Nearly 150,000 rocky bodies have been discovered around the Sun, smaller than planets, which we call asteroids. Most of them orbit between Mars and Jupiter, but a good number, about 20,000, follow trajectories that at some point in their orbits come close to Earth. Of course, these figures correspond to known asteroids, that is, those that have been identified, observed and whose orbit has been precisely calculated. “Practically all of them are known to be the largest ones, those over a kilometer in diameter,” says Julia de León, our guest at Hablando con Científicos, “but it is believed that only 40% of those with a size between 100 and one kilometer in diameter. Knowing them precisely and monitoring them is essential because, let us remember, the impact of an object of about 15 km was enough to destroy most of the living creatures on Earth at the end of the Cretaceous, 66 million years ago, among them, the dinosaurs.
Didymos is the name given to an asteroid with a diameter of 780 meters that takes two years to go around the Sun completely. In its comings and goings, at certain moments, it approaches tens of millions of kilometers from Earth, far enough away to pose no threat. Didymos is not alone, it has a small moon of just 160 meters, known as Dimorphos, which has become the protagonist of one of the most impressive stories of today. On September 26, the ship DART crashed violently into Dimorphos, a crash planned by NASA to test the ability of current technology to cause a change in the trajectory of a body, something that one day, perhaps, can save us from a catastrophe like the one at the end of the Cretaceous.
The crash was followed by scientists and amateur astronomers from all over the planet, including our guest, Julia de León, a researcher at the Instituto de Astrofísica de Canarias and a participant in the mission. DART. Julia comments today during the interview what those moments were like and the consequences of the collision that have been observed so far.
From Earth it is not possible to observe Dimorphos directly, it is too small for ground and space telescopes. Scientists knew of its existence because, throughout its orbit, it passes in front of and behind Didymos, causing small decreases in the brightness of the light that it reflects from the Sun. Thanks to these small variations, the scientists determined the mass, its period and other orbital parameters of the small moon. However, they did not have any image of it, nor data on its composition, so, as Julia de León comments, they did not know what the consequences of the collision with DART. It was the suicide ship itself that provided the first images as it approached its objective, images that helped it carry out the necessary maneuvers to complete its mission autonomously.
Ten days before the crash, DART released a small secondary spacecraft called LICIACube (Light Italian CubeSat for Imaging of Asteroids), which moved alongside the mother ship, capturing images of the event and sending them back to Earth.
The impact occurred on September 26, 2022, at 11:14 p.m. UTC. At that time the ship had a mass of half a ton and was moving at more than 22,000 km/h. The crash was so violent that it raised a cloud of dust and debris that was captured by the largest telescopes on Earth. The impact produced a reduction in the speed of Dimorphos, an effect that resulted in a change in the period of its orbit around Didymos.
The analysis of the data obtained during the last weeks has made it possible to specify that the rotation period has gone from 11 hours and 55 minutes to 11 hours and 23 minutes, a change of 32 minutes that, according to Julia de León comments during the interview, exceeds more than pre-impact calculations, which estimated the change in 10 minutes maximum. This demonstrates, for the first time, that humanity has the technology and ability to deliberately change the motion of a celestial object.
Investigations continue, Julia de León and members of her team use the X-shooter instrument of the UT3 8.2-meter telescope of the ensemble VLT of THAT to obtain spectroscopic images of the matter ejected during the impact in a wide range of wavelengths from the near ultraviolet to the infrared.
In a parallel collaborative project the European Space Agency is developing the Hera mission, a spacecraft that, if all goes as planned, will be launched in the direction of Dimorphos in 2024 in order to carry out a more detailed evaluation of the impact outcome.
I invite you to listen to Jullia de León, researcher at the Instituto de Astrofísica de Canarias, member of the Solar System Group and participant in the mission DART.