Hera Frequently Asked Questions
1. What is ESA’s Hera mission?
Hera is an ESA-led mission for planetary defence, developed as part of a larger international endeavour, the Asteroid Impact and Deflection Assessment (AIDA) collaboration. In the first part of AIDA, NASA’s DART mission impacted the Dimorphos asteroid to change its orbit around the larger Didymos asteroid. Now Hera will visit this asteroid pair to gather additional data on Dimorphos and the DART impact outcome, to help turn this ‘kinetic impact’ method planetary defence into a well-understood and potentially repeatable technique. As well as serving planetary defence, Hera will also demonstrate new technologies in deep space – including the inter-satellite links connecting a spacecraft and its two CubeSats – as well as gathering bonus asteroid science.
2. Where is Hera headed?
Didymos is a binary near-Earth asteroid system in solar orbit extending out beyond Mars. The primary mountain-sized body has a diameter of around 780 m and a rotation period of 2.26 hours, whereas the Dimorphos secondary body, about the size of the Great Pyramid of Giza has a diameter of around 150 m and orbits the primary at a distance of around 1.2 km from the primary surface in 11 hours and 55 minutes (initially). First dubbed ‘Didymoon’, the International Astronomical Union assigned Dimorphos its current name in 2020, from the ancient Greek for ‘having two forms’. This reflects its status as the first body in the Solar System to have had its orbit measurably changed through human action, by the DART impact, expected to have reshaped much of the asteroid’s surface.
Initially astronomers studied the Didymos system purely through the observation of ‘light curves’ – slight changes in the combined light of the two bodies over time along with radar observations during its close approach to earth in 2003. But DART’s approach showed Didymos had the ‘spinning top’ shape observed in many asteroids, while the smaller Dimorphos had a much rockier surface, although the boulders seen are surprisingly large, typically the size of cars to houses. The majority of 1 km-class near-Earth asteroids in our Solar System have been detected and are tracked, but the majority of asteroids in the Dimorphos size class (and there are up to 30 000 of them) are yet to be discovered, despite their ‘city-killing’ potential.
3. What happened when NASA’s DART impacted Dimorphos?
On 26 September 2022 the vending-machine-sized, approximately half-tonne DART struck the 150-m diameter asteroid at 6.1 km/s. The collision was witnessed by Italy’s LICIACube nearby, plus images from the James Webb and Hubble Space Telescopes, as well as terrestrial observatories. Observations show a giant plume of debris that extended more than 10 000 km into space and persisted for months, as well as a total of 37 metre-sized boulders sent flying away from Dimorphos. Light curve observations from Earth confirm this experiment was a success: the 11-hour 55 minute orbit of Dimorphos around its parent asteroid Didymos was shortened by around 33 minutes.
4. What remains to be discovered about the DART impact?
While the DART kinetic impact was a success in changing the orbit of Dimorphos around its parent body, many unknowns remain, which will need to be resolved to turn this grand-scale experiment into a well-understood technique that could be repeated if ever needed to safeguard Earth.
To start with researchers still don’t know how the asteroid as a whole reacted to the spacecraft impact. How much material was thrown off into space by the impact? The material displaced from the asteroid by the impact gives an extra ‘kick’ to the overall efficiency of momentum transfer. This value is known as the ‘beta factor’. But to interpret the impact outcome, including the beta factor – and to be able to scale that outcome to another asteroid if needed – requires an accurate measurement for the asteroid’s mass, as well as its make-up and structure, all of which can only be obtained by visiting the Didymos system.
Furthemore, we do not know yet whether the DART impact left a crater on Dimorphos or whether the asteroid was entirely reshaped, as suggested by impact simulations and recent ground-based observations. Even our current measurements of Dimorphos’s altered orbit are stuck with a 10% residual uncertainty. Hence the need for Hera, to perform a close-up ‘crash site investigation’ of Dimorphos.
5. What instruments does Hera carry aboard?
Hera carries a total of 12 instruments overall (including those aboard its two miniature CubeSats). These comprise:
1●Asteroid Framing Camera – With two baffle-protected cameras for redundancy, the AFC is the eye of the mission, with a 1020x1020 monochrome visible-light imager, to be used for both navigation and scientific data gathering. Produced by Jena-Optronik in Germany.
