Asteroid Didymos and its small moonlet make up what’s called a binary asteroid system. The two asteroids are not a threat to Earth, but because they do pass close to the Earth, they were chosen as the target for NASA’s Double Asteroid Redirection Test (DART) mission — the space agency's first mission to test planetary defense technology. This technology could one day be used to deflect hazardous asteroids on a collision course with Earth.
Didymos — which means "twin" in Greek — was discovered on April 11, 1996, by researcher Joseph Montani of Spacewatch at Kitt Peak National Observatory in Tucson, Arizona. Montani also suggested the name.
Suspicions arose that Didymos might have a moon after scientists spotted multiple echoes in data from NASA's Goldstone Solar System Radar, located in the Mojave Desert near Barstow, California. The suspicions were confirmed by analyzing optical light curves – telescopic observations showing the brightness of an object over a period of time – along with radar images from the Arecibo Observatory in Puerto Rico taken on Nov. 23, 2003.
NASA's DART mission is scheduled to launch toward Didymos no earlier than July 22, 2021, on a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California. DART will intercept the smaller moonlet asteroid in September 2022 at a distance of about 6.8 million miles (11 million kilometers) from Earth.
The DART probe, consisting of the main spacecraft with its antennas, thrusters, and other instrumentation — about 6.9 × 6.2 × 8.5 feet (2.1 × 1.9 × 2.6 meters) in size — and two long solar panels extending 62 feet (18.9 meters) tip to tip, will impact the moonlet at high speed — about 4 miles or 6.6 kilometer per second.
The goal of the mission is to determine how much the impact alters the moonlet’s velocity in space, by measuring the change in its orbit around Didymos. Scientists think the collision will change the speed of the moonlet by a fraction of one percent and alter its orbital period around the larger asteroid by several minutes — enough to be observed and measured by telescopes on Earth.
An international campaign coordinated by Northern Arizona University’s Cristina Thomas — DART’s Observations Working Group Lead — is using powerful Earth-based telescopes to study the asteroid system.
Researchers will get to see the asteroid system up close using images returned from DART’s only onboard instrument, called DRACO (Didymos Reconnaissance and Asteroid Camera for Optical navigation). DRACO is a high-resolution imager based on the New Horizons spacecraft’s LORRI instrument. DRACO will help DART navigate to the Didymos system, and, in its final minutes, will stream images back to Earth at the rate of one per second, enabling the DART team to measure the moonlet’s size and shape to determine the impact site.
DART also will have a passenger. It will carry a small spacecraft contributed by the Italian Space Agency (ASI). The shoebox-sized LICIACube (Light Italian CubeSat for Imaging of Asteroids) will separate from DART several days before DART’s impact with the moonlet. LICIACube will capture images of the impact and material (ejecta) kicked up from the moonlet. It will attempt to photograph the crater caused by the impact, although the ejecta may get in the way. And it will image the back side of the moonlet, which DRACO will never get a chance to see.
Size and Distance
The larger asteroid of the binary pair — also called Didymos A — is about a half mile (780 meters) in diameter. The moonlet, or Didymos B, is about 520 feet (160 meters) in diameter.
As it orbits the Sun, the asteroid comes close to Earth's orbit, occasionally approaching relatively close to our planet; in 2003, it passed only 0.048 AU from Earth. (One astronomical unit, abbreviated as AU, is the distance from the Sun to Earth.) At its farthest, when Didymos is on the opposite side of the Sun from Earth, a bit beyond the orbit of Mars, it is about 3 AU away.
Orbit and Rotation
The orbit of Didymos ranges from just outside the orbit of Earth (about 1 AU) to a bit beyond the orbit of Mars (about 2.27 AU) and is slightly inclined with respect to the plane of the planets (called the ecliptic), by about 3 degrees. It takes 2.11 years to make each trip around the Sun.
Didymos is classified as a member of the Amor group of asteroids, named after asteroid 1221 Amor. (For the technically minded, the Amors are near-Earth asteroids with orbits outside the orbit of Earth in which half the long axis of their orbital ellipse, referred to as the semi-major axis, is between 1 AU and 1.3 AU [that is, interior to Mars’ orbit].)
Didymos spins rapidly — rotating about once every 2.26 hours. The moonlet revolves around the larger body about once every 11.9 hours. The main asteroid and its moonlet orbit each other about 0.62 miles (1 kilometer) apart.
Didymos is shaped like a spinning top with a raised ridge running along its equator — a shape that’s common among binary asteroids. It’s thought that the asteroid’s rapid spin drives material toward the equator causing the bulge in the middle.
Not much is known about the structure of the moonlet except that it appears to be somewhat elongated.
Whether binary asteroids all form the same way or by many different processes isn’t yet known. The rapid spin of Didymos suggests that the moonlet may have been formed through a process called rotational fission, in which material is shed from the asteroid due to its fast rotation. Didymos is thought to have started spinning faster because infrared light was emitted unevenly from its Sun-warmed surface, resulting in a twisting force, or torque. Over millions of years, this process could have built up enough momentum to release material from the surface, which then gathered into the moonlet. But other possibilities haven’t been conclusively ruled out.
The surface of Didymos cannot be seen in great detail from Earth, but scientists think it may be like the surfaces of similar shaped asteroids that have been visited by spacecraft, like asteroids Bennu and Ryugu. Both of these asteroids have extremely rough surfaces full of boulders of various sizes. They lack the fine-grained regolith, or loose, dust-rich outer material, that is seen on Earth’s Moon and on other asteroids.
Didymos has one moon.
Potential for Life