Didymos and dimorphos

Planetary defense concept

Millions of asteroids frequent the region of space between Mars and Jupiter. All these objects are subject to the laws of celestial mechanics and their ballet around the Sun is chaotic by nature. The slightest disturbance is enough to upset their orbit.
Asteroids are close neighbors and in one way or another they are linked to our destiny especially the NEOs that cross our orbit.
There are about 20,000 asteroids that cross Earth's orbit, 15% of which are binary.
About 4700 asteroids are classified as potentially dangerous because they are at least 140 m in diameter (disaster threshold on the scale of a region or a small country).
Asteroid impacts are low probability but high consequence, which is why scientists are anticipating feasible ways to avoid this kind of major hazard.

In order to perform a planetary defense test, a pair of asteroids was chosen to measure the effects of an impact by a probe on the orbits of the binary system.
The couple chosen is the near-Earth asteroid Didymos (in Greek "twin") which has a diameter of 780 m and its satellite Dimorphos (in Greek "which has two shapes") which has a diameter of 163 m and which orbits in synchronous rotation around Didymos in 11 hours and 55 minutes at a distance of 1 180m.
Didymos is the target of the AIDA (ESA) probe of the DART mission (NASA) which took off on November 23, 2021 for an expected impact between September 26 and October 1, 2022.
The Hera mission (ESA) will be launched in 2024 for an arrival on Didymos in January 2027. It will observe the dynamic effects of the impact of the DART mission.

NB: NEOs are constantly monitored by an automated system (Sentry) which analyzes most of the dangerous asteroids and estimates the possibilities of future impact with the Earth, during the next 100 years.
The largest known NEO Ganymede (1036) is over 31 km long. It will pass 55 million km from Earth on October 13, 2024.

Image: Despite being a pair of near-Earth asteroids, Didymos and Dimorphos pose no threat to Earth for the next 100 years, including after DART crashed on its surface.
Credit: NASA / Johns Hopkins APL

DART Kinetic Impactor

The DART (Double Asteroid Redirection Test) mission launched a space probe weighing around 600 kg to crash into Dimorphos in 2022.
The objective is to evaluate the effects of a kinetic impact (6 km/s) by a spacecraft on the trajectory of a small asteroid because it is possible that in the future we will need to deviate the trajectory of a potentially dangerous asteroid.
The choice to target Dimorphos, the satellite of Didymos, makes it possible to accurately measure the change in trajectory of a small asteroid. Given the mass ratio and despite the very high speed of the spacecraft, the change in trajectory of Dimorphos should modify its orbital period only very slightly (about 1%). Moreover, the scientists do not want to modify the trajectory of Didymos because it could become dangerous whereas it is not today.
Even if the mission works as planned, it is not at all sure that the trajectory of Dimorphos will be modified because we do not know its composition and its internal structure.

If the ground is soft the impact will be null, if the object is compact, tons of ejecta will be sprayed, a crater will appear and its trajectory should be slightly modified.
The shoebox-sized LICIACube nano-satellite, attached to the probe during the journey to the target, will separate from the probe before impact so as to fly over Dimorphos three minutes after impact and transmit the images. .
In 2022, more than ten of the most powerful telescopes on the planet will be mobilized for several nights to observe the effect of the impact on Dimorphos.
In 2027, the consequences of the impact will be filmed by the Hera mission, which is to place a probe in orbit around Didymos. Thus we will have a perfectly documented real experience which will allow to validate the numerical simulations of impacts.

Image: In 2022 Hera will study the consequences of the impact caused by NASA's DART spacecraft on the smaller body of the Didymos/Dimorphos couple.
Credit: ESA – Office of Science