2●Thermal Infrared Imager – Imaging in the mid-infrared spectral region to chart the temperature on Dimorphos's surface. By charting the ‘thermal inertia’ of surface regions – or how rapidly their temperature changes – physical properties such as particle size distribution and porosity can be constrained. TIRI is equipped with filters that will provide information about the composition of Didymos and Dimorphos. It can also be used for navigation on the night-side of Dimorphos. Supplied by the Japan Aerospace Exploration Agency JAXA, from a design previously deployed on Japan’s Haybusa2 asteroid mission.
3●HyperScout H – Observing in a range of colours beyond the limits of the human eye, equalling 25 visible and near-infrared spectral bands, the HyperScout-H will help prospect the asteroid’s mineral makeup. The shoebox-sized imaging spectrometer comes from cosine Research in the Netherlands.
4●Spacecraft Monitoring Camera – This compact, low power and high reliability camera gives a view of Hera’s entire ‘Asteroid Deck’ where its instruments are hosted. Equipped with a 4 Mpixel CMOS image sensor, the SMC will be used in particular to observe the deployment of Hera’s two CubeSats and inspect them before separation. The SMC was developed for ESA by TSD-Space in Italy with Italy’s Optec SpA providing the optics.
5●Laser Rangefinder – Determining the distance to the asteroid surface by determining the time of flight of ~1550-nm wavelength laser beam pulses bounced back to a receiver telescope with a distance accuracy of better than 1 m and it has a working distance of 10 m to 20 km. Used for both navigation and scientific study, this altimeter instrument is being supplied by Jena-Optronik in Germany
6●Radio Science Experiment – Hera’s main 1.13-m diameter X-band High Gain Antenna antenna will also perform science in its own right. Doppler shifting in its signals – and the inter-satellite links connecting Hera to its two CubeSats will be used to measure the gravity fields of Didymos and Dimorphos. The HGA was manufactured by HPS in Germany and Romania.
And aboard the Juventas CubeSat:
7●Juventas Radar – the smallest radar instrument ever flown in space, which will perform the first radar sounding inside an asteroid. JURA will unfurl a quartet of 1.5-m long antennas larger than the CubeSat itself to peer up to 100 m within Dimorphos to a resolution of 15 m. Its synthetic aperture radar design takes advantage of the low velocity and distance to the target asteroid to send the same signal multiple times to boost its signal to noise ratio, compensating for the CubeSat’s lack of power. The signal is coded in such a way that the reflections can be disentangled back on Earth to compute a three-dimensional picture. JURA has been developed by France’s Institut de Planétologie et d'Astrophysique de Grenoble at the Université Grenoble Alpes and Technical University Dresden, with electronics coming from EmTroniX in Luxembourg and antennas from Astronika in Poland.
8●Gravimeter for the Investigation of Small Solar System Bodies – the first instrument to directly measure gravity on the surface of an asteroid, GRASS will come into play once Juventas touches down on the surface of Dimorphos, which will constitute the first landing of a CubeSat on such a small body. It should show how gravity levels on Dimorphos change over the course of its orbit due to the influence of its parent asteroid Didymos.
9●Juventas Narrow Angle Camera – to be used for navigation of the CubeSat as well as scientific study.
And aboard the Milani CubeSat:
10● Asteroid Spectral Imager – A descendant of compact hyperspectral imagers originally designed to fly aboard terrestrial drones, ASPECT will image Dimorphos in visible, near and shortwave infrared bands to perform close-up prospecting of the mineralogy of the asteroid surface and individual boulders. It was developed by the VTT Technical Research Centre of Finland.
11●Volatile In-Situ Thermogravimetre Analyser – based on piezoelectric quartz microbalances, the 5-cubic-cm sized VISTA detects dust particles smaller than 5-10 micrometres (thousandths of a millimetre) and volatiles such as water in their makeup. Developed by Italy’s National Research Council National Institute for Astrophysics and National Institute of Space Astrophysics and Planetology and Politecnico di Milano.
12●Milani Narrow Angle Camera – to be used for navigation of the CubeSat as well as scientific study.
