In depth | Didymos – NASA Solar System Exploration
The asteroid Didymos and its smaller moon Dimorphos form what is called a binary asteroid system, which means that the smaller moon (Dimorphos) orbits the larger body (Didymos).
The two asteroids are not a threat to Earth, but because they pass relatively close to Earth, they were chosen as the target for NASA’s Double Asteroid Redirection Test (DART) mission – the first mission of the agency to test planetary defense technology. This technology could one day be used to deflect dangerous asteroids on a collision course with Earth.
DART deliberately hit Dimorphos on September 26, 2022. On October 11, 2022, NASA announced that analysis of data obtained by the DART survey team showed the spacecraft’s kinetic impact with its target asteroid, the moonlet Dimorphos, successfully changed the orbit of the smallest asteroid. It was the first time that mankind had deliberately altered the motion of a celestial object, and it was the first large-scale demonstration of asteroid deflection technology.
Didymos – which means “twin” in Greek – was discovered on April 11, 1996 by Spacewatch researcher Joseph Montani at the Kitt Peak National Observatory in Tucson, Arizona. Montani also suggested the name.
Indications emerged that Didymos could have a moon after scientists spotted several echoes in radar data from NASA’s Goldstone Solar System, located in the Mojave Desert near Barstow, California. The suspicions were confirmed by analyzing optical light curves – telescopic observations showing an object’s brightness over a period of time – as well as radar images from the Arecibo Observatory in Puerto Rico taken on November 23, 2003.
NASA’s DART mission launched to Didymos at 1:21 a.m. EST on November 24, 2021, on a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California.
DART intercepted the smaller lunar asteroid at 7:14 p.m. EDT on September 26, 2022. DART hit Dimorphos at high speed – about 4 miles or 6.6 kilometers per second. Dimorphos was about 6.8 million miles (11 million kilometers) from Earth at the time of the DART impact.
The purpose of the mission was to determine how much the DART impact changed the moon’s speed through space by measuring the change in its orbit around Didymos.
Before DART impact, it took Dimorphos 11 hours and 55 minutes to orbit Didymos. After DART intentionally collided with Dimorphos on September 26, 2022, astronomers used telescopes on Earth to measure just how much that weather had changed. The survey team confirmed that the impact of the spacecraft altered Dimorphos’ orbit around Didymos by 32 minutes, shortening the orbit from 11 hours and 55 minutes to 11 hours and 23 minutes. (This measurement has a margin of uncertainty of approximately plus or minus 2 minutes.)
Prior to his encounter, NASA had defined a minimum successful orbit period change of Dimorphos as a change of 73 seconds or more. Early data showed that DART exceeded this minimum benchmark by more than 25 times.
“This result is an important step toward understanding the full effect of DART’s impact with its target asteroid,” said Lori Glaze, director of NASA’s Planetary Science Division at NASA Headquarters in Washington. “As new data arrives daily, astronomers will be able to better assess if, and how, a mission like DART could be used in the future to help protect Earth from collision with a asteroid if we ever find one heading our way.”
The survey team continues to acquire data from ground-based observatories around the world – as well as radar facilities from the Goldstone Planetary Radar at NASA’s Jet Propulsion Laboratory in California and the Green Bank Observatory at the National Science Foundation in West Virginia. They update the period measurement with frequent observations to improve its accuracy.
Footage of the DART impact was broadcast live to Earth as the spacecraft plunged into Dimorphos. The camera was DART’s only on-board instrument. Called DRACO (Didymos Reconnaissance and Asteroid Camera for Optical navigation), the high-resolution imager was based on the LORRI instrument on the New Horizons spacecraft. Before impact, DRACO helped DART navigate to the Didymos system.
DART also had a passenger – a small spacecraft provided by the Italian Space Agency (ASI). The shoebox-sized LICIACube (Light Italian CubeSat for Imaging of Asteroids) separated from DART several days before DART’s impact with the moon. LICIACube captured images of the impact and the material (ejecta) thrown up by the moon.
Size and distance
Didymos, the largest asteroid in the binary pair (also called Didymos A) is about 780 meters in diameter. The moon, Dimorphos (Didymos B), is about 160 meters in diameter.
Orbiting the Sun, the pair of asteroids approach Earth’s orbit, sometimes approaching relatively close to our planet. In 2003, it passed only 0.048 AU from Earth. (An astronomical unit, abbreviated AU, is the distance from the Sun to Earth.) At its furthest point, when Didymos is on the opposite side of the Sun from Earth, a little beyond the orbit of Mars, it is about AU 3 away.
Orbit and rotation
Didymos’ orbit extends from just outside Earth’s orbit (about 1 AU) to just beyond Mars’ orbit (about 2.27 AU) and is slightly inclined by relative to the plane of the planets (called the ecliptic), about 3 degrees. It takes 2.11 years to make each trip around the Sun.
Didymos is classified as a member of the Amor asteroid group, named after the asteroid 1221 Amor. (For the technically minded, the Amors are near-Earth asteroids with orbits outside Earth’s orbit in which half the major axis of their orbital ellipse, called the semi-major axis, is between 1 AU and 1.3 AU. [that is, interior to Mars’ orbit].)
Didymos rotates rapidly – rotating about once every 2.26 hours. The DART survey team has confirmed that the spacecraft impact of September 26, 2022 altered Dimorphos’ orbit around Didymos by 32 minutes, shortening the orbit from 11 hours and 55 minutes to 11 hours and 23 minutes . This measurement has a margin of uncertainty of about plus or minus 2 minutes.
Didymos is shaped like a spinning top with a raised ridge along its equator – a common shape among binary asteroids. The asteroid’s rapid rotation is thought to be pulling material towards the equator, causing a bulge in the middle.
Before DART, not much was known about the structure of Dimorphos. Scientists are studying spacecraft data to learn more about the two asteroids.
It is not yet known if the binary asteroids all form in the same way or by many different processes. The rapid rotation of Didymos suggests that the moon may have been formed by a process called rotational fission, in which material is released from the asteroid due to its rapid rotation. It is believed that Didymos began to spin faster because infrared light was emitted unevenly from its solar-heated surface, resulting in twisting force or torque. Over millions of years, this process could have created enough momentum to release material from the surface, which then collected in the moon. But other possibilities have not been definitively ruled out.
As DART data and images are studied in the coming months, scientists will be able to tell us more about the surfaces of Didymos and Dimorphos. Before DART, scientists thought they might have surfaces similar to asteroids that have been visited by spacecraft, like the Bennu and Ryugu asteroids. Both of these asteroids have extremely rough surfaces full of boulders of varying sizes. They lack the fine-grained regolith, or the loose, dust-rich outer material, seen on Earth’s Moon and other asteroids.
Didymos has a moon called Dimorphos