The European Space Agency’s Hera mission takes off from Cape Canaveral on October 7, carried by a Falcon 9 rocket from the SpaceX company, to reach the double asteroid Didymos in the fall of 2026, located several hundred million miles away. kilometers from Earth.
Because, in 2022, NASA intentionally crashed its DART probe on the smaller half of this double asteroid, the “moon” Dimorphos. The idea of this first-ever asteroid deflection test is to change the trajectory of the target and observe the result. Thus, thanks to a camera on board DART, an Italian minisatellite (or CubeSat) deployed before the impact, and the joint power of terrestrial telescopes, then Hubble and JWST, we already know several things about the success of this deviation .
But we lack a lot of information to really understand what happened following the impact – information that is essential to generalize the results and develop models that would allow us to deflect other asteroids that arrive towards Earth or space facilities. (satellites, space stations, etc.).
And that’s Hera’s mission: to understand what Didymos and Dimorphos look like after the impact.
What we already know about the asteroid deflection test
This first asteroid deflection test was a complete success. First of all because the DART probe succeeded in guiding itself autonomously in the last hours before the impact to collide with a small asteroid of which we initially only knew the size, but also because the collision well deflected the moon Dimorphos.
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This deviation was the subject of a campaign of observations by terrestrial telescopes on most continents, which made it possible to measure the reduction in the orbital period of Dimorphos around Didymos (11:22 a.m. after impact compared to 11:55 a.m. before ), showing an ability to organize on an international scale to measure the consequences of a deviation.
In addition, the images taken by DART before the impact provided some knowledge of the surface properties of the Dimorphos target and its main body.
Finally, images taken by the Italian minisatellite LICIAcube, released by DART before the impact to observe it from afar, and images from the James Webb and Hubble space telescopes, which pointed for the first time on the same object, showed that a tail of dust emitted by the impact then pushed by the Sun’s light which exerts pressure on their surface (called “solar radiation pressure”) was spreading over tens of thousands of kilometers, some of which could end up in the Earth’s atmosphere in the form of shooting stars (without risk of damage because they would completely burn up in the atmosphere).
Hera and her CubeSats, three ultra-sophisticated scientific detectives at Didymos’ bedside
But to measure the effectiveness of the technique and validate the impact modeling – which must be able to reproduce this test on a real scale in order to extrapolate it to other scenarios, this information is not enough.
Answers to crucial questions remain unanswered. For example, measuring the effectiveness of deflection requires knowing the mass of Dimorphos, and understanding the impact response requires knowing more about the physical properties of Dimorphos, and in particular its internal properties. In particular, are there large areas of voids inside Dimorphos and what are the sizes of the rock blocks that constitute it, or is it a compact rock covered in surface rocks? Did the DART impact produce a crater or did it completely deform the small moon, as certain models predict and certain recent ground observations seem to indicate?
The two CubeSats will be equipped with a radio satellite link with the Hera probe, which will transmit to Earth, in order to measure the mass and gravity field of the asteroid. The Juventas CubeSat carries a low-frequency radar which will probe for the first time the internal structure of an asteroid and a gravimeter which will precisely measure, once placed on the surface of Dimorphos, its gravity field. The CubeSat Milani carries an infrared imager making it possible to measure the mineralogical properties of Dimorphos, and in particular those of the subsurface portions revealed by the DART impact, as well as a dust detector and analyzer. This is an artist’s impression, where Dimorphos appears larger than Didymos and the impact was represented by a crater (the existence of which is not yet known).
So, like a detective, Hera sets out to investigate and report what exactly happened and why.
For the first time, a probe will be inserted near a double asteroid.
Also for the first time, a space mission will explore a small body using three satellites at the same time. Hera is in fact equipped with a probe which itself carries two CubeSats, each the size of a shoebox, and equipped with their own mode of propulsion and various measuring instruments, which it will deploy in the neighborhood of the asteroid to take measurements at closer proximity.
This configuration will demonstrate the benefit of carrying smaller modules, allowing more risks to be taken by deploying them for operations at very close proximity, while the main probe remains at a distance and ensures that the essential scientific objectives are fulfilled ( the Hera probe itself carries two observation cameras in the visible domain, a hyperspectral imager offering mineralogical composition data, a thermal infrared imager provided by the Japanese Space Agency (JAXA) to determine the properties thermal and surface roughness, and a laser altimeter).
While waiting for the surprise of the first images of Dimorphos transformed by this first deviation test, we will have the opportunity to marvel at the probe’s flyover of Mars in mid-March 2025, which will allow us to calibrate the instruments in flight by observing not only the planet, but also one of its two moons, Deimos… which will perhaps provide new scientific data in passing.
Artist’s impression of Hera passing close to Mars and its moons, Phobos and Deimos, in March 2025.
Study the consequences of the impact in detail to extrapolate them to future collisions
This is also the first time that a mission returns to a small body of which we already have images, but of which we already know that these no longer have anything to do with what it has become. Based on current data – very partial, predictions are subject to large uncertainties and several outcomes are possible.
In fact, the DART data provides us with the initial conditions of the impact, but we lack the final outcome and the properties of its target that mediate its response to the impact. The models, starting from the initial conditions provided by DART and the real internal properties of the target which remain to be measured, must reproduce the final result.
The idea is to reduce as much as possible free settings to ensure that the models succeed in reproducing the impact not because unknown parameters have been adjusted to achieve the desired result but because they are valid and reliably capture the phenomenon at a scale inaccessible in the terrestrial laboratories. These validated models will allow us to better calibrate the impact energy necessary to deflect an asteroid with known properties.
For more information on the history of the two projects DART and Hera and planetary defense in general, see the book by Patrick Michel published by Odile Jacob “Meeting the asteroids: space missions and defense of the planet”.
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