6. How is ESA’s Hera connected to NASA’s DART mission?
The two missions owe their origins to a single mission concept, devised by the late Andrea Milani, professor of mathematics at the University of Pisa (who the Milani CubeSat is named after). He was a planetary defence pioneer who first devised what became ESA’s Near Earth Object Coordination Centre, based at the Agency’s ESRIN centre in Frascati, Italy. Then in 2003 he proposed the idea of a double spacecraft planetary defence mission called Don Quijote that was then recommended in 2004 by the ESA Near-Earth Object Mission Advisory Panel (NEOMAP) as the basis for ESA participation in Near-Earth Object impact-risk assessment and reduction. One spacecraft – called Hidalgo – would impact a non-threatening asteroid while another spacecraft – called Sancho – would gather data to validate asteroid impact models.
The viability of financing the two spacecraft from European funding alone was difficult to secure. Instead, following a discussion among scientists and their related space agencies emphasising the value of a dedicated planetary defence mission, NASA took on the development of the impactor spacecraft, which became DART, while ESA undertook what evolved into Hera. Planetary defence being an inherently international endeavour, the pair became part of the Asteroid Impact and Deflection Assessment (AIDA) international collaboration. Both missions are also supported by a common community of planetary scientists, organised into working groups and performing modelling and astronomical observations.
7. Which spacecraft is bigger: DART or Hera?
DART’s main body forms a box with dimensions of roughly 1.2 × 1.3 × 1.3 m and solar wings extending 18 m across. Hera measures approximately 1.6 m across per side, with solar wings that extend 11.5 m when extended. So Hera is bigger than DART but the latter has longer solar wings.
8. What technologies will Hera be testing?
Hera includes a strong technology demonstration component. The mission will be testing the first ESA CubeSats to be deployed in deep space, which will stay linked to their mothership using an radio-based inter-satellite link system, based on distributed systems. This innovative technology, provided by Tekever in Portugal, will allow Hera to serve as relay for the CubeSats, to return data to Earth and receive commands, while also pinpointing their relative locations to minimise any risk of collision. The inter-satellite link system will also have scientific return, allowing a more accurate estimate of the gravity fields of Dimorphos and Didymos through the measurement of Doppler shifting than by solely Hera radio signals.
Hera has a high degree of onboard autonomy, fusing data from different inputs to map its surroundings, in a similar approach to self-driving cars. While designed to be fully operated manually from ground, once Hera’s core mission tasks conclude, this novel autonomy will be tested out for real. Hera’s most crucial navigation data source will be its Asteroid Framing Camera, combined with inputs from its startrackers, laser altimeter to determine its distance from the asteroids, TIRI for nighttime imaging plus inertial sensors. In principle Hera should be able to navigate safely as close at 200 metres from the surface of Dimorphos, delivering 2 cm imaging resolution.
9. How will Hera contribute to asteroid science?
While primarily a planetary defence mission, Hera will also perform unique science, as the first spacecraft to perform a sustained survey of a binary asteroid system. Dimorphos is the smallest asteroid yet visited by humankind. Surveying it together with Didymos as its primary should reveal much about binary asteroid formation. If the two turn out to be made of the same material, then it might be that Didymos spun faster than its structure could endure: debris thrown from its surface congealed into Dimorphos. Didymos itself is the fastest spinning Solar System object visited by humankind so far, rotating once every 2.26 hours, potentially close to its limits of cohesion, so may be regularly flinging dust and rocks into space. This high spin rate is believed to be due to the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect – named after four different asteroid researchers. This involves differential warming by sunlight across an asteroid surface, leading to heated materials leaving the surface, inducing thrust and possibly altering the spin rate and obliquity of the asteroid.
Hera’s Juventas CubeSat will also add a new dimension to asteroid studies by peering beneath the surface of Dimorphos. Is it a rubble pile, held together mainly by gravity, with large voids within it? Or is it made of a solid core surrounded by a layer of boulders? In tandem the mass of Dimorphos will be measured by observing the ‘wobble’ of its parent Didymos asteroid. Hera will select key landmarks, such as boulders, on the larger asteroid surface then track their motion around the centre of mass of the overall asteroid system. The entire moon will be surveyed down to a resolution of a few metres, with selected areas mapped down to 10 cm resolution. Down at smaller scales, Hera’s surface observations will reveal the range of physical phenomena other than gravity that govern asteroid surfaces. Does cohesion play a significant role, for instance? Results would hold relevance for future asteroid mining as well as science.
