As ESA's Hera mission for planetary defence probes the Didymos twin asteroid system, it will be joined by a pioneering pair of breadbox-sized 'CubeSats'. Juventas will perform radar soundings while Milani will image the bodies in a wider range of colours than the human eye can see, prospecting the mineral makeup of individual asteroid boulders.

Hera will reach the Didymos system in 2027, to survey the aftermath of a collision by NASA's DART spacecraft with the smaller of the two asteroids, Dimorphos. Hera will carry with it two 'CubeSats' - nanosatellite-class missions based on standardised 10-cm boxes, making maximum use of commercial off the shelf systems.

Milani, like Juventas, will be a '6-unit' CubeSat. Named after the pioneer of asteroid risk analysis who came up with the original double-spacecraft DART-Hera concept, Milani's main instrument will be the ASPECT hyperspectral imager, combining visible and near infrared wavelengths to survey the surface down to a maximum spatial resolution of 1 m.

ASPECT will sift through sunlight reflected from Dimorphos as well as its bigger companion Didymos, looking for distinctive mineral "fingerprint' absorptions" explains Tomas Kohout of Helsinki University in Finland and the Czech Academy of Sciences. "By obtaining a full spectrum for each pixel we can identify variations in surface composition, including DART's crater and its ejecta, and link these to known meteorite samples and minerals."

Margherita Cardi of Tyvak International in Italy is managing Milani development on the industrial side: "The main objective of Milani, along with its consortium partners, is to enhance Hera's overall science return and demonstrate CubeSats technology use in the deep-space environment.

"The satellite navigation signals that most CubeSats rely on in Earth orbit will not be available, so we needed to come up with other methods of navigating around the asteroids, comprising optical cameras, inertial measurement units, Sun sensors and star trackers as well as ranging performed through our inter-satellite link with the Hera mothercraft.

"Another aspect to be considered is that direct communication with Milani CubeSat will not be possible - we are reliant on Hera itself as a relay spacecraft. This means Milani needs a high level of onboard autonomy, and also onboard data processing, to reduce the spacecraft data volume to be downloaded."

Simply to fit all necessary subsystems within 6U CubeSat means swapping traditional space-qualified parts for the latest miniaturised commercial-off-the-shelf items. These offer enhanced performance, at the cost of potentially greater vulnerability to space radiation. The Politecnico di Torino,a mission partner, is performing radiation analyses on candidate avionics batches.

After two years of cruising to the Didymos system, Milani, as the other cubesat Juventas, will be deployed from its mothercraft in a cautious manner, Margherita explains: "It will be partially deployed, exposed to outer space while still interfacing with its deployer, for a period which is in the order of days during which a complete system checkout is performed."

Upon successful checkout completion, release will follow, at a maximum velocity of 5 cm/s. Any faster ejection would expose Milani to the risks of leaving the Didymos system before the initial, critical propulsive maneoeuvre is executed.

Milani will fly a set of complex trajectories about Dydimos asteroids, with range to the main bodies reduced progressively from 10 down to 2 km. The rationale is to hover on the asteroids in order to keep their sunlit face in view, and to maintain this geometry with a number of maneuvers using cold-gas thrusters.

Milani will fly in an inclined orbit, 20 degrees off the poles, at roughly airliner altitude - a maximum 11.7 km above Dimorphos, allowing ASPECT to keep the entire asteroid within its field of view. Its working orbit is planned to keep the sunlit face of the asteroid in view, maintained with regular orbital manoeuvres using cold-gas thrusters.

The CubeSat is also designed to perform 'corrective action manoeuvres' if it risks heading into 'risk tubes' surrounding either asteroid, Juventas or Hera itself.

Along with ASPECT, Milani's secondary payload is called VISTA (Volatile In-Situ Thermogravimetre Analyser) ,an Italian-built dust detector. The instrument is devoted to detect the presence of dust particles smaller than 5-10 micrometres (thousandths of a millimetre) to detect volatiles such as water, characterise light organics and to monitor molecular contamination surrounding the CubeSat.

Milani will be launched aboard Hera in 2024 and the baseline mission duration is 12 weeks, after which comes the question of its final fate. It could be despatched into a graveyard orbit, or else - in a higher-risk move - put down on the surface of Dimorphos.

The overall Milani consortium is made up of 12 companies and research institutes in Italy, the Czech Republic and Finland, as follows: Tyvak International (Italy); Politecnico di Torino (Italy); Politecnico di Milano (Italy); Centro Italiano Ricerche Aerospaziali, CIRA (Italy); Istituto Nazionale di Astrofisica, INAF-IAPS (Italy); Aerospace Logistics Technology Engineering Company, ALTEC (Italy); VTT Technical Research Center of Finland (Finland); University of Helsinki (Finland); Institute of Geology of the Czech Academy of Sciences (Czech Republic), Reaktor Space Lab (Finland), Brno University of Technology (Czech Republic); HULD (Czech Republic).

Related Links
Hera at ESA
Asteroid and Comet Mission News, Science and Technology

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