Image of Arrokoth

The Kuiper Belt object Arrokoth is double-lobed, meaning it has two rounded pieces in contact with each other. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko | Full image and credit

10 Things in Space That Come in Twos

By Amanda Barnett

Feature | February 18, 2022

This February’s calendar has lots of twos. There was 02-02-2022 and now 02-22-2022 – which some people are calling “Twosday.” To mark this day of duos, we bring you our list of things in the solar system that come in twos, or pairs.

DART Mission
Credit: NASA/Johns Hopkins APL/Steve Gribben | Full image and caption

1. Binary Asteroids – Sometimes Asteroids Come in Pairs

Asteroids don’t always fly solo. Some have a moon – or moonlet – orbiting the larger body. This is called a binary asteroid system. NASA has a spacecraft traveling to a near-Earth binary asteroid: the Double Asteroid Redirection Test, or DART, is heading to Didymos, which means "twin" in Greek (and explains the word "double" in the mission's name). Didymos has a moonlet named Dimorphos. DART will impact Dimorphos nearly head-on to demonstrate a method of asteroid deflection.

DART’s target is NOT a threat to Earth. This asteroid system is a perfect testing ground to see if intentionally crashing a spacecraft into an asteroid is an effective way to change its course, should an Earth-threatening asteroid be discovered in the future. No known asteroid larger than 460 feet (140 meters) in size has a significant chance to hit Earth for the next 100 years.

Double Craters
Credit: NASA/JPL/University of Arizona | Full image and caption

2. Double Craters

This image shows a remarkable double crater on Mars. Scientists think the craters must have formed simultaneously. Maybe Mars was hit by a double asteroid. Or it may have been impacted by an asteroid or comet that separated into two pieces before hitting the surface.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko | Full image and caption

3. Kuiper Belt Objects – Strange, Faraway Worlds

The small Kuiper Belt object Arrokoth is the most distant and most primitive object ever explored by a spacecraft. It was discovered in 2014 by NASA’s New Horizons science team, using the Hubble Space Telescope. The New Horizons spacecraft flew by Arrokoth on Jan. 1, 2019, snapping images that showed a double-lobed object that looked like a partially flattened snowman. It’s also very red – even redder than Pluto. The object's strange shape – unlike any other object visited so far – was the biggest surprise of the flyby.

Earth and Moon
Credit: NASA/JPL-Caltech | Full image and caption

4. Earth and Its Moon – Quite the Pair

The brightest and largest object in our night sky, the Moon makes Earth a more livable planet by moderating our home planet's wobble as it spins on its axis, leading to a relatively stable climate. It also causes tides, creating a rhythm that has guided humans for thousands of years, and life in Earth's oceans for much longer. The Moon was likely formed after a Mars-sized body collided with Earth several billion years ago.

Pluto and Charon
Credit: NASA/JHUAPL/SwRI | Full image and caption

5. Pluto and Charon – Double Planet

Pluto has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. The largest of those moons, Charon, is about half the size of Pluto. That makes it the largest satellite relative to the planet it orbits in our solar system. It orbits Pluto at a distance of just 12,200 miles (19,640 kilometers). For comparison, our Moon is a quarter the size of Earth and 20 times farther away from Earth. Pluto and Charon are often referred to as a double planet. This composite of enhanced color images of Pluto (lower right) and Charon (upper left), was taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015.

Credit: NASA/JPL-Caltech | Full image and caption

6. Two Interstellar Travelers

NASA’s twin Voyager 1 and 2 spacecraft are exploring where nothing from Earth has gone before. Launched in 1977, each probe is much farther away from Earth and the Sun than Pluto. In August 2012, Voyager 1 made the historic entry into interstellar space, the region between stars, filled with material ejected by the death of nearby stars millions of years ago. Voyager 2 entered interstellar space on November 5, 2018. Both spacecraft are still sending scientific information about their surroundings through NASA's Deep Space Network, or DSN.

Credit: NASA/JPL-Caltech

7. Double Stars

Sirius is the brightest star in our sky. It’s nicknamed "the Dog Star" because it's the brightest star in the constellation Canis Major ("the Greater Dog" in English). The main reason Sirius is so bright is that it's one of the closest stars to our Sun, at just 8.6 light-years away. Sirius is actually a binary star system, with a tiny, white dwarf companion (although you'd need a decent-sized telescope to see it). Sirius is super easy to locate: Just look for the constellation Orion. The three bright stars that make up Orion's belt point downward, toward Sirius. (Unless you're in the Southern Hemisphere, and then they point *up* toward Sirius.) Bonus note: Voyager 2 spacecraft is actually headed in the direction of Sirius. It'll pass within 4.3 light-years of the bright star in about 300,000 years.

Planet with two suns
Credit: NASA/JPL-Caltech | Full image and caption

8. A World With Two Suns

NASA's Kepler mission discovered a world where two suns set over the horizon instead of just one. The planet, called Kepler-16b, is the most "Tatooine-like" planet yet found in our galaxy and is depicted here in this artist's concept with its two stars. (Tatooine is the name of Luke Skywalker's home world in the science fiction movie “Star Wars.”) In this case, the planet is not thought to be habitable. It is a cold world, with a gaseous surface. NASA missions have since discovered at least a dozen other planets orbiting two suns.

Two moons
Credit: NASA/JPL/Space Science Institute | Full image and caption

9. Seeing Double

Nope, that’s not two images of the same moon. Saturn's sibling moons, Rhea and Dione, appear to be side by side in this image captured by NASA’s Cassini spacecraft. The image was taken in visible light on Nov. 1, 2005, when Cassini was at a distance of approximately 1.1 million miles (1.8 million kilometers) from Rhea, the top image, and 800,000 miles (1.2 million kilometers) from Dione.

Rovers on Mars
Credit: NASA/JPL-Caltech | Full image and caption

10. Rovers on Mars

NASA has two rovers operating on Mars: Perseverance (Mars 2020), which just passed the one-year mark on the Red Planet; and Curiosity (Mars Science Laboratory), which landed Aug. 5, 2012. This isn’t the first time NASA has had two rovers on Mars. The twin Opportunity and Spirit rovers landed on Mars in 2004. Spirit lasted 20 times longer than its original design until its final communication to Earth in March 2010. Opportunity’s mission ended in February 2019. ​