10. Why is Hera bringing CubeSats with it?
Hera will deploy a pair of CubeSats once it reaches the Didymos system. CubeSats are small, low-budget satellites assembled from standardised 10 cm boxes. They were originally developed for educational purposes but are increasingly finding operational roles in Earth orbit. Further out, in 2018 NASA deployed twin CubeSats in the vicinity of Mars and in 2022 DART’s impact of Dimorphos was recorded by Italy’s LICIACube.
Hera will carry two ‘6-unit-XL’ CubeSats, each about the size of shoeboxes. Think of Hera like an aircraft then the CubeSats will operate more like drones, flying closer to the target asteroids and taking more risks. Hosting added instruments, they will give different perspectives and complementary investigations of this exotic binary asteroid system. They will also provide Europe with valuable experience of close proximity operations relayed by the Hera mothercraft in extreme low-gravity conditions. This should also be very valuable to many future missions.
11. What particular discoveries will Hera’s CubeSats make?
The Juventas CubeSat, developed by GomSpace in Luxembourg, will perform the first internal radar sounding of an asteroid, revealing the inner structure of Dimorphos. Once it lands it will also measure the asteroid’s gravity field, and how it shifts during each orbit of Didymos.
The Milani CubeSat, developed by Tyvak in Italy, will perform detailed spectral measurements of Dimorphos’s surface – breaking down the sunlight it reflects into individual colours to discern what materials it is made from, and highlighting its interaction with the space environment and potentially spotting any differences with its Didymos parent body. Milani will also perform a census of the dust surrounding Dimorphos, measuring its size and identifying its makeup.
12. How is the Hera mission being implemented?
A mission in ESA’s Space Safety programme, Hera was approved for development at ESA’s Council at Ministerial Level in November 2019. The mission was produced for ESA by an industrial consortium led by OHB in Bremen, Germany and supported by a science team. To ensure a complete deep space mission could be ready for testing within three years, the spacecraft was divided into two parts to allow simultaneous working: the Core Module – the brains of the mission, hosting its onboard computer, mission systems and instruments – was prepared at OHB while its Propulsion Module – incorporating its propellant tanks within a central carbon fibre reinforced polymer cylinder, the ‘backbone’ of the spacecraft plus piping and thrusters – was prepared at Avio near Rome in Italy. Finally the two halves were joined together in August 2023 then the complete Hera spacecraft was transported to ESA’s ESTEC Test Centre in the Netherlands to begin pre-flight testing.
13. How is Hera being powered?
Hera will be powered by solar panels in the form of two 5-m long wings, made up of three hinged panels each. These add up to an overall area of approximately 14 square meters in all, with more than 1,600 gallium arsenide solar cells in total. Azur Space in Germany manufactured the solar cells, which were then interconnected and arranged into working arrays by Leonardo in Italy onto panels provided by Beyond Gravity in Switzerland. These arrays are manoeuvred towards the Sun with solar array drive mechanisms linking them to the body of the spacecraft.
Designed and qualified to operate at temperatures between -100°C and +140°C, the panels will continue working even with the Sun at its furthest distance, out beyond Mars orbit, where the spacecraft will receive only 17% of sunlight compared to a satellite orbiting Earth. In the phases of the mission in which Hera will be most distant, the solar panels will generate around 800 watts, equal to the energy needed to power a small microwave oven.
14. How is Hera being launched?
Hera will be launched in October 2024 by SpaceX Falcon 9 launcher from the Cape Canaveral Space Force Station in Florida, USA, with a launch window opening on 7 October and closing on 27 October.
15. How is Hera being controlled from the ground?
Hera will be controlled from the European Space Operations Centre, ESA’s mission control centre in Darmstadt, Germany. Contact will be maintained using ESA’s trio of 35 m-diameter Deep Space Antennas, located in Cebreros (Spain), Malargüe (Argentina) and New Norcia (Australia). During Hera’s Launch and Early Operations Phase additional coverage will be offered from NASA’s Deep Space Network stations at Goldstone (US) and Canberra (Australia). Once Hera deploys its Juventas and Milani CubeSats, these will be overseen from a purpose-built facility at ESA’s European Space Security and Education Centre, at Redu in Belgium. Hera will serve as a relay for signals to and from the CubeSat pair.
