AIDA (mission)

The Asteroid Impact and Deflection Assessment (AIDA) mission is a proposed pair of space probes which will study and demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon. The mission is intended to test and validate impact models of whether a spacecraft could successfully deflect an asteroid on a collision course with Earth.
The original plan called for a European spacecraft, the "Asteroid Impact Mission (AIM)", to operate in synergy with a large NASA impactor called "Double Asteroid Redirection Test" ("DART") and observe the immediate effects of the impact. "AIM" was cancelled in 2016 when Germany was unable to fund its portion, and after some backlash within ESA, "AIM" was replaced in 2018 with a smaller spacecraft called "Hera" that will launch five years after "DART" to orbit and study the crater on the asteroid.
"DART" is currently planned to impact in 2022 in the small moon of asteroid 65803 Didymos, while "Hera" will arrive at Didymos in 2027, five years after "DART" impact.
History.
Initially, "Hera" role was to be realized by a much larger spacecraft called "Asteroid Impact Mission" ("AIM"), that would have observed the plume, the crater, and the freshly exposed material to provide unique information for asteroid deflection, science and mining communities. In December 2016, the European Space Agency (ESA) cancelled the development of the "AIM" spacecraft after Germany decided to fund the ExoMars project only. Germany offered to cover only 35 million of the 60 million needed for the "AIM" portion to continue, and this was not enough to continue development. Had "AIM" been developed, its notional requirements in 2012 were:
Under the original proposal, "AIM" would have launched in October 2020, and "DART" in July 2021. "AIM" would have orbited the larger asteroid and studied the composition of it and its moon. "DART" would then impact the asteroid's moon in October 2022, during a close approach to Earth. "AIM" would have studied the asteroid's strength, surface physical properties and internal structure, as well as measured the effect on the asteroid moon's orbit around the larger asteroid.
Nevertheless, NASA has continued development of the "DART" mission to 65803 Didymos, and plans to measure the effects of the impact from ground-based telescopes, and from an Italian CubeSat "DART" will bring along. Following "AIM" cancellation, ESA director Jan Wörner stated his intentions to revive the European mission in some form. Etienne Schneider, Luxembourg's deputy prime minister, expressed regret at "AIM" cancellation, and commented that his country would continue to advocate for the realization of the mission.
Status.
By March 2018, "Hera" proposal was in Phase B1, where the preliminary design was being drawn up. On 7 January 2019, the "Hera" team announced the selection of two CubeSats to piggyback on the mission: "APEX" and "Juventas". ESA officials approved "Hera" in November 2019 for a 2024 launch.
The Italian Space Agency (ASI) decided in 2018 to contribute to NASA a secondary spacecraft called "LICIA" ("Light Italian CubeSat for Imaging of Asteroids"), a 6-unit CubeSat that will piggyback with "DART" and will separate shortly before impact to acquire images of the ejecta as it drifts past the asteroid.
Collaboration.
The AIDA mission is a joint international collaboration of the European Space Agency (ESA), the German Aerospace Center (DLR), Côte d'Azur Observatory (OCA), NASA, and Johns Hopkins University Applied Physics Laboratory (JHU/APL). 
The miniature lidar instrument on board "Hera" will be provided by a consortium of teams from Portugal, Poland, and Ireland. Two CubeSats will be deployed by "Hera" while at Didymos: The "APEX" (Asteroid Prospection Explorer) CubeSat is being developed by Sweden, Finland, Czech Republic and Germany. The "Juventas" CubeSat is being developed by GomSpace and GMV's Romanian division. 
Along with surveying "DART" impact crater, "Hera" may also carry a Japanese impactor that would be a replica of the Small Carry-on Impactor (SCI), on board the "Hayabusa2" asteroid sample return mission. NASA's "DART" will bring along a 6U CubeSat called "LICIA" which is being developed by the Italian Space Agency (ASI) to image the ejecta plume.
"Hera".
"Hera" is the European component of the ESA–NASA AIDA mission. The "Hera" spacecraft, approved on 29 November 2019, will focus on key measurements to validate impact and asteroid deflection models such as the detailed characterisation of the impact crater made by the "DART" impactor. "Hera" will also measure the DART impact outcome, such as change in the binary system orbit, and will enable detailed characterisation of the Dimorphos volume and surface properties, as well as measure the volume and morphology of the "DART" impact crater.
The baseline payload of "Hera" is still in flux, and it includes a camera, a miniaturized lidar and two 6U CubeSats dedicated to asteroid characterisation. The spacecraft design allows for of additional payload mass, including the Small Carry-on Impactor (SCI) proposed by Japan's space agency JAXA. Other options, such as a small lander, are being considered. "Hera" will be launched on an Ariane 6 in 2024.
Proposed payload.
The notional payloads on "Hera" are:
DART.
DART or Double Asteroid Redirection Test is a impactor that hosts a single camera, Didymos Reconnaissance and Asteroid Camera for OpNav (DRACO) to support autonomous guiding to impact the moon of (Didymos B) through its center. It also carries an Italian-built cubesat called LICIACube that will be released pre-impact to image the event. It is estimated that the impact of the DART at will produce a velocity change on the order of 0.4 mm/s, which leads to a small change in the orbit of the Didymos B, but over time, it leads to a large change in the orbital position (or orbital phase).
Mission design.
AIDA will target 65803 Didymos, a binary asteroid system in which one asteroid is orbited by a smaller one. The primary asteroid is about in diameter; its small satellite is about in diameter in an orbit about 1.1 km from the primary. Didymos is not an Earth-crossing asteroid, and there is no possibility that the deflection experiment could create an impact hazard to Earth.
The impact of the "DART" spacecraft at 6.25 km/s will produce a velocity change on the order of 0.4 mm/s, which leads to a significant change in the mutual orbit of these two objects, but only a minimal change in the heliocentric orbit of the system. AIDA will provide a great benefit obtaining the size of the resulting impact crater in addition to the momentum transfer measurement, as the effects of porosity and strength of the target are needed to calculate the momentum transfer efficiency.
"DART" is currently planned to impact in 2022 on the small moon of asteroid Didymos, while "Hera" will arrive at Didymos in 2027, five years after "DART" impact, so in order to maximize scientific outcome, the AIDA team proposes to delay "DART" launch so that "Hera" would arrive at the asteroid first, enabling it to study "DART" impact, the plume, the crater, and the freshly exposed material. While most of the initial objectives of AIDA would still be met if "Hera" arrives after "DART", as a drawback data from direct observation of the impact and ejecta will not be obtained.