# Hera (space mission)

Hera is a spacecraft developed by the European Space Agency for its space safety program. Its primary mission objective is to study the Didymos binary asteroid system that was impacted four years earlier by the NASA Double Asteroid Redirection Test (DART) spacecraft and contribute to validation of the kinetic impact method to deviate a near-Earth asteroid from a colliding trajectory with Earth. It 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.
The spacecraft was launched on 7 October 2024 aboard a SpaceX Falcon 9 launch vehicle and will study the results of the DART impactor, four years after impact. DART impacted the asteroid Dimorphos, the smaller of two objects forming the binary asteroid 65803 Didymos, on 27 September 2022. The launch vehicle, B1061, previously used for the Crew-1 mission, was expended on this flight.
Hera has a mass of 1,128 kg (2,487 lb) and carries a payload of cameras, an altimeter, and a spectrometer. It is carrying two small CubeSat spacecraft, called Milani and Juventas.
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.

## Project history
### AIDA the joint project with NASA

AIDA is 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 is to test the use of an impactor-type device to deflect an asteroid that might strike the Earth. This program provides 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 must measure the effects of the impact. After an evaluation phase in the two space agencies, the European Space Agency 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 project would evolve thereafter by incorporating the LICIACube nano-satellite, released before the impact and responsible for taking and retransmitting the first 100 seconds of it.

### Renaissance of the European project

In 2017, at the request of several member states of the European Space Agency, the ESA resumed studies of a replacement for AIM, specifically, a mission that was named Hera (named after the Greek goddess of marriage Hera). Hera must fulfill all the objectives assigned to AIM that Earth-based observatories could not, using the components of AIM as much as possible. Hera was planned to be launched in October 2024 to catch a workable planetary/asteroid alignment and study the effects of the DART impact on Dimorphos, the satellite of Didymos, 4 years after it occurred. The Hera mission was subsequently approved by the ESA Ministerial Council in November 2019. In September 2020 the European Space Agency 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. The mission moved to final testing in March 2024.

## Goals

The main objective of the Hera mission is to evaluate the kinetic impactor method for deflecting a near-Earth object that threatens to crash into Earth. This deflection method consists of modifying the trajectory of the asteroid by launching a spacecraft at a speed of a few kilometers per second. Of all the methods, this is the most mature because it relies on the use of available and inexpensive spacecraft technologies. To fulfill this objective, Hera must 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.
Hera also has high scientific objectives. It must collect the characteristics of the two asteroids: surface characteristics, internal porosity and internal structure. In particular, Hera will be the first mission to measure the subsurface and internal structures of an asteroid. For this, it will use the JuRA low-frequency radar on board the CubeSat Juventas (see below). 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 of Didymos will be estimated with high accuracy, allowing a direct estimate of the momentum transfer efficiency from DART impact.
The mission also includes several technological objectives. The most important thing is the production of a 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 must also embark two CubeSats which will be dropped once the asteroid is reached. These CubeSats are:
* Juventas, which must 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 and
* Milani, with the mission of collecting spectral data from the surface of the two asteroids (surface composition) and identifying the presence of dust in the surrounding space.

## Mission profile

The DART mission, launched on 24 November 2021 at 06:21 UTC by a Falcon 9 rocket from Vandenberg Space Force Base 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. In order to be successful, the impact needed to change the orbital period of Dimorphos around Didymos by a minimum of 73 seconds from its pre-impact value of 11.9 hours, which should be observable by terrestrial telescopes.
Hera 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 are 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 with a second stage deorbit burn happening 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.
Following the launch, the spacecraft conducted a deep-space maneuver in November. Hera then performed a gravitational 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 also spent some time observing the Marian moon Deimos, imaging it from distance of 1,000 km (620 mi) and having its closest approach from 300 km (190 mi) away. The spacecraft will reach the binary asteroid 65803 Didymos on 28 December 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 phase :
| Phase | Mission phase | Duration | Mission |
| 1 | Early Characterization | 6 weeks | The global shape and mass/gravity, as well as thermal and dynamical properties, of both asteroids will be determined |
| 2 | Payload Deployment | 2 weeks | The release of the two CubeSats |
| 3 | Detailed Characterization | 4 weeks | Meter-scale mapping of the asteroids and determination of thermal, spectral, and interior properties |
| 4 | Close Observation | 6 weeks | High-resolution investigations of a large fraction of the surface area of Dimorphos, including the DART impact crater |
| 5 | Experimental | 6 weeks | Morphological, spectral, and thermal properties of Dimorphos |

## Spacecraft

The main bus of Hera has a box-shape based on a central tube and adapter cone of 1.6 × 1.6 × 1.7 meters. Two solar panel wings extend from opposite sides, and a high-gain dish antenna is mounted on one face. Total launch mass of the spacecraft is approximately 1214 kg, the dry mass is 696 kg. Spacecraft deployed dimensions are 2.2 x 11.4 x 2.2 meters. The solar panels have an area of about 13 square meters. The spacecraft will use 712 W at the nominal 2.4 AU distance. Bi-propellant chemical propulsion is used for 16 orbit control thrusters and 6 reaction control thrusters, all 10-N motors. Total available delta-V is about 1300 m/s. Communications with the ground are X-band (~8.4 GHz), with two low gain antennas in addition to the high-gain dish. S-band communications (~2.2 GHz), using patch antennas, will be used to communicate with the two CubeSats named Juventas and Milani, with a range of 60 km. Spacecraft orientation is maintained by 4 reaction wheels, gyroscopes, using star trackers and solar sensors, as well two Asteroid Framing Cameras (AFC). Attitude guidance is through the Planetary Altimeter (PALT).

