm spring night for a night to remember. No one expected a visitor from outer space to explode above their heads.
At 11:46 p.m. in Portugal, a fireball streaked across the sky, leaving a trail of smoldering incandescent graffiti in its wake. Footage shared on social media shows the dark night momentarily giving way to daylight, sparkling in jaw-dropping hues of snowy white, otherworldly green and arctic blue.
Rocky asteroids self-destruct with some frequency in Earth's atmosphere, causing streaks that extend into the sky. But over the weekend, the projectile hurtled toward Earth at an astonishing speed, about 100,000 miles per hour, more than twice the speed expected of a typical asteroid. Experts said its orbit was unusual, not consistent with the orbits of nearby space rocks.
Because the intruder was not an asteroid. It was a fragment of a comet, an icy object thought to have formed in the early days of the solar system, that lost its battle with Earth's atmosphere 57 miles above the Atlantic Ocean. According to the European Space Agency, it is unlikely that any of the objects ever reached the ground.
“This is an unexpected interplanetary fireworks show,” said Meg Schwamb, a planetary astronomer at Queen's University Belfast.
It is not uncommon for comets to create shooting stars. “We see notable meteor showers throughout the year, which are the result of Earth passing through certain comet debris clouds,” Dr. Schwamb said. For example, the Perseid meteor shower, which occurs every August, is the result of Swift and his Tuttle comet sweeping the world of debris left behind.
These meteor showers and the weekend's only meteor shower will light up the sky in a similar way. The air in front of the object is compressed and heated, causing the debris to heat up, erode, crack, and disappear. That destructive process emits light and, if the projectile is large enough, it emits a powerful shock wave as it releases its enormous kinetic energy into the sky.
Dr. Schwamb said, “The weekend blob was likely a little larger than a good portion of the meteors seen during a meteor shower, so this was a bigger light show.”
In addition to its flashy performance, the breakup of the comet's debris also served as a dry run for experts hoping to protect Earth from large killer asteroids.
One of the principles of planetary defense is to find space rocks before they find them. Then the guardians of the earth can try to do something against them. But the debris, which spans Portugal and Spain, was never spied on until it disintegrated.
“It would have been better if we could have detected the object before it hit Earth,” said Juan Luis Cano of the European Space Agency's Planetary Defense Directorate.
The fear is that objects just slightly larger than Saturday's missile could again evade detection and explode over unsuspecting and unsuspecting cities, with deadly effects. For example, a small 55-foot meteorite that exploded over the Russian city of Chelyabinsk in 2013 was not discovered until it arrived. The midair explosion was equivalent to approximately 500,000 tons of TNT, causing widespread damage and injuring people. At least 1,200 people.
But advances in ground and space technology have made it possible to detect even small, innocuous objects from around the solar system, like this weekend's icy visitor, which experts estimate is several feet in diameter. It is expected that this will be a practical example of planetary defense. Researchers are searching the skies for a common but elusive rock the size of a soccer field that could destroy cities.
Fortunately, a series of next-generation observatories are scheduled to become operational in the coming years, including the Vera C. Rubin Observatory in Chile, named after the American astronomer. This will lead to the discovery of millions of previously undiscovered faint asteroids.
For now, the sights in Spain and Portugal are a reminder that Earth is a participant in the solar system's never-ending game of planetary billiards, and the task of finding as many killer space stones as possible is the most important mission.