Optical tracking of deep-space spacecraft in Halo L2 orbits and beyond: The Gaia mission as a pilot case

The probe has been located, along its projected nominal path, with quite high precision, within 0.13_±0.09 arcsec, or 0.9_±0.6 km. Spacecraft color appears to be red, with (V -R_c) =1.1_±0.2 and a bolometric correction to the R_c band of (Bol -R_c) = -1.1_±0.2 . The apparent magnitude, R_c =20.8_±0.2 , is much fainter than originally expected. These features lead to suggest a lower limit for the Bond albedo α =0.11_±0.05 and confirm that incident Sun light is strongly reddened by GAIA through its on-board MLI blankets covering the solar shield.

Relying on the GAIA figures, we found that VLT-class telescopes could yet be able to probe distant spacecraft heading Mars, up to 30 million km away, while a broader optical coverage of the forthcoming missions to Venus and Mars could be envisaged, providing to deal with space vehicles of minimum effective area A ⩾10⁶ cm². In addition to L2 surveys, 2 m-class telescopes could also effectively flank standard radar-ranging techniques in deep-space probe tracking along Earth's gravity-assist maneuvers for interplanetary missions.