Free Astronomy Magazine July-August 2019

42 JULY-AUGUST 2019 SPACE CHRONICLES Painstaking calibration is required to transform the observations, in which Gaia is just a speck of light among the bright stars, into meaningful or- bital information. Data from Gaia’s second re- lease was used to iden- tify each of the stars in the field of view, and al- lowed the position of the spacecraft to be calcu- lated with astonishing precision — up to 20 mil- liarcseconds. “This is a challenging process: we are using Gaia’s measurements of the stars to calibrate the position of the Gaia spacecraft and ultimately improve its measure- ments of the stars,” ex- plains Timo Prusti. “After careful and lengthy data processing, we have now achieved the accuracy required for the ground-based observations of Gaia to be implemented as part of the orbit determination,” says Mar- tin Altmann, lead of the Ground Based Optical Tracking (GBOT) cam- paign at the Centre for Astronomy of Heidelberg Univer- sity, Germany. The GBOT information will be used to im- prove our know- ledge of Gaia’s orbit not only in observa- tions to come, but also for all the data that have been gathered from Earth in the previ- ous years, leading to improvements in the data products that will be included in future releases. T his image, a composite of several observations captured by ESO’s VLT Survey Telescope (VST), shows the space observatory Gaia as a faint trail of dots across the lower half of the star-filled field of view. These observations were taken as part of an ongoing collaborative effort to measure Gaia’s orbit and improve the accuracy of its unprecedented star map. The position of Gaia is shown in this annotated image. [ESO] “Gaia observations require a special observing procedure,” explained Monika Petr-Gotzens, who has coor- dinated the execution of ESO’s ob- servations of Gaia since 2013. “The spacecraft is what we call a ‘moving target’, as it is moving quickly relative to background stars — tracking Gaia is quite the chal- lenge!” “The VST is the perfect tool for picking out the motion of Gaia,” elaborated Ferdinando Patat, head of the ESO’s Ob- serving Programmes Office. “Using one of ESO’s first-rate ground-based facilities to bol- ster cutting-edge space obser- vations is a fine example of scientific cooperation.” “This is an exciting ground- space collaboration, using one of ESO’s world-class telescopes to anchor the trailblazing ob- servations of ESA’s billion star T his video summarizes how ESO’s VLT Survey Telescope (VST) helps map our galaxy, by tracking Gaia in motion as a faint series of dots trailing across the night sky. [ESO] surveyor,” commented Timo Prusti, Gaia project scientist at ESA. The VST observations are used by ESA’s flight dynamics experts to track Gaia and refine the knowledge of the spacecraft’s orbit. !