Asteroids constantly zoom around our solar system and sometimes smash into planets, including Earth. So, in the name of planetary protection NASA tested, called DART (Double Redirected Asteroid Test), to deflect a space rock like the one that wiped out the dinosaurs (or the one that Bruce Willis blew up at the end of the movie Armageddon).
Not all asteroids are planet killers, however. While some are monstrous, others are tiny. So what are the biggest and the smallest asteroids on record in the solar system?
In short, Ceres is the largest asteroid in the solar system, nearly 590 miles (950 kilometers) across, about one-quarter the size of the moon. The smaller asteroid is harder to pin down; the smallest asteroid officially recognized by NASA (opens in a new tab), known as asteroid 2015 TC25, is 6 feet (2 meters) across. That is to say in some definitions asteroids must be at least 3 feet (1 m) across (opens in a new tab)but not all scientists agree on this size limitation.
Related: What are the largest impact craters in the world?
Probably the term “asteroid”. coined by the German astronomer William Herschel (opens in a new tab) in 1802. But as it turns out, this word is a misnomer — meaning “star-like” in Greek. Rather than glowing balls of plasma, asteroids are small rocky bodies that carry dust, ice and sometimes metal. “Obviously they have nothing to do with stars,” Simone Marchi (opens in a new tab), an astronomer at the Southwest Research Institute in Boulder, Colorado, told Live Science. “So it’s kind of a vague term and definition at first.”
Asteroids are thought to have formed in the early days of the Solar system, when the sun was surrounded by a thick cloud of dust. As this dust cooled over thousands of years, it began to coalesce into larger and larger clumps, which gained mass as they collided and stuck together. Eventually, some of these bodies gained enough mass to become planets. But a smattering of leftovers remained “like crumbs on a table,” said Marchi. Some of these “crumbs” became asteroids.
Today, most asteroids in the solar system are found in the asteroid belt, a 140-million-mile-wide (225 million km) area of rocks and dust orbiting in the space between Mars and Jupiter. Scientists have recorded nearly a million of these objects so far. And there are probably millions more, depending on how they are counted.
As the largest known asteroid, Ceres is easy to see with a telescope. In 1801, it became the first known object in the asteroid belt, NASA reported (opens in a new tab). In 2006, the giant space rock – which is more spherical than other asteroids and has a thin atmosphere – was categorized as a dwarf planet, the same designation given to Pluto. (But Pluto, at 1,473 miles (2,370 km) in diameter (opens in a new tab)larger and 14 times larger than Ceres, according to NASA.) Based on its size and structure, scientists believe that Ceres could tell us a lot about how planets form (opens in a new tab). If you disqualify Ceres based on its dwarf planet status, then Vesta is the largest asteroid, at 326 miles (525 km) across.
Meanwhile, the smallest asteroid is a matter of debate. “There is no official cut,” he said Ian Fernandez (opens in a new tab)an astronomer at the University of Central Florida, “so different people have different ideas about what an asteroid is.”
Telescopes have seen asteroids as small as 3 feet across, and there are tons of smaller objects orbiting in the asteroid belt. However, objects this small tend to ignite if they enter the relatively thick atmosphere of a planet like Earth, according to NASA (opens in a new tab). According to this metric, they can be categorized as meteoroids, small bits of space debris that become meteors as they streak across the sky (and meteorites if they reach the planet’s surface).
To make matters more confusing, asteroids don’t always stay the same size. “Out of spin, there are different ways for asteroids to lose things,” Fernandez told Live Science. Some crack from thermal stress as they go near the sun. Others collide with each other, breaking off chunks of rock in the process. Still others begin to rotate on their axis, gradually spinning so fast that they release some of their dust into the void.
Sometimes pieces of larger asteroids qualify as asteroids in their own right, but other times they are downgraded to meteoroids or simply space dust. So how do experts draw the line between an asteroid and a meteoroid? For Marchi, if there is a rocky object in space, it will be counted as an asteroid — it is only considered a meteoroid when it enters the planet’s atmosphere.
Fernandez sees it as a matter of observation. “If you can find it with a telescope … look at it in orbit and what not, I would count it as another asteroid in the asteroid pile,” he said. “But just my opinion, right?”