  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.
