Milani (ex APEX)

Milani [ESA]

ESA's Milani (formerly known as APEX (Asteroid Prospection Explorer)) probe is a 6U CubeSat mission developed by Tyvak International srl to perform a multispectral mineral survey of the makeup of the binary asteroid system (65803) Didymos / Dimorphos.

Its 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. It will also carry VISTA,an Italian-built dust detector. The instrument is devoted to detect the presence of dust particles smaller than 5-10 µm, to detect volatiles and light organics charactersation and to monitor the molecular contamination in the surrounding of Mliani. It is a miniaturised hyperspectral imager extending from the visible up to shortwave infrared (SWIR) wavelengths. In contrast to more traditional spatial scanning imaging spectrometres, the Asteroid Spectral Imager takes 2D snapshots at a given wavelength. When multiple snapshots are combined, a spectral datacube is formed, where the wavelength bands are separated in the time domain. The instrument is split into four measurement channels, one in the visible (VIS), and two in the NIR and one in SWIR. The VIS and NIR channels are imaging spectrometres, while the SWIR channel only measures a single point. The spectral separation is done by a tunable Fabry-Perot Interferometre (FPI). All four channels have dedicated FPIs optimised for the desired wavelength range. The imaged wavelengths are freely selectable within these ranges, and the targeted spectral resolution is ca. 10 - 50 nm. All four channels can be operated simultaneously and are independent of each other: even if a single image sensor or FPI is lost, the mission can still carry on with limited capabilities.

The other instrument is VISTA (Volatile In-Situ Thermogravimetre Analyser). It is a micro-thermogravimetre (sensor head including Proximity Electronics) devoted to detecting the presence of dust particles smaller than 5-10 µm; to detect volatiles (e.g., water) and light organics characterisation (e.g. carboxylic acids with low carbon chain) and monitoring the molecular contamination in support of other instruments on-board cubesat or satellites. VISTA has been designed with a miniaturised configuration (as an environmental sensor) for cubesat and other instrument packages. The device is based on micro-oscillators with a detection system made up of piezoelectric crystals with a metal electrode that acts as a collector of micron and sub-micron size particles. An important challenge of VISTA is the presence of a built-in heater and a built-in Resistance Temperature Detector (RTD) integrated onto the crystal. This special design dramatically reduces the instrument mass and the power required to perform thermal cycles to characterise the deposited mass on the sensor head.

Milani is planned to launch with Hera in October 2024 and rendezvous with asteroid Didymos in 2026. It was baselined for an Ariane-62 Astris launch, but in October 2022 a SpaceX Falcon-9 v1.2 (Block 5) was selected.