Observe Gaia from the ground

Gaia Ground Based Optical Tracking (GBOT)

Ever since Gaia was launched into space, the spacececraft has been observed from the ground. This task is known as the Ground Based Optical Tracking (GBOT) of Gaia, and was necessary to ensure the full accuracy of the ultra-precise Gaia astrometry could be reached, since the conventional means of determining a spacecraft's position and motion were not sufficient in the case of Gaia.

Soon after launch, it was discovered that Gaia was about 3 magnitudes fainter than predicted, and the GBOT team adapted their approach. Supplemented by another technique called Delta Differential One-Way Ranging (Delta DOR), previously only used for interplanetary missions, GBOT was nonetheless able to fulfil its task of observing Gaia from ground.

Most of the time Gaia was visible at about 21 magnitude in the red passband, but there were some occasions where Gaia was far brighter. During the initial testing and commissioning phase in the first half of 2014, Gaia was swerved so that the sun shield assembly directly faced the Sun (instead of the 45 degrees solar aspect angle it keeps during its nominal operations). Other rare occasions were when Gaia avoided the Draconid storm with a commanded safe-attitude manoeuvre, this changed Gaia's attitude with respect to the Sun and a brightness of about 14 to 15 magnitude in the same passband was observed by GBOT. It is assumed that this brightness will be reached during the end of observation testing, when Gaia is repositioned to similar angles with respect to the Sun.

Read more about this topic in the article on observing Gaia from Earth to improve its star maps.

Gaia Ground-Based Optical Tracking measurements showing apparent magnitude of Gaia as a function of time, with a clear peak at the start in 2014 when Gaia was in its commissioning period, and another clear peak around 2018 when Gaia was oriented differently with respect to the Sun with a commanded safe-attitude manoeuvre to avoid the Draconid storm. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: Gaia GBOT team.

Perform Gaia observations yourself

While Gaia has been visible from ground for dedicated ground-based observatories, it was very faint and difficult to follow-up for amateur astronomers (at a magnitude of about 21). During the Gaia technology tests following the end of Gaia observations, Gaia's solar aspect angle will change and its brightness will change accordingly (to a magnitude of about 15). This will give the unique opportunity to view Gaia with also smaller telescopes from ground.

To follow-up from the ground, a good knowledge of where to find Gaia in the sky is essential. A tool called the Gaia ephemeris service is available to make predictions. It can derive topocentric ephemerides for every city worldwide above 100,000 inhabitants. The server is updated on a weekly basis.

Track where Gaia is in the sky with the Gaia ephemeris service

The visibility will change because the solar aspect angle (SAA) of Gaia will change. Below an overview is given of the planned solar aspect angle changes over time. This schedule can still change and more detailed information on how to observe Gaia from ground in this period will be given soon.

Table with draft schedule for Gaia's changing solar aspect angle following the planned technology tests. (*) The Gaia solar aspect angle (SAA) is the angle between the spacecraft spin axis and the Sun. (**) The brightness refers to the brightness of Gaia in apparent magnitudes (visible light). (***) NSL refers to the nominal scanning law, EPSL refers to the Ecliptic Plane scanning law. Last updated 12 November 2024

Gaia's expected attitude profile following the end of Gaia science observations on 15 January 2025. For the planned timing, the table above is leading. This visual is merely an illustration. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: Gaia Flight Operations Team.

Gaia scanning law explained, as published here. You can see clearly the Gaia solar aspect angle of 45 degrees highlighted. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

Share your Gaia observations

We would love to see your observations of the Gaia spacecraft.

If you want to share your observations with the astronomical community, and/or if you want to help the GBOT-team with their brightness campaign, we welcome your data. Below we describe where to deposit them, dependent on the type of data you share. Please be aware that we can only accept fully reduced data, as we do not have the manpower to reduce the data ourselves.

In order to be a flexible as possible, there are three repositories:

1. All image (and short video clip) files, i.e. jpeg, png, tiff, mpeg, gif, ..., can be sent directly to gaia-helpdesk@esa.int. A selection of these files will be shown on Gaia Cosmos. When sharing your files, please mention a credit line to go with the image, and if possible, the date, time and location of observation. Please do read point 3 in case you wish to send very large files.
2. All FITS data can be put on this VO-type repository (link will be provided soon). Note that this portal ONLY accepts FITS files. Please adhere to the instructions given on the entry page. Please be reminded to only submit fully reduced data.
3. All other files, i.e. ASCII, CSV, VO-table, etc. can be put on a cloud, based at the University of Heidelberg, Germany. This service will not be open access, but will be granted upon request, to prevent misuse. Details on how to request access will be added later, once this is finalised. Image/video files, which are too large to be accepted by the gaia-helpdesk email account can also be put there, with the same mode of access applying.