It was a day when the Earth was caught in a cosmic crossfire. The big rock came from the south, the smaller one from the east. They were unrelated objects, with different orbits, one the size of an apartment building, the other slimmer but with better aim.

The larger asteroid missed by 17,000 miles, as expected, but the Russian meteor stole the show Friday, fireballing across the Ural Mountains in spectacular fashion and exploding into fragments, creating a powerful shock wave that blew out windows, collapsed roofs and injured 1,200 people, mostly from broken glass.

 It was surely the most thoroughly documented meteor in human history — captured by countless crack-of-dawn Russian drivers who own dashboard cameras.

The spectacle capped an extraordinary day for the planet. The object, which exploded over the industrial city of Chelyabinsk, caused the largest such impact in more than a century and was the first to inflict significant human casualties, with at least 48 victims hospitalized.

The asteroid that was supposed to show up Friday, the much-hyped 2012 DA14, passed by harmlessly, just as the experts had promised it would.

But they had no way of seeing the other rock heading toward Russia. The explanation from NASA scientists, when asked why they hadn’t spotted it, boiled down to two simple facts: It was small, and the sun was in their eyes.

“This was the largest object observed to hit the Earth since 1908,” said Margaret Campbell-Brown, an astronomer at the University of Western Ontario. That’s when another space rock exploded over Siberia, leveling 800 square miles of forest in what became known as the Tunguska event.

On Friday, a global network of sensors recorded the space rock’s object’s descent and revealed its stunning power. It measured about 50
feet wide, weighed more than a nuclear-powered submarine and screamed in at 40,000 miles per hour, said Campbell-Brown, who examined data from sonic sensors deployed by the Comprehensive Nuclear-Test-Ban Treaty Organization to detect nuclear detonations.

In its 30-second shallow-angle dive into the thickening atmosphere, the meteor shed energy equivalent to more than 20 Hiroshima-size atomic bombs. Most of that energy was dissipated many miles above the surface, and, in a sense, the atmosphere saved the day, preventing catastrophic damage from a major surface impact.

Initial estimates from Russian authorities sketched a much smaller and weaker object, but scientists say the nuclear-sensor network provides the best measure an incoming asteroid’s size and power.

Intense heat and pressure shattered the object into dozens of large pieces during its descent. Russian officials said they believed they had identified meteorite fragments on the ground 50 miles west of Chelyabinsk and had reports of pieces stretched out over another 75 miles.

Searchers also found a circular hole in the ice, 15 to 20 feet across, in a lake west of Chelyabinsk, and roped it off.

The meteor that passed through Russian skies on Friday was the largest to reach Earth in a century. Its explosion was brighter than the sun and powerful enough to send damaging shockwaves to the Earth’s surface. Other meteors have impacted the world in big and small ways.

A transcript from a meeting of Russian emergency officials indicated about 3,000 buildings suffered damage.

The region’s governor, Mikhail Yurevich, said the biggest worry after the incident was the cold, with single-digit temperatures forecast overnight. “Our main task now is to preserve the heat in offices and homes where windows were shattered, to prevent the heating system from freezing,” he said.

Chelyabinsk, a city of 1.1 million people, has a high concentration of defense industries, and arsenals in its vicinity have occasionally exploded, but the meteor’s arrival appears not to have set off any. The roof of a zinc factory, however, came crashing down, triggering a spike in global zinc prices.

Russian President Vladi­mir Putin said, “Thank God no large objects fell in populated areas. However, there were still people who were injured.” The Interior Ministry, meanwhile, mobilized 10,000 police to deal with the incident.

The event immediately generated conspiracy theories. One
anti-Western member of Russia’s parliament, Vladimir Zhirinov­sky, claimed that the meteor was actually a U.S. weapons test.

Scientists say the object was instead a small asteroid. NASA’s Bill Cooke said it flew in from the asteroid belt, a band of space rocks circling the sun beyond Mars and the source of all near-Earth asteroids.

History has recorded occasional injuries from meteorites, but the number hurt Friday is unprecedented, said Timothy McCoy, who studies meteorites at the Smithsonian’s National Museum of Natural History. “I can’t think of a burst this size over a city before,” he said.

Amateur footage showed at least two orange flashes as the meteor streaked over apartment buildings. A series of booms trailed the space rock. As it exploded, the meteor briefly blazed brighter than the sun.

And no one saw it coming.

A weather satellite’s camera snapped the meteor’s dive, but a global network of asteroid-spotting telescopes funded by NASA failed to detect it. The sun was in the way, the telescopes blinded by a dayside approach.

“An asteroid such as this is virtually impossible to see ahead of time,” said Paul Chodas of the Jet Propulsion Laboratory in Pasadena, Calif.

The twin asteroid encounters on Friday — one benign, the other malign — laid bare both the uncertain reality of life in the cosmos — you never know when a space rock might come crashing down — and our planet’s lack of defense against such threats.

NASA, the world’s lead agency for detecting asteroid hazards, boosted its budget for the task from $6 million in 2011 to $21 million last year. And a NASA effort launched in 1998 has found 95 percent of potential “planet killers” at least a mile wide; none is headed for Earth. But still, many say the agency, and the world, are not doing enough.

Two congressmen seized the moment to promise hearings on the planet’s space-threat readiness. Lamar Smith (R-Tex.), chairman of the House Science, Space and Technology Committee, said in a statement that developing technology to track asteroids “is critical to our future,” while Dana Rohrabacher (R-Calif.) called asteroid impacts the “only true preventable natural disaster.” Even if we find one that will hit us, he said, “we might not be able to deflect it.”

A private effort by two former astronauts to build a space telescope to spot smaller Earth-bound asteroids has raised a few million dollars, but is years away from launch — if it ever gets off the ground.

Plans to deflect asteroids are even more nascent. Proposals range from the sublime — spraying an asteroid with reflective paint so the sun nudges it — to the extreme — nuking it. With early detection and a few decades of lead time, even a tiny nudge could push an asteroid out of the tiny “keyhole” in space that would otherwise send it crashing to Earth.

The European Space Agency and NASA are in the early stages of plotting a mission to smash a spacecraft into an asteroid to see if humans can, in fact, push around a sizeable space rock. The project, called AIM-DART, has no timeline and no real budget.

In another cosmic coincidence, three of the nation’s top asteroid trackers met in Vienna on Friday with international counterparts under the auspices of the United Nations Committee on the Peaceful Uses of Outer Space to better coordinate planetary defense.

“This is a what-sort-of-things-are-we-doing-to-protect-the-Earth meeting,” said Timothy Spahr, director of the Minor Planet Center at Harvard University, the international clearinghouse for asteroid tracking. He said enthusiasm for building a better detection network runs high among the attendees from dozens of nations.

As for meeting on the day when the reality of extraterrestrial threats burst into view over Russia, Spahr said, “Yeah, pretty crazy timing.”

Englund reported from Moscow. Caitlin Dewey and Olga Khazan in Washington contributed to this report.