German-built ESA space probe Hera is ready to take a closer look at near-Earth asteroids

Planetary defence against asteroids

Asteroid impacts on Earth are very rare but can have serious consequences. On 15 February 2013, around 1500 people were injured when a small asteroid measuring 20 metres across entered Earth's atmosphere near Chelyabinsk, a Russian city with a population of over one million. Most of it vaporised, but what remained triggered an explosion at an altitude of around 30 kilometres, sending a shock wave that caused injuries in the city from countless shattered window panes. "Chelyabinsk was an event that serves as a warning, and to prevent dangerous incidents in the future, we need the data from the Hera mission," explains Manuel Metz, Hera project manager at the German Space Agency at DLR. "The consequences of an impact by a larger celestial body would be much more severe and could even threaten entire ocean coasts or continents. This would have existential consequences for the survival of humanity," emphasises Stephan Ulamec of MUSC, who is involved in the Hera mission’s science tea. “The remains of the almost 200-kilometre-wide Chicxulub crater in what is now Mexico are a testament to this."

Preparing to ward off near-Earth objects

The good news is that none of the approximately 36,000 known near-Earth objects (NEOs) with a diameter of more than 100 metres are currently on a collision course with Earth. The Apophis asteroid, discovered in 2004, will fly past Earth in 2029 and at its closest approach will be just 31,750 kilometres away, closer than geostationary satellites. Apophis is around 350 metres in diameter, which would have extreme consequences in the event of an impact. Based on what we know today, such a collision can be ruled out for the 21st century, but the fact that Apophis will pass so close to Earth shows that we must always be prepared for such events. In order to develop methods for effectively countering such dangers, NASA and ESA are conducting the ‘Asteroid Impact and Deflection Assessment' (AIDA) joint project, consisting of NASA's DART and ESA's Hera missions.

The DART and Hera missions

NASA began the project with the DART (Double Asteroid Redirection Test) space probe. The chosen target was the Didymos and Dimorphos binary asteroid system, where the much smaller Dimorphos (around 150 metres in diameter) orbits the larger Didymos (around 800 metres in diameter). The aim was to influence the time that it takes for the asteroids to orbit each other. To do this, DART collided with Dimorphos at a controlled speed of over six kilometres per second (22,500 kilometres per hour) on 26 September 2022. Measurements with telescopes were able to determine that the orbital period had been shortened by 33 minutes from the original 11 hours 55 minutes, surpassing the 10-minute reduction predicted by prior modelling.

Hera will now be sent to the binary asteroid system to investigate exactly how the orbital period and shape of the asteroid has changed. The probe is equipped with 12 measuring instruments for this purpose. Among the most important ones are the two Asteroid Framing Cameras (AFC), two redundant monochromatic cameras built in Jena, which will be used to determine the position of the space probe in the asteroid system. They are essential for the navigation of the spacecraft and will also contribute to further exploration of the asteroids.

Digital terrain model of the asteroid

The Hera science team will use the images from the AFCs to calculate a digital terrain model of the asteroid and search for changes caused by the DART impact on Dimorphos (in fact the asteroid's name derives from the Greek for 'two forms', referring to its changed shape after the impact). "Was a crater created on Dimorphos? Was the entire asteroid altered? Was Didymos' surface also affected by ejected material? We want to answer these questions using our digital terrain model," says Principal Investigator for the cameras Jean-Baptiste Vincent from the DLR Institute of Planetary Research.

CubeSats land on Dimorphos

Hera will also carry two CubeSats, named Juventas and Milani, each the size of a shoebox. The nanosatellites will observe Dimorphos at close range and ultimately attempt to land on it in the final phase of the mission to measure its surface characteristics, interior structure and gravitational field. The measurements are intended to determine the exact mass of Dimorphos, which will have already been determined by the AFCs. The data obtained will then be used to calculate how other celestial bodies could be deflected, forming the basis of a planetary defence mission in the event of an asteroid on a collision course with Earth. The data collected will also mark a further milestone in asteroid research more generally.

German technology featured in a European joint project

Germany is the largest contributor to the ESA mission, providing around 130 million euros (37 percent of the total). The Hera spacecraft was developed and built by OHB SE in Bremen. A newly developed antenna made of carbon-fibre-reinforced plastic was produced by Munich-based HPS, while the two AFCs are from Jena-Optronik. The TUD Dresden University of Technology is heavily involved in the development of the radar experiment on the Juventas CubeSat. German researchers are also working on the Hera Science team to scientifically evaluate the data obtained from the mission. The German Space Agency at DLR is coordinating all of these German contributions to the mission with funding from the Federal Ministry for Economic Affairs and Climate Action (BMWK).

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