Hera is the first planetary defence spacecraft developed by the European Space Agency (ESA). Its primary mission objective is to study the Didymos binary asteroid system that was impacted in 2022 by NASA's DART spacecraft and contribute to validation of the kinetic impact method to deviate a near-Earth asteroid from a colliding trajectory with Earth. Hera has a mass of and carries a payload of cameras, an altimeter, and a spectrometer. It is carrying two small CubeSat spacecraft, called Milani and Juventas. The spacecraft was launched on 7 October 2024 by Falcon 9. Hera is the first mission of ESA's Space Safety Programme.

Mission goals Hera will measure the size and morphology of the crater created as well as the momentum transferred by an artificial projectile impacting an asteroid, which will allow measuring the efficiency of the deflection produced by the impact. It will also analyze the expanding debris cloud caused by the impact. Hera is intended to fully characterize the composition and physical properties of the binary asteroid system including, for the first time, the sub-surface and internal structures. It will also perform technological demonstrations linked to operations in the vicinity of a small Solar System body and the deployment of and communication with CubeSats in interplanetary space.

Planetary defense The main objective of the Hera mission is to evaluate the kinetic impactor method for deflecting a near-Earth object. This planetary defense method consists of modifying the trajectory of the asteroid by launching a spacecraft at a speed of a few kilometers per second. To fulfill this objective, Hera will determine:

how much momentum transfer depends on the density, porosity and characteristics of the asteroid's surface and internal structure and what proportion of the kinetic energy is transferred in the fragmentation and restructuring of the asteroid or in the kinetic energy of the ejected materials.

Science Scientific objectives of the mission include exploring the properties of the two target asteroids: surface characteristics, internal porosity, and internal structure. The entire moon Dimorphos will be mapped with a spatial resolution of a few meters and the vicinity of the impact with a resolution of 10 centimeters. The mass of the moon will be estimated with high accuracy, allowing a direct estimate of the momentum transfer efficiency from DART impact.

Technology The mission's technological objectives include the production and testing of guidance software which, by using data from several sensors, will make it possible to reconstruct the surrounding space and thus to independently define a safe trajectory around the asteroid.

Hera also carries two CubeSats which will be dropped once the asteroid is reached:

Juventas will carry out subsurface and internal structure measurements, contribute to the determination of the gravity field, and provide information on the mechanical response of the surface when landing on Dimorphos Milani will collect spectral data from the surface of the two asteroids to study their surface composition and identify the presence of dust in the surrounding space

Software laboratory After Hera completes its primary mission in mid-2027, it will be used as a flying software laboratory for testing innovative autonomy-focused approaches. One core of the onboard computer's dual-core LEON3 processor will operate the spacecraft while the other will be used as a software sandbox.

Timeline AIDA, the joint project with NASA AIDA was the first operational program whose objective is to test a method of deflecting near-Earth asteroids. It was set up in 2013 jointly by scientists supported by NASA and ESA. Its objective was to test the use of an impactor-type device to deflect an asteroid that might strike the Earth. This program provided for the launch to the binary asteroid 65803 Didymos of two spacecraft: the DART impactor developed by NASA responsible for crashing at high speed on the smaller of the two asteroids and the AIM orbiter developed by ESA, which would measure the effects of the impact.

After an evaluation phase in the two space agencies, ESA decided at the end of 2016 to abandon the development of AIM due to lack of sufficient financial support from member states. NASA, for its part, decided to continue the development of DART. In this new context, terrestrial observatories were responsible for taking over partially the role of AIM.

The DART mission was launched on 24 November 2021 by a Falcon 9 rocket from Vandenberg Space Force Base and reached the binary asteroid 65803 Didymos on 26 September 2022, colliding with its satellite Dimorphos at 23:16 UTC at a relative speed of about 6.6 km/s. The project incorporated the Italian nano-satellite LICIACube, responsible for documenting and retransmitting the first 100 seconds of the impact.

Renaissance of the European project In 2017, at the request of several member states of ESA, the agency resumed studies of a replacement for AIM, specifically a mission that was named Hera (after the Greek goddess). Hera is intended to fulfill all the objectives assigned to AIM that Earth-based observatories could not, using the components of AIM as much as possible.

The Hera mission was subsequently approved by the ESA Ministerial Council in November 2019. In September 2020, ESA awarded the construction of the spacecraft to a consortium of companies led by OHB, under a contract of 129.4 million euros. It also formalized the scientific team of the mission, made up of a principal investigator, scientific council, and four working groups covering all aspects of the mission and the scientific managers of the instruments.

