Originally perceived by astronomers as a space mission that was unlikely to ever happen, the European Space Agency’s Gaia space observatory is currently midway through the commissioning and calibration phase of its mission, preparing for the start of science observations which are scheduled to begin later this year.
The idea for a new space observatory to follow the success of the Hipparcos astrometry mission of the late 1980s was originally conceived in the early 1990s as part of the agency’s Horizon 2000 Plus scientific program, which also included such notable missions like the XMM-Newton, Planck, and Herschel space telescopes. The Gaia mission proposal steadily progressed through various levels of ESA, gaining Science Programme Committee approval in October 2000 and entering the hardware construction phase—with EADS Astrium as its prime contractor—in February 2006.
Following a successful night-time launch atop a Soyuz STB/Fregat-MT rocket from ESA’s Guiana Space Centre in Kourou, French Guiana, on Dec. 19, 2013, Gaia spent the next several weeks cruising toward its destination, the L2 Sun–Earth Lagrangian point 930,000 miles (1,500,000 km) away from Earth, and entering into a Lissajous 180-day period orbit around L2, after arriving there in mid-January.
Gaia’s optical instruments consist of two telescopes, with a set of 10 different mirrors of various shapes and sizes. The light gathered by the telescopes will be directed toward three different science instruments: an astrometry instrument to precisely determine the positions, distances, and proper motions of stellar objects, called ASTRO; a photometric instrument to gather spectral data, or BP/RP; and a radial-velocity spectrometer, or RVS, for determining the velocities of stars across the field of view of the telescopes. Gaia’s mission, according to ESA, is “to make the largest, most precise three-dimensional map of our Galaxy by surveying more than a billion stars.” This number of stars represents approximately 1 percent of the Milky Way’s total stellar population, giving astronomers a very important and diverse dataset of objects by which to test current models of our galaxy’s evolution and overall structure. But before that can happen, mission controllers on the ground must first go through a process of calibrating the space telescope’s mirrors and science instruments, so that they can operate as planned.
Even before arriving at its final destination Gaia had begun acquiring a series of test images of different star fields to better guide mission controllers on their task of properly focusing and aligning the spacecraft’s telescopes and mirrors. A week before entering orbit around the Sun-Earth L2 point, Gaia opened its electronic eyes for the first time while observing a total of approximately 18,000 stars—a feat that was completed in less than three hours! One of these stars was α Aquarii, also known by its Arabic name of Sadalmelik. Although the star appeared saturated in these first test images, engineers on the ground were able to get their first taste of the space observatory’s overall capabilities.
A series of tests was also made to the onboard high-gain antenna, responsible for transmitting the bulk of Gaia’s science observations to Earth. To that end, the spacecraft was aimed toward NGC 1818, a young star cluster located approximately 164,000 light-years away inside the Large Magellanic Cloud, a satellite galaxy of the Milky Way. “As part of that process, the Gaia team have been using a test mode to download sections of data from the camera, including this image of NGC 1818, a young star cluster in the Large Magellanic Cloud,” stated ESA during the press release of the NGC 1818 test image. “The image covers an area less than 1% of the full Gaia field of view. This test picture, taken as part of commissioning the mission to ‘fine tune’ the behaviour of the instruments, is one of the first proper ‘images’ to be seen from Gaia, but ironically, it will also be one of the last, as Gaia’s main scientific operational mode does not involve sending full images back to Earth.”
The famous Cat’s Eye Nebula, or NGC 6543, also posed for some calibration images during late January. The test pictures obtained of the planetary nebula helped engineers to fine-tune the optical instruments to the spacecraft’s rate of spin of four rotations per day.
Although the spacecraft have proved to be in excellent health during the initial commissioning phase of operations, some minor issues have nevertheless cropped up during testing. Calibration images have shown that a small amount of stray light coming from the Sun finds its way to the telescopes’ focal plane assembly, resulting in a diffuse illumination that could hinder the spacecraft’s ability for precise and detailed observations of very faint objects. The cause of the problem, according to the engineering teams, is believed to be the current tilt of Gaia’s sunshield relative to the Sun. The sunshield is one of Gaia’s most important components, responsible for protecting all of the spacecraft’s sensitive optical instruments and electronics from the Sun’s intense glare, which would otherwise make any observations impossible. By lowering the sunshield’s tilt of 45 degrees down to 42 degrees during the course of the next few weeks, engineers hope to mitigate the amount of stray light from the Sun that reaches the telescopes.
Following the beginning of its five-year science mission later this summer, Gaia will start analyzing each of its target stars no fewer than 70 times. The space telescope’s observations will lead to the creation of precise three-dimensional maps of star motions, yielding detailed physical properties, characterizing luminosities, pegging effective temperatures, and compiling databases of gravitational and elemental compositions. In addition to stars and galaxies, half a million quasars will fall under Gaia’s gaze, together with around 1,000 extrasolar planets and objects within our Solar System, including the mysterious “Apohele” asteroids, which lurk between Earth and the Sun. “Our Gaia discovery machine will keep us busy throughout the mission, with the final results coming only after the five years of data have been analysed,” says Timo Prusti, Gaia Project Scientist at ESA. “But it will be well worth the wait, ultimately giving us a new view of our cosmic neighbourhood and its history.”
Considering the huge amounts of data that it will be able to provide to the scientific community, there is no doubt that Gaia will turn out to be one of the most important space missions of the whole decade.
Video Credit: ESA
This article was co-written by AmericaSpace writers Ben Evans and Leonidas Papadopoulos.
Below are more calibration test pictures from Gaia:
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