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, with quite high precision, within 0.13±0.090.13±0.09 arcsec, or 0.9±0.60.9±0.6 km. Spacecraft color appears to be red, with (V-Rc)=1.1±0.2(V-Rc)=1.1±0.2 and a bolometric correction to the RcRc band of (Bol-Rc)=-1.1±0.2(Bol-Rc)=-1.1±0.2. The apparent magnitude, Rc=20.8±0.2Rc=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±0.05α=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⩾106A⩾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.