The serious asteroid watchers know the big ones by name, diameter, composition, orbit, spin and potential troublesomeness.

There’s Apophis, for example, a rock bigger than the Pentagon, which on April 13, 2029, will pass close enough to the Earth to be seen with the naked eye and quicken the pulse. There’s Toutatis, mountain-sized tumbling and, fortunately, at a safe distance. There’s 2005 YU55, a quarter of a mile wide, which passed the Earth at roughly moon distance a little over a year ago.

And now comes 2012 DA14, a rock the size of an apartment building that on Friday will buzz the Earth at a distance of about 17,200 miles. It poses no hazard. It won’t generate the slightest breeze or disturb a single mote of dust meandering in a sunbeam.

DA14 is too small to see with the naked eye. Though it will pass just inside the orbit of communications satellites, the chance that it will obliterate anyone’s favorite TV show is vanishingly remote.

Scientists point out that we have never detected an asteroid this large passing so close to our planet. The key word here is “detected.” Astronomers have honed their ability to see things that just a few decades ago would have been invisible. The universe keeps coming into focus, and it’s messier out there than the ancient stargazers could have imagined.

An asteroid the size of an office building will narrowly miss Earth, but it’s just one of millions out there. The Post’s Brian Vastag stops by to tell us about the “giant cosmic shooting gallery” in which our planet sits and what scientists are doing to protect us. (The Fold/The Washington Post)

“Thirty years ago, we would not have been able to find the thing,” said Michael Busch, a planetary astronomer at the National Radio Astronomy Observatory in Socorro, N.M.

Busch estimates that an object this size passes this close to Earth every decade or two, on average. He is part of a team of researchers that will use four telescopes, arrayed from California to New Mexico, to bounce radar signals off DA14 in an attempt to understand how it is spinning.

The precise size and shape of the rock are unclear. In telescopes, DA14 is just a bright dot. That will change Friday as astronomers observe it coming and going.

DA14 isn’t an old asteroid. It likely is the remnant of a collision in the asteroid belt within the past few million years, Busch said. Such rocks have limited life spans: As sunlight strikes their uneven surfaces, they spin faster and faster, until finally they explode, a process known as the YORP effect (for the scientists who studied it — Yarkovsky, O’Keefe, Radzievskii and Pad­dack).

Most asteroids are in the famous belt between Mars and Jupiter. Some asteroids are big enough to have their own moons. At least two asteroids orbit the sun in the opposite direction of everything else in the solar system; they’re wrong-way drivers. Some asteroids spin neatly on a single axis; others, such as Toutatis, tumble chaotically, as if someone had knocked them silly.

Any asteroid that comes within 50 million kilometers — about 31 million miles — of the Earth’s orbit is categorized as a near-Earth object. There are hundreds of thousands of NEOs, and scientists believe they’ve now mapped roughly 95 percent of the largest NEOs, the ones that would be very hazardous to human civilization should one strike the planet. None of the asteroids found so far is on a collision course with Earth.

The only known impact of a sizable asteroid in human history occurred in 1908 — the “Tun­guska event.” It’s unclear whether an object struck the Earth or exploded in the atmosphere. The impact leveled a vast area of forest in remote Siberia. Researchers in 2012 claimed to have found a small lake that is an impact crater, but that finding remains controversial.

A timeline of the Mars rover Curiosity through its two-year mission on Mars.

This week’s visitor, DA14, is slightly larger than the asteroid that struck the Tunguska region. Such an asteroid probably hits only about once every 1,200 years on average, scientists estimate.

Given enough time, a major impact becomes a certainty. The asteroids are patient and operate on time scales different from mere mortals.

“It’s not the type of thing you panic over,” says David Kring, a geologist at the Lunar Planetary Institute in Houston. “But it’s a real threat, and we should take prudent steps to understand the hazard better. What are those prudent steps? First, a geological one — you study the geological past, measure the consequences of past impact events.”

For a long time, geologists overlooked impact craters on Earth because they were intellectually reluctant to embrace anything that smacked of catastrophism. The theory of “uniformitarianism” saw the world in terms of slow, gradual processes — a view that helped Charles Darwin develop his theory of evolution.

Catastrophism made a comeback after the father-son team of Luis and Walter Alvarez announced in 1980 their discovery of iridium in a narrow band of sediment marking the end of the Cretaceous Era, when dinosaurs and half the species on the planet vanished in a mass extinction. Iridium is rare on Earth but relatively common in asteroids. A decade later, scientists said they had matched the end-Cretaceous impact to a massive crater beneath the tip of the Yucatan peninsula near the town of Chic­xulub.

“Part of that revolution was to realize that just a relatively small impact, from a 10-mile object, was able to redirect the course of evolution,” said David Morrison, a NASA scientist and a major force in the mapping of NEOs.

“In other words, the leverage was huge,” he said. “This object, geologically, was quite minor, and had a tremendous impact on biology.”

Lurking somewhere on Earth, eroded and buried and scrubbed by time, are mega-craters, remnants of what scientists refer to as the Late Heavy Bombardment of about 4 billion years ago. We don’t know if life had yet appeared when 70-mile-wide rocks slammed into the planet, but it wouldn’t have been a good time to be alive.

“The oceans boil,” said Jay Melosh, a planetary scientist at Purdue University and an expert on impact craters. “Most of the oceans boil away, the Earth has a steam atmosphere that lasts for thousands or tens of thousands of years, and the surface, of course, is very, very hostile.”

There is in the study of ancient craters a glimpse of our blessings as a planet. Such craters are hard to find because this is not a static world but a living, dynamic place.

Every time we look at the cold, gray, cratered moon, we see what might have been.