16. How long will Hera take to reach its destination?
Once launched, Hera will begin a two year cruise phase. An initial deep space manoeuvre in November 2024 will be followed by a Mars swingby (and Deimos flyby) in March 2025. A second deep space manoeuvre in February 2026 will bring Hera on course to the Didymos system. An ‘impulsive rendezvous’ in October 2026 will bring Hera into the vicinity of the asteroid system for orbit insertion. Relative distances change continuously as everything in the Solar orbits around the Sun, but on the day Hera reaches Didymos it will be 195 million km away from Earth.
17. How will Hera maintain its position in the ultra-low gravity of the Didymos system?
Didymos’s gravity is estimated to be about 40 000 times weaker than Earth’s while Dimorphos’s gravity is approximately 200 000 times weaker. This is so low that Hera will remain in orbit around their ‘barycentre’ (or common centre of gravity in the Didymos system) at a typical relative velocity of around 12 cm per second. Even so, this orbit will need ongoing corrections to be maintained at the desired distance, rather than gradually drifting away. The result resembles a set of flybys thrown regularly into reverse; Hera borrows this ‘hyperbolic arc’ technique from ESA’s Rosetta mission. Short arcs of three days’ duration are interleaved with long arcs taking four days, creating a weekly cycle, bringing the spacecraft within 20-30 km of the asteroid. Because Hera’s overall velocity remains low, it takes little propellant to perform each individual manoeuvre. The technique is inherently safe because if any single manoeuvre fails then Hera will fly further out into space, rather than risk an impact.
The Milani CubeSat will operate similarly but at lower altitudes, from 10 down to 2 km. The Juventas CubeSat will enter a unique orbit around Didymos, perpendicular to the orbit of Dimorphos around Didymos, lined up with the Sun in parallel with the terminator line along Didymos.
18. Why is Hera going to swing by Mars?
In March 2025 Hera will perform a ‘swingby’ of Mars and flyby of Martian moon Deimos. The spacecraft will come to within 5000 – 8000 km of the Martian surface, closer than both Martian moons, and its trajectory will be tweaked to observe Deimos from as close as 1000 km away. This swingby will lend Hera extra momentum to rendezvous with the Didymos system. It is not part of the mission’s core objectives but will give an opportunity for serendipitous science discoveries and for extra calibration of the instruments aboard. Hera will use its visual Asteroid Framing Camera, visual-near infrared Hyperscout-H imaging spectrometer and its Thermal Infrared Imager to observe Mars and Deimos during the swingby.
Orbiting 23 460 km from Mars, Deimos is the further and smallest of the two Martian moons. The lumpy body has a diameter of 12.4 km across has a dark surface reminiscent of C-type asteroids. One theory is that both Deimos and its fellow Martian moon Phobos are in fact captured asteroids from the main Asteroid belt, although their surface characteristics have features in common with the planet below them, which would conversely suggest an impact-based origin.
As well as observing in synergy with ESA’s own Mars Express and Trace Gas Orbiter missions, Hera will also gather data along with the Emirates Mars Mission ‘Hope Probe’, which launched in July 2020 and entered orbit around Mars in February 2021. Hope performs regular Deimos flybys, but Hera’s Hyperscout-H observes in a wavelength range not covered by Mars Hope. Hera’s flyby of Deimos will also gather data of use for planning the JAXA-led Martian Moons eXploration mission, MMX, which is due to launch in 2026. MMX will survey both moons while also landing a small rover called IDEFIX developed by CNES and DLR on Phobos and acquiring samples from Phobos to return to Earth in 2031.
19. Which countries are involved with the Hera mission?
The Hera mission has 18 participating ESA Member States plus Japan (supplying the TIRI instrument). Notably German industry is leading the mission while Italy is providing the propulsion and Spain and Romania developed Hera’s innovative guidance, navigation and control system. The Hera Science Team involves scientists from all ESA Member States, Japan, the US and other non-European countries.
20. What is the mission cost?
Hera’s overall mission cost is €363 million at 2022 economic conditions, which includes spacecraft and payload development, launcher procurement and operations.
