Gaia technology tests

The cold gas to keep the Gaia spacecraft spinning at its steady rate will be depleted early 2025. On 15 January 2025, the Gaia spacecraft will stop taking new science data and the final amount of cold gas, estimated to be an equivalent of about 15 days of nominal Gaia operations, will be used for technology tests.

The Gaia spacecraft has been constructed using a wide range of technologies which have been combined to create a unique machine that operates in a very stable environment. The spacecraft's stability is essential for the science observations. These technology tests would have disrupted the spacecraft for an extended period and therefore could not be performed during the normal science observation campaign. The goal of these technology tests is to learn more about the spacecraft's instruments onboard Gaia, to study the behaviour of certain instruments and the spacecraft as a whole. This could help to improve Gaia calibrations for future data releases, but also the design of future space missions. Some of the Gaia technologies have already been re-used, for example the mirror-drive electronics and cold-gas thrusters on EUCLID. Future science missions such as the gravitational wave observatory LISA or the potential next generation GaiaNIR are going to be ever more sensitive and are good candidates that might profit from the Gaia experience.

A list of planned tests is given below.

Calibration Tests

Even though throughout the mission regular calibration data was acquired, a few additional, one-off calibration tests are planned following the end of the Gaia science observation phase. One of them is to use the special high-density virtual object patterns for a longer period of time to map things like straylight, CCD response, and the wings of the point spread function. Another test involves high rate pre-scan pixels sampling. The data will be immediately useful to better understand some known issues, but can potentially also help to better map the temperature variation across the focal plane.

More information on the applied calibration models used for Gaia Data Release 3 can be found here.

Basic Angle Variation Tests

Gaia performs wide-angle astrometry, and does so by having two telescopes with their fields of view separated by a constant angle of 106.5°. This angle is referred to as the basic angle. Small variations in the basic angle are monitored by the 'basic angle monitor'.

To better understand the observed variations of the basic angle, a few tests are planned to study how the basic angle varies following certain changes in the spacecraft configuration, in an attempt to identify the root cause for these variations. These tests can provide important information for the design of future space missions.

Illustration of the Gaia torus with mounted mirrors as published here.

The majority of the tests on basic angle variation require disturbances to the spacecraft's thermal stability which means these tests could not be performed during Gaia's science observation phase, where Gaia is kept at a very stable temperature of around -110º Celsius.

What will be tested?

One probable cause for the basic angle variations would be thermal changes to the spacecraft. The thermal state of the spacecraft is most easily influenced by changing the Sun Aspect Angle (SAA), which is nominally 45º. Over the course of a few weeks, the Gaia Sun Aspect Angle will be changed. Below, an overview of the planned states is given. The changes in the Gaia SAA will make the spacecraft more easily visible from the ground. Astronomers are encouraged to follow-up and observe Gaia.

For a few weeks, data will be obtained from both the nominal and the redundant Basic Angle Monitor, a test referred to as the "Dual OSE/BAM Operation", which will allow a comparison of the output. This test hence focuses on the working of the basic angle monitor itself.

Since it is known that the basic angle variation is correlated with the spacecraft rotation, which in itself seems to affect the spacecraft's temperature, a few tests focus on investigating this connection. One of the tests called "CPS Usage Zone Investigations" will focus on orienting the spacecraft in different ways and subsequently allowing the spacecraft to reach thermal equilibrium at each atttitude. During routine operations, Gaia was already oriented differently for avoiding the Draconids, and from this different orientation, an effect on the basic angle was seen. Different attitudes are now tested as well.

Another test called "Micro-propulsion free-drift" investigates the relation between the micro-propulsion that is used by Gaia for its steady scanning of the sky and the variation seen in the basic angle.

More information on the basic angle of Gaia can be found from the Data Release Documentation for Gaia DR3, as well as from the Gaia mission paper.

Technology Tests

These tests aim to study the technology used onboard Gaia. The results can help to improve future mission design and can help us understand better the behaviour of certain technology following the more than 10 years in space.

Gaia transmits data to Earth using its transponder. On 18 February 2018, Gaia's first transponder experienced an anomaly. Gaia was recovered at that period from its safe mode and has been using its second transponder since. After Gaia's science observation campaign ends, a test is performed to see if the first transponder can be switched on and used nominally. Even though Gaia's transponders are quite unique, there are some similarities to transponders used for other spacecraft and investigating the behaviour of the first transponder after many years of not being used can be useful for current and future missions using a similar device.

Also the Phased-Array Antenna (PAA) of Gaia will be tested. The PAA has been used in a certain way throughout Gaia's operational life referred to as “beam steering mode”. In this mode, the azimuth of the beam follows the spin of the spacecraft such that the beam is always Earth directed. A new contingency mode, referred to as "torus mode", was implemented during the mission in which the device radiates equally in all azimuth directions but this mode was never tested so far. Now, this mode will be tested in view of gaining experience if used in future missions. Another test involving the Phased-Array Antenna of Gaia will be to change the set-points of some of the heaters in its vicinity. This may allow insight into the aging and performance after 10 years in space of some of the materials used to provide protection against the Sun on-board the spacecraft.

Finally, also some tests will be done on the central data management unit (CDMU) and the micro-propulsion system (MPS). Also these tests can give some valuable information for current and future missions.

More information on the spacecraft can be found from the Gaia mission paper as well as from the section on the spacecraft status from the Data Release Documentation of Gaia's latest data release.