8 Astounding Facts About the Asteroid Belt

An artist's conception of the dwarf planet Ceres in the main asteroid belt.
An artist's conception of the dwarf planet Ceres in the main asteroid belt.
ESA/ATG medialab

The asteroid belt tells the story of the creation of the solar system. When it formed, not everything coalesced into a planet. Like LEGO bricks that never made it out of the bucket, these objects were organized by physics into a ring located between the orbits of Mars and Jupiter. This is the asteroid belt. Here are some things you might not know about it.


The asteroid belt in black and white.

Over a half-million asteroids have been discovered by scientists, with hundreds of thousands yet to be found. They are generally divided into three classes: S-type (for stony); C-type (chondrites, largely composed of carbon, the most common—and perhaps the oldest—of the bunch); and M-type (metallic). The asteroids range in length from 30 feet to 330 miles. For the most part, they are oddly shaped, and, like the planets, spin (though not always so eloquently). Some asteroids have moons; some have two. Not every asteroid is located in the asteroid belt. Some, called Trojans, share the orbit of Jupiter. Some lurk perilously close to Earth. These are called near-Earth asteroids.


A night sky with a shooting star.

The first asteroid was discovered in 1801 by Giuseppe Piazzi, though he didn't know it at the time. He thought he had discovered the long-sought planet between Mars and Jupiter. He named his discovery Ceres, after the Roman goddess of harvest (and namesake to the word cereal, though that word wouldn't be coined until 1818).

Just over a year later, another "planet" was discovered. And another. And another. Eventually, there were so many planets that astronomers threw up their hands and gave the whole group a new classification: asteroid, or "star-like" in Greek. The name was chosen because they appeared, well, star-like in telescopes; they would not resolve as discs as planets do. Ceres's run as a planet lasted about 60 years before it was demoted to asteroid. In 2006, the International Astronomical Union reclassified it again, this time as a dwarf planet. This is the same decision that designated Pluto as a dwarf planet, though there is some debate on this point.


An artists' rendering of the asteroid belt.

It's likely that illustrations you've seen of the asteroid belt are not drawn to scale. If all of the objects in the asteroid belt were brought together and made into a single ball, its combined mass would only be 4 percent of the size of the Moon. In fact, Ceres alone accounts for one-third of the total mass of the asteroid belt. 


A little girl plays with a toy rocketship in a space system.

What’s the biggest misconception that people have about the asteroid belt? Mental Floss asked Dante Lauretta, the principal investigator of the OSIRIS-REx mission to the asteroid Bennu (which once lived in the main asteroid belt before being jarred loose by Saturn and sent on a course for the inner solar system). His response: The Empire Strikes Back. People imagine "that it's this tumbling, highly energetic boulder field with things crashing into each other constantly, and Han Solo has to dodge and weave to avoid collisions," Lauretta says.

But the asteroid belt is downright spacious. If the arcade game Asteroids were real, it would pretty much consist of a ship and a black screen and … nothing to shoot. "When you fly a spacecraft through the asteroid belt, it’s a real challenge to actually get close enough to an asteroid to see it," Lauretta explains. "You have to specifically target it." There are, on average, 620,000–1.8 million miles between asteroids.


An artist's concept of the solar system.

Back in the 1980s, scientists really set about spectrally classifying each object in the asteroid belt, and they discovered a compositional gradient. There were a lot of dark, carbonaceous objects in the outer asteroid belt, and brighter, "ordinary chondritic," S-type material in the inner asteroid belt. Spectral surveys today are starting to get really detailed, and scientists are getting a good look at objects as small as 6 miles across. As they get into asteroid compositions, they are discovering a fine structure in the compositional pattern, and placing it in the context of the dynamic state of the early solar system.

"Organics and ices are going to be stable farther out [in the belt], and metals and rocks are going to be stable farther in. You expect to see that," says Lauretta. "But now you're starting to see that there really are organic-rich and water-rich materials in the main belt. It's a small fraction of it, but it's a significant fraction. And there's rocky and bright material in the outer belt." That distribution of material is a record of migration of giant planets and the dynamical evolution of the solar system.

