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

We tackle the problem of accurate optical tracking of distant man-made probes, on Halo orbit around the Earth-Sun libration point L2 and beyond, along interplanetary transfers. The improved performance of on-target tracking, especially when observing with small-class telescopes is assessed providing a general estimate of the expected S/N ratio in spacecraft detection. The on-going Gaia mission is taken as a pilot case for our analysis, reporting on fresh literature and original optical photometry and astrometric results. The probe has been located, along its projected nominal path, 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 a = 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 Aeff >= 10^6 cm^2. In addition to L2 surveys, 2m-class telescopes could also effectively flank standard radar-ranging techniques in deep-space probe tracking along Earth's gravity-assist maneuvers for interplanetary missions.