The target of the AIM mission is asteroid 65803 Didymos (1996 GT), an Apollo-type near-Earth object (NEO) with a perihelion (minimum distance to the Sun) that is just below the aphelion radius (maximum distance to the Sun) of Earth orbit. Didymos is a binary asteroid; the primary body has a diameter of around 775 m and a rotation period of 2.26 hours, whereas the secondary body (informally called Didymoon) has a diameter of around 165 m and rotates around the primary at a distance of around 1.2 km in around 12 hours.
In 2015 telescopes around the globe homed in on the Didymos system to study the shape and size of the double asteroid and try to ascertain the orbital pole of Didymos and refine the shape model of the primary, among other things, to help plan ESA’s AIM mission proposal, which is part of the AIDA project.
The orientation of Didymoon’s orbit with respect to the ecliptic is one of the key factors involved in planning the AIM/AIDA mission. It is important, for example, in calculating the approach of the spacecraft to the binary system, as well as to know when the asteroid moon will fall into shadow – needed for scheduling payload operations. The information obtained during the latest observation period was put together with already known data to refine the orbital pole of the primary, its shape and other dynamical parameters, putting constraints on the bulk density of the two bodies.
A preliminary shape model of the Didymos primary could be extracted from previous radar observations in combination with new lightcurve data.
When the AIM spacecraft has arrived at the binary system it will collect extensive amounts of data on both the primary and secondary asteroid, including precise measurements of the asteroid’s mass and shape and thermal imaging of the surface to understand the structure and mineralogical composition of the soil.
A variety of measuring techniques will be available through the instruments on board AIM and its lander/cubesats (see Payload) to comprehensively detail the Didymos system before DART, the impactor, will hit Didymoon. In particular, direct information on the internal structure of an asteroid will be obtained for the first time. AIM will watch the impact and will again collect masses of data after impact to be able to calculate the effect of the impact on both the shape, structure and orbit of Didymoon around the primary.
The thorough study of Didymoon and its impact by DART will be the first fully documented impact experiment at asteroid scale and will offer valuable insights into the origin and history of our Solar System that need a better understanding of the impact process at large scales. It will also provide scientists invaluable data to develop and assess the efficiency of planetary defense strategies against any incoming asteroids in the future.
Despite Didymos' close encounter with the Earth in 2022, this asteroid poses no threat as there is no risk of impact. It is therefore the ideal candidate to run an asteroid mitigation precursor experiment whose objective is to change the orbit of Didymoon around its primary main body thus leaving the binary asteroid's path around the Sun unchanged. Such experiment will give us crucial information about whether the kinetic impactor technique would work to deflect an asteroid if ever one was discovered to be headed our way.