21. How is Hera linked to ESA’s Rosetta comet mission?
Following a decade-long odyssey across the Solar System, in 2014 Rosetta rendezvoused with Comet 67P/Churyumov-Gerasimenko in deep space. It studied the nucleus of the comet and its surrounding environment for nearly two years, orbiting the comet on its journey through the inner Solar System, measuring the increase in activity as the icy surface was warmed up by the Sun, and mapping its surface features and composition. Rosetta also set down its Philae lander on the comet’s surface and performed the first radar sounding of a cometary interior.
Hera is about about two thirds of Rosetta’s size and a third of its mass. However Hera has various elements in common with its comet-chasing predecessor. The spacecraft will fly the same kind of hyperbolic arcs around its target asteroid as Rosetta and will use surface feature tracking as a means of navigation in the same way as Rosetta (although processing of this data took place on the ground with Rosetta, and will take place autonomously onboard in the case Hera). In addition, Hera’s Juventas CubeSat carries a scaled-down ‘monostatic’ version of Rosetta’s radar instrument (meaning Rosetta’s radar receiver picked up signals transmitted from the Philae lander, while Hera’s radar will pick up reflected signals from the same instrument).
22. Is there any danger to Earth from the DART impact?
It important to note that neither Dimorphos nor Didymos pose any hazard to Earth before or after DART’s controlled collision with Dimorphos. The Didymos asteroid pair only came to within 10 million km of Earth during their 2022 close encounter. The impact only slightly altered the orbit of Dimorphos around Didymos, making them the ideal candidate for such orbital deflection experiments and crucial science.
23. What will happen to the CubeSats and Hera itself at their end of mission?
Juventas and Milani will both land on Dimorphos once their scientific goals are achieved while the concept of Hera itself touching down on one of the poles of Didymos is currently under consideration. None are specifically designed for landing, but will limit their velocities to a few centimetres per second. The hope is that all they continue to operate after landing.
Juventas will spend its working lifetime in orbit around Didymos to perform radar soundings of Dimorphos. Once sufficient radar and radio science data has been gathered, the CubeSat will have its orbit shifted around Dimorphos. As it descends Juventas will use its cameras to image Dimorphos surface features and hopefully the DART landing site in the highest possible resolution. Before landing it will turn on its 3-axis GRASS gravimeter, accelerometers and gyros to record the dynamics of the impact or landing event. Once Juventas comes to rest, it will use its gravimeter payload – the first to operate from the surface of an asteroid – to measure local gravity. The goal would be to stay operational for one terrestrial day, equivalent to approximately two orbits of Dimorphos around Didymos, showing how local gravity changes over time.
Milani will already be in orbit around Dimorphos once its scientific mission is concluded. It will similarly perform an uncontrolled soft landing on Dimorphos; the dynamics of its surface interaction will be captured through its accelerometers and gyros. Assuming a stable landing, Milani’s VISTA instrument will be able to gather data on the makeup of the asteroid’s surface dust. Results from both CubeSats will be gathered by Hera via its inter-satellite links.
24. What is ESA’s Space Safety programme?
ESA’s Space Safety programme aims at mitigating and preventing the effects of hazards from space, protecting our Pale Blue Dot, its inhabitants, and the vital infrastructure − like satellites in orbit and power grids on the ground − on which we have become so dependent. Living so close to an active star, in a Solar System filled with ancient and fast-moving space rocks, on a planet that is becoming increasingly surrounded by discarded satellites and their debris, comes with a plethora of possibilities for something to go wrong.
Along with Hera, the Space Safety programme includes a plan to develop a European space weather monitoring system, a planned network of Flyeye telescopes and a new in-space asteroid detection mission in support of ESA’s existing Near-Earth Object Coordination Centre – the central access point to a network of European asteroid data sources and information providers – and a automated ‘space traffic management’ system to monitor and manage space debris, including an Automated Collision Avoidance System to allow the safe operation of satellites and constellations. Space Safety also includes ESA’s Clean Space initiative, pioneering an eco-friendly approach to space activities. On the ground, that means adopting greener industrial materials, processes and technologies. In space, it means preserving Earth’s orbital environment as a safe zone, free of debris.