"I think we're going to be able to piece together a much more complex, chemical-dynamical coupled model of the evolution of the solar system, and see it almost like the stratigraphic layers of the geologic record. It will tell us the story of the evolution of the belt—not just the initial protoplanetary disk which established that chemistry, but how major events in the evolution of our solar system modified that distribution."


An asteroid in space.

Presently, planetary scientists are trying to identify and understand the primordial structures in the asteroid belt much in the same way that paleontologists or geologists search for the earliest signs of the origin of life in the geologic record. "That's where the biggest challenge lies," says Lauretta. "There is, in the most ancient examples, a discrete signal that you've got to pull out. It's going to be very tiny."

Scientists endeavor to understand what has been preserved from the dawn of the solar system—what they can trust as a true primordial signature of how our planetary system formed—versus what has been eroded or modified or changed over 4.5 billion years of evolution. "Trying to pick out that starting condition and targeting our scientific investigations into those areas is where the greatest challenge lies."

NASA's recently announced Lucy mission to multiple asteroids will help do this. "Everybody thinks those Trojans are the ones that hold that key to the earliest, most primitive material of the solar system," says Lauretta. "The Lucy team can get out there and do some cool science."


A snowflake in snow on a dark background.

No two are exactly alike, and each one has its own story to tell. "Every asteroid is a unique world to explore, and that is awesome," says Lauretta. "There is so much diversity and so many challenges out there. When we truly get out there and start poking around, we're going to see some stuff happening that we never even dreamed of."


An artist's drawing of Dawn firing its engines above Ceres.

NASA's Dawn spacecraft is currently in orbit around Ceres, where it continues to characterize that object and how it changes as it circles the Sun. (It previously orbited Vesta, making it the only spacecraft to orbit two extraterrestrial bodies.) NASA's OSIRIS-REx will arrive at the asteroid Bennu in August 2018. Earlier this year, the agency approved two missions to small bodies: the spacecraft Psyche will travel to the asteroid 16 Psyche, a mysterious, all-metal world. (It might once have been the core of a protoplanet.) The Lucy mission will travel to five Trojan asteroids that share Jupiter's orbit. The Japanese Space Agency's Hayabusa 2 spacecraft will arrive next year at Ryugu (a.k.a. 1999 JU3), a near-Earth asteroid. Like OSIRIS-REx, it will take a sample and return it to Earth for analysis.

And that analysis is serious business. In the case of samples and meteorites, Lauretta says, "most people don't realize that we pick apart these things grain by grain, atom by atom, isotope ratio by isotope ratio, and put together detailed stories about what happened billions of years ago in our solar system."

A Snow Moon—the Year’s Brightest Supermoon—Will Be Visible Next Week


Save the date: The next supermoon is set to light up skies on Tuesday, February 19. Because of when it's arriving, the event will also be a snow moon—a type of full moon that can only been seen this time of year, USA Today reports.

What is a supermoon?

A supermoon occurs when the moon is at its largest in the night sky. That means the Moon is not only full, but also at the point in its orbit that brings it closest to Earth—a position called perigee. On Tuesday, the Moon will appear 14 percent larger and 30 percent brighter than when it's farthest from our planet, making it the brightest supermoon of 2019.

This next supermoon will also have a fun nickname that fits the season. The full moon of each month has a special name. A harvest moon, the first full moon of September, is the best-known moniker, but there are also strawberry moons (June), sturgeon moons (August), and so on. A snow moon is the name for the full moon in February, alluding to February being the snowiest month of the year in the U.S.

When to watch the next supermoon

If the weather is clear in your area, the best time to see the super snow moon is early Tuesday morning on February 19, when the moon reaches its perigee. The Moon will become officially full six hours later at 10:53 a.m. EST. Sunday, Monday, and Tuesday nights will also offer spectacular views of a seemingly huge, nearly full moon.

Supermoons usually happen just a few times a year, but skygazers won't have to wait long for the next one: There's a super worm moon coming March 21, 2019.

