Hera (space mission)

Near-Earth Objects (NEOs) are celestial bodies (asteroids, comets) whose orbits around the Sun cross the Earth's one and may, as a result, in a more or less distant time, crash on Earth. Their impacts generate damages, depending mainly on their size, their density, their speed, their trajectory incidence and on the Earth zone hit. Probability of a large-size asteroid impact on Earth is low but its consequences on society may be particularly severe.

The more numerous NEOs, which regularly strike the Earth, are less than 30 meters in diameter and have a low probability of causing a catastrophe. Beyond a diameter of 30 meters, the impact of an NEO can range from the annihilation of a city to that of human civilization (object of more than 1,000 meters in diameter). The impact of an NEO presenting a serious threat is statistically rare (the impact frequency of an object of 1000 meters in diameter is on average 500,000 years).

Since it was scientifically identified, the threat of near-Earth objects has been neglected by society because a collision with Earth of a large asteroid is perceived as a very rare phenomenon. But between 16 and 22 July 1994, the fragments of comet Shoemaker-Levy 9 crashed spectacularly on the giant planet Jupiter. An impact of the same order of magnitude on Earth would have had planetary consequences with effects similar to those that led to the extinction of the dinosaurs. The threat is now tangible and contemporary. The United States were the first to take it into account by developing measures falling under what would later be called planetary defense.[1]

The American Congress, sensitized by the impact of comet Shoemaker-Levy 9 and advised by several scientists including Eugene Shoemaker, took a first step in 1998 to assess the danger. It included in the objectives of the American space agency (NASA) the detection of 90% of near-Earth objects having more than one kilometer in diameter. The American space agency had 10 years to identify them and determine their trajectories and their main characteristics.[1] In 2005, the Congress broadened NASA's mission by extending it to near-Earth objects over 140 meters in diameter. NASA has 15 years to achieve this goal (deadline 2020) but the budget granted by the Congress is insufficient to achieve this objective within the time limit.[2][3] Over the following years, NASA funded several terrestrial (Catalina Sky Survey, Pan-STARRS, LSST, etc.) and space (NEOWISE) telescope projects aimed at carrying out this essential census to assess the threat and prevent it. For NEOs larger than 1 km, the goal has been achieved but for smaller ones, the calendar objective is falling far behind since, in 2019, only 1.6% of NEOs over 30 meters in size (16,000 out of an estimated number of one million) and 31% of NEOs over 140 meters in diameter (about 5,000 out of 16,000) have been identified.[4] A space telescope dedicated to this census, NEO Surveyor, is to be launched in 2026.

During April 2021 none of the asteroids identified and whose orbit is known presents a threat to Earth. The threat will therefore come from asteroids that have not yet been discovered.[5]

In 2021, there is no operational method for diverting a near-Earth object that threatens to collide with the Earth. Several techniques are envisaged but they need to be tested. Generally it is a question of slightly modifying the orbit of the near-Earth object by applying thrust to the celestial body so that it avoids the Earth. If the thrust is punctual, it must be applied when the body is near its aphelion (apogee). One can also choose to exert a weaker but continuous thrust. The more the correction of the trajectory is anticipated, the less it needs to be significant. To avoid an impact with the Earth, it is therefore necessary to identify as soon as possible all the near-Earth objects likely to threaten the Earth and to estimate their trajectories with great precision for the decades to come. The second condition for success is to be able to set up a space mission allowing the threat to be diverted with a very high probability of success. The main trajectory modification methods are as follows:[6]

•    The first deflection method, already implemented for a completely different purpose by NASA's Deep Impact space probe, is to launch a spacecraft against the NEO. The speed of the asteroid is modified due to the law of conservation of momentum:

M1 x V1 + M2 x V2 = (M1 + M2) x V3

with M1 mass of spacecraft, M2 mass of comet, V1 velocity of spacecraft, V3 velocity of comet after impact, M1 and M2 respective mass of spacecraft and comet. Velocities are vectors.

