One of the reasons that the operational orbit at the second Sun-Earth Lagrange point (L2) was selected for Gaia is because it provides the thermal stability that is needed to make the extremely precise measurements of star positions for which the spacecraft was built. Satellites in Earth orbit experience deep eclipses where either the Earth or the Moon passes in front of the Sun and the thermal disturbances caused by these eclipses would affect the Gaia measurements. The Gaia orbit at L2 has been designed to have no Earth transits across the face of the Sun although there are occasional transits of the Moon which cause small thermal changes onboard.

The Gaia spacecraft temperature is passively controlled, this means that there are no switching heaters used to control the temperature, and it is allowed to reach a temperature where the solar heating of the structure is balanced by the blackbody radiation to space. Because of this operational strategy, the temperature of Gaia is sensitive to the received solar power which varies over a year as the distance to the Sun changes slightly due to the ellipticity of the Earth's orbit.

The power of the solar radiation itself is known to be very stable over long periods of time, but it is not constant and has some deviations during an eleven year solar cycle. On a shorter timescale, it has been known for hundreds of years that the visible face of the sun can show sunspots, and it is understood that these sunspots are areas of the photosphere of the Sun which appear to be black when viewed in comparison to the surrounding surface because they are much cooler. Observed from the Earth, these sunspots are seen to move across the face of the Sun in about 13 days due to a combination of the rotation of the Sun and the orbit of the Earth around it.

The Sun has been under continual surveillance from space by a fleet of spacecraft for a few decades. For Gaia, one of the interesting parameters that is collected, because of the potential to effect the thermal balance, is the total solar irradiance (TSI) which is a measure of the solar output power over a wide band of the electromagnetic spectrum. This value is normally quoted at the average distance of the Earth from the Sun and has a value of about 1361 W m^-2.

The observed TSI can be influenced by the appearance of a sunspot where the cooler area of the spot emits less energetic photons. The largest sunspot of the current solar cycle, numbered AR12192, was as big as the planet Jupiter and appeared during the early routine operations phase of the Gaia mission in October 2014. Images of the Sun's surface in visible light were acquired at this time by the Solar Dynamics Observatory (SDO) spacecraft (Figure 1). The images of this sunspot show it rotating into view on about 18 October, crossing the centre of the Sun on the 23^rd and departing out of view on the 28^th of the month. During this time, the sunspot did not appear to evolve, although it did emit a number of X-class flares including a class X3.1 flare on 24 October.

The TSI data for this period were acquired by the Solar Radiation and Climate Explorer (SORCE) spacecraft. Starting on 18 October, these data show a slow decrease in the TSI over a few days with the lowest TSI value about 3 W m^-2 (0.22%) less than the normal reached on 23 October (Figure 2). The reason for this slow decrease in observed output is that as the spot rotates across the spherical surface of the Sun its apparent area increases and therefore so does its influence on the measured solar output.

All temperature changes to the Gaia spacecraft have the potential to influence the scientific products, so engineers at the control centre in Darmstadt were curious to know how much this sunspot had affected the temperatures around the spacecraft. A Gaia temperature monitor (thermistor) located in the spacecraft sunshield was selected for investigation. Although this thermistor is not intended for high precision measurements, because of the stability of the thermal environment, it is possible to calculate long-period running averages that provide high resolution data for slow changes such as the transit of this sunspot. Such computations revealed a cooling of the spacecraft sun-shield by up to 0.15 °C caused by the   sunspot AR12192. This is a small effect but within the resolution of the method.

As expected, similar analyses of the data from the payload, which is strongly thermally isolated from the sun-shield, show that there was no significant effect on the instrument from this sunspot.

Acknowledgement: contribution by Edmund Serpell, a Gaia Operations Engineer working in the Mission Operations Centre at ESA's European Space Operations Centre in Darmstadt, Germany.