ESA Hera Asteroid Impact Investigation

ESA's Hera spacecraft, launched in October 2024, is scheduled to arrive at the Didymos-Dimorphos binary asteroid system in November 2026 — approximately one month earlier than originally planned due to trajectory optimisation. Hera is the European follow-up to NASA's DART mission, which deliberately impacted Dimorphos on September 26, 2022, altering its orbital period around Didymos by 33 minutes. While DART proved that kinetic impact deflection works, it left critical questions unanswered: How large is the crater? How much mass was ejected? What is Dimorphos's internal structure? Hera will answer all of these. The spacecraft has a mass of 1,128 kilograms and carries a suite of advanced instruments including a wide-angle camera, a thermal infrared imager, an asteroid framing camera, and the PALT lidar altimeter. Crucially, Hera also carries two CubeSat companions deployed once in the Didymos system: Milani, which will conduct a detailed mineral survey of both asteroids' surfaces, and Juventas, which will perform the first-ever radar sounding of an asteroid's interior — a technique capable of mapping internal voids and structural weaknesses. The scientific significance of Hera's work extends well beyond satisfying curiosity. Planetary defence requires not just the ability to deflect an asteroid but the ability to predict the deflection outcome with enough confidence to avoid an overcorrection. Hera's detailed characterisation of the DART crater, the ejecta plume structure, and Dimorphos's bulk density will calibrate the models that planetary defence agencies worldwide will use for any future threat response. The European Space Agency invested approximately 363 million euros in Hera, reflecting the strategic value of planetary defence capability for Europe and the broader international community.