IoW_20160106 - Gaia

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Gaia intersects A Perseid Meteoroid

	Figure 1: Corrective torques applied to recover spacecraft attitude after a meteoroid strike.
In orbit at the second Sun-Earth/Moon Lagrange point (L2), Gaia is exposed to a flux of high-velocity meteoroids. When a meteoroid strikes Gaia, these particles carry enough momentum to disturb the spacecraft attitude and at the same time cause local heating and deformation of the spacecraft structure. The Gaia measurements of a single meteoroid strike from 2015 are presented here. To perform its astrometric mission, Gaia is equipped with extremely sensitive sensors for measuring its attitude and movement, and so the deviations due to a meteoroid strike are detected within a few seconds and corrected by use of the fine attitude control cold gas thrusters. Figure 1 shows the torque applied to correct for the above mentioned meteoroid strike. Some information about the direction of origin of the meteoroid can be extracted from the magnitude of the applied corrective torques, but because a meteoroid hit can occur at any point on the spacecraft, it is not possible to know the forces involved because the lever arm length is generally unknown. Gaia is also equipped with many sensors which are used to continually monitor the spacecraft temperature. If the point of impact is sufficiently close to one of these temperature sensors, then the effects of the impact are detectable as small increases in the temperature which coincide with the attitude disturbances. Figure 2 shows the temperature change as measured on the Gaia sunshield (DSA) due to the same meteoroid strike. These impacts, which are also detected by the temperature sensors, are interesting for study because they provide more information about the nature of the impact. Firstly, because the location of the temperature sensor is known, it is possible to reduce the possible originating direction of the impactor to a 180 degree arc. Secondly, the lever arm length is known to allow calculation of the forces of impact from the torques. Finally, an estimate of the energy of the impact can be calculated from the measured heating of the structure.

Figure 2: Localised temperature increases are occasionally observed to coincide with meteoroid strikes.
The temperature sensors on the DSA are mounted in a thermally isolated box made from 350 g of aluminium with a heat capacity of just under 1 J/gK. To heat this box by the observed 0.25 K requires 84 J of energy which is the lower limit of the kinetic energy of the impactor relative to Gaia as the efficiency of energy transfer is unknown. For comparison, 84 J is about half the energy of a .22 calibre bullet as it leaves the muzzle of a gun. Meteoroids are known to mostly originate from comets and recently the ESA spacecraft Rosetta has provided spectacular images of dust separating from the comet 67P Churyumov-Gerasimenko. After ejection from the parent body, these dust particles continue to orbit the Sun following approximately the same orbit in a stream. The example meteoroid impact might be special because it occurred on 18 August 2015 which is within the time period where the Earth, and therefore Gaia, traverses the meteoroid stream that causes the Perseid shooting stars. The Perseid meteoroids have been ejected from comet 109P Swift-Tuttle and are known as Perseids because the shooting stars that they generate appear to originate from the constellation Perseus. Figure 3 shows in blue the outline of the Perseus constellation with the apparent point of origin (radiant) of the Perseids on 18 August 2015 as a red dot. The green line shows the arc of possible origins of the Gaia impactor as determined by the torque and temperature measurements with the green dot representing the central (zero radial component) direction.

Figure 3: All-sky map showing constellation Perseus and arc of possible origins of Gaia impactor.
Although it has not been possible to prove that the meteoroid originated from comet 109P with the current analysis, it is tempting to imagine that it may have done so and to calculate the mass of such a particle. The Perseid meteoroids are known to be travelling at 59 km s^-1 relative to the Earth and Gaia, and a particle at this speed needs to have a mass of only 48 μg to carry 84 J of kinetic energy. These serendipitous measurements from Gaia will allow scientists to study the meteoroid environment for future missions to L2 and beyond, including manned missions to asteroids and Mars. Due to the incredible precision of its instruments, Gaia is able to join Giotto and Rosetta as one of ESA's comet explorers! Acknowledgement: these figures were prepared by Edmund Serpell, a Gaia Operations Engineer working in the Mission Operations Centre at ESA's European Space Operations Centre in Darmstadt, Germany.
Credits: ESA/Gaia-CC BY-SA 3.0 IGO http://www.esa.int/Services/Creative_Commons_Attribution-ShareAlike_3.0_IGO_CC_BY-SA_3.0_IGO_licence [Published: 06/01/2016]