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

Abstract

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. <P />The probe has been located, along its projected nominal path, with quite high precision, within 0.13<SUB>±0.09</SUB> arcsec, or 0.9<SUB>±0.6</SUB> km. Spacecraft color appears to be red, with (V -R<SUB>c</SUB>) =1.1<SUB>±0.2</SUB> and a bolometric correction to the R<SUB>c</SUB> band of (Bol -R<SUB>c</SUB>) = -1.1<SUB>±0.2</SUB> . The apparent magnitude, R<SUB>c</SUB> =20.8<SUB>±0.2</SUB> , is much fainter than originally expected. These features lead to suggest a lower limit for the Bond albedo α =0.11<SUB>±0.05</SUB> and confirm that incident Sun light is strongly reddened by GAIA through its on-board MLI blankets covering the solar shield. <P />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 ⩾10<SUP>6</SUP> cm<SUP>2</SUP>. 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. <P />