[h/t USA Today]

11 Photos From the Opportunity Rover's Mission on Mars


In 2004, the rover Opportunity landed on Mars. Originally intended to serve a mere 90-day mission, the rover instead beamed back scientific discoveries for 15 years. But since a massive dust storm in 2018, the rover Opportunity ceased sending data—and now, NASA has declared its groundbreaking mission complete. (Its twin rover, Spirit, ended its mission in 2011.) Opportunity is the longest-serving robot ever sent to another planet. Let's celebrate Opportunity's Mars mission with a look at the images it captured.

1. Opportunity rover gets its first 360° shot.

Rover Opportunity's 360° photo of Mars

This 360° panorama, comprised of 225 frames, shows Mars as it was seen by the Opportunity rover on February 2, 2004. You can see marks made by the rover's airbags, made as Opportunity rolled to a stop. Here's a larger version of the photo.

2. Opportunity rover finds a meteorite.

Opportunity rover's photo of a meteorite on Mars

This meteorite, found by Opportunity on January 19, 2005, was the first meteorite ever identified on another planet. The rover's spectrometers revealed that the basketball-sized meteorite was composed mostly of iron and nickel.

3. Opportunity rover shoots the Erebus Crater and drifts.

Opportunity rover's photo of Erebus craters and drift

On October 5, 2005—four months after Opportunity got stuck in an area NASA nicknamed "Purgatory Dune"—the rover skirted wind-deposited drifts in the center of the Erebus Crater, heading west along the outcrop (the light-toned rock) on the crater's rim, and snapped this photo with its PanCam.

4. Opportunity rover captures Martian rock layers.

Opportunity rover's photo of layers on Mars

Located on the western ledge of the Erebus Crater, this ledge—called "Payson"—has a diverse range of primary and secondary sedimentary layers formed billions of years ago. According to NASA, "these structures likely result from an interplay between windblown and water-involved processes." Opportunity snapped this photo on April 5, 2006.

5. Opportunity rover comes to Cape Verde.

Opportunity rover's photo of Cape Verde

On October 20, 2007, Opportunity celebrated its second Martian birthday (one Martian year = 687 Earth days) by snapping this photo of Cape Verde, a promontory that juts out of the wall of the Victoria Crater. Scattered light from dust on the front sapphire window of the rover's camera created the soft quality of the image and the haze in the right corner.

6. and 7. Opportunity rover is hard at work on Marquette Island.

Opportunity rover's photo of Marquette Island

This photo shows Opportunity approaching a rock called "Marquette Island" on November 5, 2009. Because its dark color made it stick out, the rover team referred to the rock—which investigations suggested was a stony meterorite—as "Sore Thumb." But it was eventually renamed, according to NASA, using "an informal naming convention of choosing island names for the isolated rocks that the rover is finding as it crosses a relatively barren plain on its long trek from Victoria Crater toward Endeavour Crater."

On November 19, 2009, the rover used its rock abrasion tool to analyze a 2-inch diameter area of Marquette, which scientists called "Peck Bay."

8. Opportunity rover encounters SkyLab Crater.

Opportunity rover's photo of SkyLab Crater

Opportunity snapped a photo of this small crater, informally called Skylab, on May 12, 2011. Scientists estimate that the 30-foot crater was formed within the past 100,000 years. Click the photo for a larger version. You can also see the crater in stereo if you have a pair of anaglyph glasses!

9. Opportunity rover sees its shadow.

Opportunity rover's selfie

On its 3051st day on Mars (August 23, 2012), Opportunity snapped this photo of its own shadow stretching into the Endeavour Crater.

10. Opportunity rover sees its first dust devil.

Opportunity rover's photo of a dust devil
NASA/JPL-Caltech/Cornell University/Texas A&M

Though its twin rover, Spirit, had seen many dust devils by this point, Opportunity caught sight of one for the first time on July 15, 2010.

11. Opportunity rover snaps a selfie.

Opportunity rover's self-portrait
NASA/JPL-Caltech/Cornell University/Arizona State University

A girl sure can get dusty traversing the Martian plains! Opportunity snapped the images that comprise this self-portrait with its panoramic camera between January 3 and January 6, 2014, a few days after winds blew off some of the dust on its solar panels. The shadow belongs to the mast—which is not in the photo—that the PanCam is mounted on.