Can we deflect an asteroid from hitting Earth? The success of NASA’s DART mission suggests that we can, but only after ESA’s HERA mission – now in the launch phase – verifies the results will we know if we are truly able to protect the planet from this threat.
2023: Nearly 200 space rockets lifted off from various points on Earth to put satellites and other spacecraft into orbit. 2024: The number grew to nearly 250, most of them from Elon Musk’s SpaceX. Among all these launches, carried out with almost complete indifference from traditional media, there was one on October 7 that could turn out to be one of the most important ever: the European Space Agency’s (ESA) HERA mission, which aims to show us whether and how much we are capable of deflecting an asteroid’s trajectory. In November 2021, NASA launched the Double Asteroid Redirection Test (DART) mission. The target was Dimorphos, a small rock 177 m in diameter orbiting a larger asteroid, Didymos. The DART probe would deliberately collide with Dimorphos to see if it could change its orbit.
The mission was designed to test a diversion technique called kinetic impactor (in short, smashing one thing into another) – and it worked. The probe hit its target at a speed of about 6,6 km/s in September 2022, changing the time of its orbit around Didymos by 33 minutes, much more than expected. It was a historic moment: the first time we had intentionally changed the trajectory of a celestial body.
“DART showed how effective a kinetic impactor can be at moving and deflecting small asteroids. It was a very successful mission,” says Prof. Alan Fitzsimmons, an astronomer at Queen’s University Belfast, Northern Ireland, who specializes in asteroid research. To properly interpret NASA’s success, astronomers need to collect a number of key data points about the target: for example, the internal structure of Dimorphos and how it responded to the impact. That’s where HERA comes in. Arriving at the Didymos–Dimorphos system in 2026, HERA will conduct a detailed post-impact investigation of both asteroids: it will capture high-resolution images, measure the asteroids’ masses, and study the aftermath of DART’s impact.
HERA SPACE PROBE
About the size of a car and weighing 1,2 tons, HERA carries five scientific instruments to collect the data needed to characterize Dimorphos: two cameras (one in visible light, one in infrared); a hyperspectral imager to split the light into small bands for geological and compositional analysis; a laser altimeter to measure HERA’s distance from the asteroid’s surface; and a radio science experiment to determine the asteroid’s masses and gravitational fields.
Its construction involved 18 countries and the Italian contribution was significant, both scientifically and industrially. INAF designed the VISTA instrument to study dust particles, while the University of Bologna is participating with a radioscience project. Italian companies such as Thales Alenia Space, Leonardo, OHB Italia, Avio and TSD Space provided essential components.
HERA will also deploy two CubeSats (nanosatellites) for additional investigations. One, built by Tyvak International in Turin, is named Milani in honor of Prof. Andrea Milani, a pioneer in asteroid threat monitoring, and will allow the discovery of surface mineralogy. The other, Juventas, carries a radar that will allow scientists to “see inside” asteroids to determine their internal structure and composition. Together, the data will be essential for understanding deflection techniques and their potential uses in future planetary defense.
Asteroids, in fact, can be of two types: monolithic or “rubble pile”. The former are single blocks of once-melted rock; the latter are conglomerates of debris without a solid structure. Each would react differently to a kinetic impactor. Dimorphos is thought to belong to the latter type. “When we get the first images from HERA, in late 2026, we will quickly understand how Dimorphos is structured internally, how it reacts and what its physical properties are,” says Fitzsimmons.
GLOBAL EFFORTS
In the past 20 years, planetary defense has become a major focus for space agencies around the world, reflecting growing awareness of the asteroid threat. In addition to NASA and ESA, China is also working on a diversion test with an as-yet-unnamed mission, scheduled for 2027. The likely target is asteroid 2015 XF261, a small rock 17–78 km across. “China aims to combine DART and HERA in a single launch with two probes: one will collide with the asteroid, the other, on a different trajectory, will make follow-up observations,” explains Andrew Jones, a journalist specializing in the Chinese space industry. Unlike DART and HERA, the Chinese observation probe would arrive first, study the asteroid for months, broadcast the impact in real time, and immediately analyze the consequences.
In addition to advancing methods to protect Earth, these missions also offer scientific gains: asteroids are early remnants of the Solar System and carry key clues to the formation of planets.
PREVENTIVE ALARM SYSTEM
Although the deflection tests get a lot of media attention, the foundation of planetary defense is detection and tracking. That’s why there are international structures that geolocate the orbits of known asteroids and detect unknown ones. NASA’s Planetary Defense Coordination Office (PDCO) and the International Asteroid Warning Network (IAWN) use powerful telescopes and ground-based radars to continuously scan the sky for objects that could come dangerously close to Earth. Dozens of structures around the world contribute, and two stand out in particular: Pan-STARRS at the Haleakalā Observatory (Maui, Hawaii) and the Catalina Sky Survey (CSS) in the Santa Catalina Mountains (Arizona, USA). Together, they have discovered over 17.000 NEOs (near-Earth objects).
A major achievement of the CSS was the discovery of asteroid 2024 RW1, a 1m-wide rock, detected on 4 September 2024. ESA immediately analysed the data and realised it was heading towards Earth, posting on X (formerly Twitter): “A 1m-wide asteroid will enter Earth’s atmosphere over the Philippines, near Luzon Island, at 17:08 UTC today, 4 September. The object is harmless, but people in the area may see a spectacular fireball! Discovered this morning by the Catalina Sky Survey, this is only the ninth asteroid that humanity has identified before impact.” And indeed, the rock disintegrated, producing a very bright meteorite, filmed by many cameras. Although the time between detection and entry into the atmosphere was around eight hours, according to Prof. Fitzsimmons says this shows the progress in impact prediction: “The important thing is that it was identified; the orbit was calculated, tracked, and we knew it was going to hit; the time and location of impact were determined with great precision before it entered the atmosphere. 20–30 years ago, this would not have been possible.”
In other words, telescope detection and subsequent calculations are now sensitive and fast enough to predict impacts. If we can do this for a 1m-wide “pebble” found just a few hours before impact, we should be able to detect objects 50m across a week earlier, or rocks the size of Dimorphos about a month (or more) before impact. This is a crucial aspect: the earlier an asteroid is detected, the less force is needed to push it into a non-threatening trajectory.
EYES FROM THE SKY
To increase the capacity for detecting NEOs, new structures are being built. ESA is working on a ground-based telescope called Flyeye, inspired by the compound eyes of insects, that will observe a very wide area of the sky in a single view. It will be built in Sicily, on top of Monte Mufara (Palermo province), 1865 m above sea level. The Vera C. Rubin Observatory on Cerro Pachón, Chile, is expected to become operational in 2025. With a tracking telescope 8,4 m in diameter, it is expected to be an extraordinary resource for detecting NEOs.
Several space telescopes are also planned. NASA is funding the NEO Surveyor, due to launch around 2027–28. With a 50 cm aperture, it is designed to detect and classify most potentially hazardous asteroids and comets within 30 million miles of Earth’s orbit. Operating at two infrared wavelengths, it will provide good estimates of the size of the rocks, as well as data on their composition, shape, and rotation rate. (BBC)