Construction and testing Hera's Core Module and Propulsion Module were connected at OHB Bremen in Germany in August 2023. In November 2023, the spacecraft completed acoustic testing at ESTEC in the Netherlands. The Milani CubeSat, integrated in Italy, and the Juventas CubeSat, integrated in Denmark, were delivered to ESTEC for integration with Hera in March 2024. The spacecraft moved to final testing in March 2024. In September 2024, Hera was transported to Florida by Antonov Airlines's AN-124 cargo aircraft. Launch

Hera was launched on 7 October 2024 at 14:52 UTC by a Falcon 9 rocket lifting off from Space Launch Complex 40 (SLC-40) at Cape Canaveral Space Force Station. Usually, Falcon 9 launches for beyond geostationary transfer orbit were made from the nearby Launch Complex 39A (LC-39A) at Kennedy Space Center due to SLC-40's focus on Starlink launches. However, LC-39A was in the midst of preparing for the launch of NASA's Europa Clipper on a Falcon Heavy later in the week, which locked Hera into using SLC-40. Additionally, the launch occurred while Falcon 9 was grounded due to a mishap during a second stage deorbit burn that happened on the SpaceX Crew-9 mission the previous month. As Hera was to be put into a heliocentric orbit however, the Federal Aviation Administration waived the grounding for its launch as an exception. The booster used for the launch was expended to help with the trajectory; it previously flew 22 times, including on missions like SpaceX Crew-1 and the IXPE.

Cruise towards Mars On 10 and 11 October 2024, Hera tested three of its science instruments (AFC, TIRI, and HyperScout H) by using them to take pictures of Earth and the Moon from a distance of more than one million km. In October 2024, the two CubeSats aboard Hera were briefly activated to confirm their nominal status and test their ability to communicate with Earth. This marked the first operation of CubeSats in deep space by ESA. Following three successful test burns performed after launch, the spacecraft conducted its first deep-space maneuver on 27 October 2024, firing its three orbital control thrusters for 100 minutes and changing its velocity by ~146 m/s, and second on 6 November 2024, firing for 13 minutes and changing its velocity by ~20 m/s.

Mars flyby Hera performed a gravity assist at Mars on 12 March 2025, coming at a distance of 5,000 km (3,100 mi) from the surface. As part of the flyby, the spacecraft spent some time observing the Martian moon Deimos, imaging it from distance of 1,000 km (620 mi) and having its closest approach at 300 km (190 mi) away. During the flyby, Hera also autonomously locked onto impact craters and other surface features on Mars to track them over time, in a full-scale test of the autonomous navigation techniques it will use to navigate around its targets.

Hera's observations of Deimos revealed previously unobserved outlines of ancient craters buried beneath a dust layer. These data were used in a study, published in Nature Astronomy in 2026, that simulated various scenarios of the moon's past and composition. The researches concluded that Deimos is most likely a rubble pile body which has been dramatically reshaped by a slanting impact of a 320-m-wide asteroid, resulting in a 10 km basin around the moon's south pole and a thick layer of dust on its surface.

Cruise through the main asteroid belt On 11 May 2025, the mission has captured images of the asteroid (1126) Otero from a distance of approximately 3 million km and on 19 July 2025 of the asteroid (18805) Kellyday from a distance of approximately 6 million km in order to perform instrument tests and to demonstrate agile spacecraft operations useful for planetary defence. In October 2025, ESA announced that Hera was on track to arrive at Didymos in November 2026—a month earlier than originally planned. Between 25 October and 1 November 2025, Hera was passing through the cometary tail of the third known interstellar object, the comet 3I/ATLAS. The spacecraft flew 8.2 million km from the tail's central axis.

In March 2026, Hera successfully performed its second deep-space maneuver, burning 123 kg of hydrazine and changing its velocity by 367 m/s. This was the mission's largest maneuver in terms of fuel consumption. Over two weeks in June and July 2026, ESA upgraded Hera's software, making the spacecraft ready for its asteroid phase.

Arrival at Didymos The spacecraft will reach the binary asteroid (65803) Didymos in November 2026, four years after DART, to begin six months of investigation. Hera will be the first to make a rendezvous with a binary asteroid. Once close to the double asteroid, five stages will follow:

+Mission phasesPhase Mission phase Duration Mission1Early Characterization 6 weeks The global shape and mass/gravity, as well as thermal and dynamical properties, of both asteroids will be determined2Payload Deployment 2 weeks The release of the two CubeSats3Detailed Characterization 4 weeks Meter-scale mapping of the asteroids and determination of thermal, spectral, and interior properties4Close Observation 6 weeks High-resolution investigations of a large fraction of the surface area of Dimorphos, including the DART impact crater5Experimental 6 weeks Morphological, spectral, and thermal properties of Dimorphos

Spacecraft