If all goes well, the spacecraft that NASA plans to launch Tuesday will smash itself to bits against an asteroid.

If all goes absolutely perfectly, that impact will jostle the asteroid into a slightly different orbit, meaning that for the first time, humans will have changed the trajectory of a celestial object.

Making history, however, is incidental. The real mission is to defend the planet.

No need to panic: The target space rock has no chance of striking Earth, nor does any other known asteroid for at least half a century. This NASA mission, operated by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., is testing a technique for redirecting an asteroid in case future Earth folk really need to bat one out of the way.

The basic idea could not be simpler: Hit it with a hammer! But the degree of difficulty is high, in part because no one has ever actually seen the asteroid NASA plans to nudge. It is a moonlet named Dimorphos that is about the size of a football stadium.

Sky watchers operating the world’s highest-powered telescopes detect the moonlet only as a shadow that crosses the larger asteroid it orbits, Didymos, as the two circle the sun together. The pair make up a “double asteroid,” a common arrangement in our solar system.

Here’s how the $330 million Double Asteroid Redirection Test (DART) is supposed to work:

Why just bump it instead of blowing it apart, “Armageddon”-style? Because exploding a pile of ancient rock — especially one that may contain metal or giant boulders, as many asteroids do — would be messy and unpredictable, said Nancy Chabot, a planetary scientist and the mission’s coordination lead. The deflection method assumes we have time for a bit of finesse: A small nudge now could ensure that an asteroid sails well wide of Earth many years down the road.

“You don’t want, necessarily, to make this more complicated than it has to be, right? You would do this well ahead of time, like decades — 10, 20, 30 years ahead,” she said. “Small changes add up to big changes in that amount of time.”

The asteroids in our neighborhood

Thousands of asteroids are large enough and come close enough to Earth’s orbit that researchers need to keep an eye on them.

[The chances of this asteroid hitting Earth are tiny, NASA says — but not zero]

No known asteroid large enough to cause damage on the ground has any significant chance of reaching our planet in the next 50 years, according to Paul Chodas, director of NASA’s Center for Near-Earth Object Studies. His team catalogues and tracks asteroids and comets whose orbits bring them into Earth’s general neighborhood, defined as within 121 million miles of the sun.

Most of these known asteroids were identified by ground-based optical telescopes, and some were located by an infrared space telescope named NEOWISE that detected their heat signatures from its perch in low Earth orbit.

Almost two-thirds of those are so small that they would burn up in Earth’s atmosphere if they came our way. But, of course, some asteroids are huge and dangerous — just ask any dinosaur.

Chodas said scientists have discovered 95 percent of near-Earth asteroids that are large enough to create global catastrophe, meaning a kilometer (about six-tenths of a mile) or wider. The largest is about four miles across, much smaller than the six-mile behemoth that wiped out the dinosaurs.

The unknown ones are the wild cards.

Asteroids that are just a bit smaller but could still do a lot of regional damage are tougher to detect with current technology. Models estimate that we have found just 40 percent of those that are 460 feet wide (140 meters) and larger, such as Didymos and its moonlet. That is well below NASA’s goal of identifying at least 90 percent.

“Some asteroids are sneaky, and they have orbits that make an asteroid very hard to find,” Chodas said.

Some may be in orbits that don’t often bring them close to Earth. Some are made of dark material that doesn’t reflect much light, making it difficult for ground-based telescopes to detect them. Others may lurk on the opposite side of the sun.

The truck-size rock that caused a fireball and shock wave over Russia in 2013 arrived with no warning because it came from the direction of the sun, a huge blind spot for existing telescopes.

[Don’t panic: Scientists are practicing for a killer asteroid impact]

Fortunately, more high-powered eyes are on the way.

In 2026, NASA plans to launch a very sensitive infrared telescope called NEO Surveyor, which will have a wide view of the skies from a stable vantage point about a million miles up between the Earth and the sun. Like its predecessor NEOWISE, it will detect heat signatures rather than visible light.

Amy Mainzer, principle investigator on the Surveyor team, said it should be able to spot a 460-foot asteroid from at least 50 million miles away.

Around the same time, a new ground telescope in Chile is expected to become operational with a massive 28-foot mirror that will be able to detect objects that are much fainter and farther away than any current ground telescope.

“The two together will get us to 90 percent very quickly,” Chodas said.

Why NASA picked this asteroid

The moonlet Dimorphos is an ideal target because of how ordinary it is and how extraordinary its location will be.

It is probably chondrite, Chabot said, a common type of asteroid made of rock and metal rubble left over from when planets were formed 4.5 billion years ago. No one knows its shape, but it is the size of something people would definitely want to redirect if it were headed toward Earth.

About a sixth of all near-Earth asteroids are linked by gravity in pairs or small groups the way Dimorphos is linked to Didymos. That is how we know the moonlet exists: Ground-based telescopes detect the regular dimming and brightening of Didymos as the moonlet passes in front of it and behind it every 11 hours 55 minutes.

The spacecraft’s head-on collision is expected to slow the moonlet enough that Didymos’s gravity will pull it a bit closer, speeding up its orbit. The plume of rock that flies out of the crater on impact may provide an extra push as well.

The contact will occur about 6.7 million miles from Earth, roughly 28 times the distance between the Earth and the moon. That’s close enough for high-speed data transmission and for telescopes on the ground to detect a change in the moonlet’s orbit, but it’s far enough away that the whole endeavor presents a significant technological challenge.

If the craft misses, the asteroid won’t be nearby again for decades.

The tech that will be tested

The DART spacecraft carries quite a bit of sophisticated equipment, including some that NASA is testing for future missions.

What’s next? We’ll see.

In 2024, the European Space Agency will launch a spacecraft named Hera to visit Dimorphos and investigate the crater that — fingers crossed — will be left by DART. What it discovers will help planetary defense experts figure out how the deflection technique can be refined, and perhaps they will gain some insight into what other methods might work as well.

Future techniques might include using gravity to tug asteroids out of orbit, zapping them with lasers, or even moving them with tractor beams, said NASA planetary defense officer Lindley Johnson said in a pre-mission news conference.

“This,” he said, “is just a start.”