•    Another method is to cause a nuclear explosion intended to fragment the asteroid. This solution is technically feasible but its effects are uncontrollable. This would be a solution to be considered as a last resort.
•    A more effective method would be to detonate a nuclear charge on the surface or at a short distance from the NEO in such a way as to transmit an impulse to it without fragmenting it.
•    The gravity tractor is a method that uses the mutual gravitational attraction between the NEO and a spacecraft.
•    The use of the Yarkovsky effect, which is a force produced by the discrepancy between solar absorption and thermal emission by radiation. This force which contributes permanently to shape the orbit of the near-Earth object can be modified for example by interposing a screen between the Sun and the asteroid or by modifying the albedo of this one (for example by depositing a black or white coating on its surface). The intensity of this force is very low but it can, over time, make it possible to obtain the desired deviation. While elegant on paper, this solution is complex to implement in a space mission.

European Space Agency logo

The European Space Agency (ESA) was the first to embark on the study of an experimental mission aimed at evaluating a method of deflecting a near-Earth object. In 2005-2007, following the recommendations of its NEOMAP committee (Near-Earth Mission Advisory Panel) made up of 6 European experts (Willy Benz, Alan Fitzsimmons, Simon F. Green, Alan W. Harris, Patrick Michel , Giovanni Valsecchi), it defined the specifications of the Don Quijote mission, the objective of which is to demonstrate that it is possible to deflect an asteroid using the kinetic energy provided by an impactor. The program did not materialize for reasons of cost and lack of a dedicated program. But the need to perform such a test remained and the Don Quijote concepts served as a reference in many reports dedicated to planetary defense.

From 2012, the European Union became involved in planetary defense and funded four studies relating to this for a total amount of around €16 million over the decade. Their objective is to develop the various aspects of a system ensuring the protection of the Earth against an impact of a near-Earth asteroid: detection, feasibility of the deflection process, modeling of the impact, guidance of the impactor, methods observations from Earth, etc. These four studies are:

• NEOShield (de) (2012-2015), piloted by the German Space Agency (DLR),
• NEOShield 2 (de) (2015-2018), piloted by the German establishment of Airbus Defense and Space,
• NEO-MAPP (2020-2023), piloted by the French CNRS,
• NEOROCKS (2020-2023), led by the Italian National Astrophysical Institute (INAF).

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 are responsible for taking over partially the role of AIM. The DART project will evolve thereafter by incorporating the LICIACube nano-satellite, released before the impact and responsible for taking and retransmitting the first 100 seconds of it.

In 2017, at the request of several member states of the European Space Agency, the latter resumed the studies of a replacement for AIM that was named Hera (named after the Greek goddess of marriage Hera). Hera must fulfill all the objectives assigned to AIM, but in the meantime optimizing all the components of the mission as much as possible. Hera shall be launched in October 2024 and study the effects of the DART impact on Dimorphos, the satellite of Didymos, 4 years after it occurred. The Hera mission was eventually approved by the ESA Ministerial Council in November 2019. In September 2020 the European Space Agency entrusted 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, a scientific council, four working groups covering all aspects of the mission and the scientific managers of the instruments.

The Hera satellite is cubic in shape, 1.6 × 1.6 × 1.7 meters and has a mass of approximately 1128 kg. Its energy is provided by solar panels with an area of 13 m². It includes an inter-satellite link to communicate with the two nano-satellites.

The satellite is stabilized on 3 axes. The attitude is maintained by 4 reaction wheels, gyroscopes, star trackers, solar sensors and two Asteroid Framing Cameras (AFC). Attitude guidance is through the Planetary Altimeter (PALT).

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.

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 the Cosine company. This is a specific version developed for Hera, different from the standard Hyperscout.

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.

TIRI is a thermal infrared imager provided by 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.

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.

Two CubeSat type nano-satellites, 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 affecting radio links 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.

The CubeSat 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.

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).