USE OF MULTI-SITE OPTICAL MEASUREMENT FOR JOINT PHOTOMETRIC AND ASTROMETRIC OBSERVATIONS Marco Acernese 1 , Veronica Marini 1 , Stefania Melillo 2,3 , Leonardo Parisi 1 , Fabrizio Piergentili 1 , Fabio Santoni 4 1 Dipartimento di Ingegneria Meccanica e Aerospaziale, Universit` a Sapienza, 00184 Rome, Italy 2 Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy 3 Dipartimento di Fisica, Universit` a Sapienza, 00185 Rome, Italy 4 Dipartimento di Ingegneria Astronautica, Elettrica ed Energetica, Universit` a Sapienza, 00138 Rome, Italy ABSTRACT The increasing population of space debris and NEO ob- jects is becoming a severe threat for all the on-ground and in-space infrastructure, because of the risk of poten- tial collisions that may seriously damage these active sys- tems. It is therefore of paramount importance to maintain updated catalogues containing estimates of the orbital pa- rameters of objects belonging to the whole trackable pop- ulation. Space Surveillance activities, such as in-orbit collision avoidance and re-entry campaigns, are typical based on publicly available orbital parameters (TLE), ac- cessible only for catalogued and unclassified space ob- jects. Unfortunately, TLEs are generally characterized by a few days or even faster degradation, which makes the information provided not completely reliable: object positions may be affected by errors of the order of sev- eral kilometers, mainly in the in-track direction, making the orbital prediction unreliable both at short (few hours in the specific case of objects at the end of their orbital life) and at large term (few days). In this paper we propose a relatively cheap resource to improve the short and large term orbital prediction us- ing multi–site optical measurements, i.e. collecting op- tical data of the same objects simultaneously from two or more sites. We plan to use optical measurements for joint astrometric and photometric observations: merging the astrometric information of the multiple sites we accu- rately retrieve the objects positions in the 3D space, while merging the photometric information we accurately re- trieve the objects attitude. On one side, by reconstructing the 3D position of the objects using multi–site measure- ments we drastically reduce the in-track error on the orbit parameters (from kilometers to tens of meters depending on the experimental set-up) producing accurate TLEs at epoch and as a consequence improving the quality of the TLEs predictions both at short and large terms. On the other side, using objects lightcurves from different point of views, we may retrieve their attitude which much more details than when using the lightcurve from one optical measurement only and we may figure out the shape and the orientation of the object we are looking at. These two elements (accurate 3D position and attitude) are essential for the large term prevision, where a force model estima- tion has to be fed with information on the orientation, on the shape and on the position of the objects. Keywords: Debris, Observations, Multi-site, Optical. 1. INTRODUCTION From the beginning of human space activities on 4th Oc- tober 1957, approximatively 5.000 rocket launches have placed more than 8.000 satellites into Earth orbit, of which only 1.800 are still active [1]. The disruption of these satellites, together with uncontrolled collisions have produced a incredible high number of fragments, which according to the Inter-agency Space Debris Coordination Committee (IADC) are defined as space debris: Space debris are all man-made objects including fragments and elements thereof, in Earth orbit or re-entering the atmo- sphere, that are non-functional. The number of these de- bris is increasing day by day, as shown in Fig.1, where the evolution of the debris number is plotted as a func- tion of time for all the orbits. ESA estimates approxima- tively 29.000 debris larger than 10cm, 750.000 smaller than 10cm and larger than 1cm and 166 millions of frag- ments smaller than 1cm. These objects may reach very high relative speed (∼16km/s) and as a consequence, even objects as small as 1cm are characterized by a suf- ficient kinetic energy to severely damage or even catas- trophically destroy spacecraft or active satellites after col- lision events, eventually causing the spreading of addi- tional fragments, capable to produce further collisions. The first known collision between two satellites occurred in 2009, involving the US operating satellite Iridium 33 and the non-functional Russian satellite Kosmos-2251. Beside the destruction of both satellites, more than 2000 additional objects were produced in the catastrophic col- lision. Furthermore, since one of the two satellites was Proc. 1st NEO and Debris Detection Conference, Darmstadt, Germany, 22-24 January 2019, published by the ESA Space Safety Programme Office Ed. T. Flohrer, R. Jehn, F. Schmitz (http://neo-sst-conference.sdo.esoc.esa.int, January 2019)