## Scientific instruments
### Asteroid framing cameras (AFC)

The main instruments of Hera are the two AFC cameras (Asteroid Framing Cameras), developed by the company JenaOptronik. Identical and redundant, they each have a FaintStar panchromatic sensor of 1020 x 1020 pixels with a telephoto lens. The field of view is 5.5 x 5.5 degrees, and the spatial resolution reaches one meter at a distance of 10 kilometers. These cameras are to provide physical characteristics of the surface of the asteroid Didymos and Dimorphos as well as the crater created by DART and the Juventas landing zone.

### Hyperspectral imager – HyperScout-H

HyperScout-H is a hyperspectral imager that must provide images in a spectral range between 665 and 975 nm (visible and near infrared). The instrument makes its observations in 25 distinct spectral bands. It is developed by cosine Remote Sensing. This is a specific version developed for Hera, different from the standard HyperScout.

### Planetary altimeter (PALT)

PALT is a micro-Lidar planetary altimeter using a laser emitting an infrared light beam at 1.5 microns. Its track on the ground is 1 meter at an altitude of 1 kilometer (1 milliradian). The altitude measurement accuracy is 0.5 meters. Its frequency is 10 Hertz.

### Thermal Infrared Imager (TIRI)

TIRI is a thermal infrared imager provided by the JAXA, the Japanese space agency. The spectral range observed is between 7 and 14 microns and it has 6 filters. Its visual range is 13.3 x 10.6°. The spatial resolution is 2.3 meters at a distance of 10 kilometers.

### X-Band Radio Science (X-DST)

The mass of the two asteroids making up the binary system, the characteristics of their gravity field, their rotational speed, and their orbits will be measured using radio wave disturbances caused by the Doppler effect. The measurements relate to the radio exchanges between Hera and Earth stations but also between Hera and the CubeSats. Due to the low orbit in which the CubeSats will circulate, these last measurements are crucial to determine the gravity of Didymos.
Main characteristics of the instruments :
| | AFC | HyperScout-H | PALT | TIRI |
| Type | Visible Imager | Spectral imager | Altimeter | Thermal infrared imager |
| Mass (kg) | <1.5 | 5.5 | 4.5 | <4.4 |
| Visual range (degrees) | 5.5 | 15.5 x 8.3 | non-applicable | 13.3 x 10 |
| Spatial resolution (microradians) | 94.1 | 133 | 1000 | 226 |
| Spectral band (nanometers) | 350-1000 | 665-975 | | 700-1400 |
| Others | | 25 spectral bands | vertical precision : 0.5 m. | 6 filters |
| Power (Watts) | <1.3 | 2.5 (average) - 4.5 (peak) | <14.5 | 20 (average) - <30 |

## CubeSats

Two CubeSat type nanosatellites, named Milani and Juventas, are transported by Hera and released before arrival in the asteroidal system 65803 Didymos. They are responsible for carrying out investigations that complement those of their carrier ship.
Both CubeSats are built around a similar platform. These are 6U-XL CubeSats with a mass (including propellant) of approximately 12 kilograms. They are 3-axis stabilized and have a cold gas propulsion system. They communicate with the mothership in S-band. The Doppler effect affected radio link is used to measure the characteristics of the gravitational field of the binary system. They have a visible light camera and star trackers which are used to determine the dynamic variations of Didymos. Finally, the two CubeSats are equipped with accelerometers which will be used to determine the properties of the surface of Dimorphos if the CubeSats land on its surface as planned at the end of their mission. Juventas is developed by GomSpace while Milani is made by Tyvak International.

### CubeSat Milani

The CubeSat Milani (named after Andrea Milani) aims to take images and measure the characteristics of the possibly present dust. It must map the two asteroids forming the binary asteroid 65803 Didymos, characterize their surface, evaluate the effects of the DART impact, contribute to the measurements of the gravitational field of the asteroids, and determine the characteristics of the dust clouds possibly located around the asteroids.
To fulfill these objectives, it carries two instruments:
* The ASPECT hyperspectral imaging spectrometer is the main instrument. It works in visible and near infrared light (0.5 to 2.5 microns). Its spatial resolution is 2 meters at 10 kilometers and its spectral resolution is less than 40 nanometers (20 nanometers in the visible). It has a total of 72 channels.
* The VISTA thermogravimeter is responsible for detecting dust (5 to 10 microns), volatiles (such as water) and light organic materials.

### CubeSat Juventas

Juventas aims to determine the geophysical characteristics of Dimorphos. The probe must map its gravity field and determine its internal structure as well as the characteristics of its surface.
To fulfill these objectives, it carries the following instruments:
* The JuRa radar operating in the 50–70 MHz frequency with a spatial resolution of 10 to 15 meters. It is the first instrument to probe the inner layers of an asteroid. It uses two dipole antennas with each branch measuring 1.5 meters. Each measurement session can last up to 45 minutes. It occupies a volume of less than 1U and its mass is less than 1300 grams.
* The GRASS gravimeter whose dynamic range is 5 x 10$^{−4}$ and sensitivity is 5 x 10$^{−7}$. Its mass is less than 380 grams.
* A camera.
* The radio link with the mother ship (measurement of the Doppler effect).