Gaia’s Mission to Map the Milky Way

Astronomers who study the Milky Way don’t have it easy. Bound to the solar system, they’re on the inside looking out, all the while whipping around the galactic center at roughly 900 000 kilometers per hour. That’s made it more than a little tricky to pin down fundamental details. It’s still unclear, for example, how massive the Milky Way is and whether it’s on a collision course with the nearby Andromeda galaxy. And there’s still a lot of uncertainty about its basic structure. “There’s been a pretty active debate in the last couple of years whether there are two or four spiral arms,” says Mark Reid, a radio astronomer at the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass. “That’s pretty basic.” But the Milky Way could soon start coming into better focus. Later this year, a European Space Agency space observatory called Gaia will launch from a spaceport near Kourou, French Guiana. From a perch some 1.5 million kilometers from Earth, Gaia will spend five years scanning the sky, taking in starlight from all over the Milky Way and processing it at the rate of as many as 8000 stars per second. The long-awaited €700 million mission aims to pin down the positions and velocities of a billion stars, about one out of every 100 stars in the galaxy. Those observations are expected to have a dramatic impact on our picture of the Milky Way. They should also significantly boost the accuracy with which we measure the distances to other galaxies, and by extension, the rate at which those galaxies recede from us as the universe expands. “It’s going to be phenomenal,” says astronomer Barry F. Madore of the Carnegie Observatories in Pasadena, Calif., who works on astrophysical distance measures. The mission will be nothing short of “foundational,” he says. “It will change everything.” Gaia’s main job will be to pinpoint the three-dimensional positions of stars. To measure distances—the trickiest component to pin down—Gaia is designed to be sensitive to parallax, the subtle shift in the position of an object against a background that occurs when the object is viewed from two different angles. You can see parallax in action if you hold a pencil in front of your nose and look at it with one eye closed and then the other. The farther away the pencil is from your nose, the smaller the change in position it exhibits with this change in perspective. Because at interstellar distances these shifts in position are tiny, Gaia needs to change the viewing angle dramatically. It will do this by following a solar orbit similar to Earth’s own and looking at the same patch of sky while on opposing sides of the sun. That method puts its “eyes” some 300 million km apart. By measuring frequency shifts in stellar spectra and subtle changes in position over time, the spacecraft will be able to get the three-dimensional velocities of stars as well. For space fans fed on a steady diet of stunning Hubble Space Telescope images and news of supermassive black holes and record-setting explosions, stellar mapping might sound a bit stodgy. “People say it’s not a very sexy topic,” admits Gaia mission scientist Timo Prusti, but that’s before they hear about Gaia’s capabilities, he says. Despite decades of effort, he says, precise distance measurements are still a rare thing; fewer than 1000 stellar distances are known with a precision of 1 percent. Gaia will be able to extend parallax measurements out much farther, measuring distances to more than 10 million stars with that same precision. “It’s going to really be a sledgehammer in fundamental astronomy,” Prusti says. Illustration: L-Dopa TWO PERSPECTIVES:Gaia will see the relative positions of stars shift at different points in Earth’s orbit [left]. Gaia’s two telescopes will observe two different patches of sky [right], which will eventually let astronomers estimate absolute distances. Gaia is only the second space mission dedicated to astrometry, the study of stellar positions and velocities. Such measurements were originally made from the ground. But about 50 years ago, improvements in precision began to run up against the limit set by atmospheric turbulence, which causes stellar positions to jitter and jump, says Michael Perryman, coproposer of the Gaia mission. To get above the atmosphere, the European Space Agency launched a dedicated astrometric telescope called Hipparcos in 1989. Hipparcos was fairly rudimentary by modern telescope standards. For example, it was designed before charge-coupled devices (CCDs) were widely available; instead, it relied on a photomultiplier tube, which could measure only one star at a time. Hipparcos was able to measure the parallaxes of nearly 120 000 stars with a precision of about a milliarcsecond, roughly equivalent to the angle subtended by an astronaut standing on the moon as seen from Earth. Gaia’s precision should be some 50 times that, or about what you’d need to see an insect crawling across the lunar surface. Part of the improvement comes from better detector technology. Instead of the photomultiplier tube, Gaia will launch with a bank of 106 CCDs measuring 1 by 0.5 meters, larger than any other focal plane yet sent to space. Each CCD boasts a high probability that an incoming photon will excite an electron and thus generate a signal, making it sensitive to very faint and distant stars. To cover the sky, Gaia will spin once every 6 hours and precess about its axis once every 70 days. Because the spacecraft must accurately pinpoint its orientation at all times, it will maintain the timing of its spin with a precision of just 20 nanoseconds, using an onboard atomic clock. Gaia’s spinning prevents it from operating in point-and-shoot mode. Instead, the CCD will gather up detected charge and pass it down the length of the detector as a star’s image moves across it, in a read-out mode called time-delay integration. This technology has been used by other astronomy missions, including the Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment, to boost signals. Like Hipparcos, Gaia will aim two main telescopes in different directions so that astronomers can get two independent perspectives on stars’ positions relative to one another. After a good deal of analysis, the observations should lead to better estimates of the absolute distances from Gaia to the stars. To save space, the light from the two telescopes will be projected onto the same bank of CCDs. Two “sky mapper” detectors at the edge of the focal plane will be used to tag stars coming from each telescope so they can be followed as they sweep across the rest of the CCDs. Long-term stability will be vital to achieving Gaia’s targeted precision. Even microscopic changes in the size or shape of the mirrors, detectors, and supports can affect the positions of stars on the detector. So Gaia’s designers built those parts from silicon carbide, a ceramic with a much lower coefficient of thermal expansion than the glass and aluminum used in Hipparcos. To reduce thermal fluctuations even further, the observatory will be sent to the second Lagrange, or L2, point, where it will orbit the sun in tandem with Earth and therefore avoid flying in and out of Earth’s shadow. Gaia will also carry a 10-meter-wide sun shield that it will unfold like an umbrella after launch to keep the observatory in permanent shadow, maintaining a constant temperature of –110 °C. Gaia will have to contend with a great deal of obscuring dust in the plane of the Milky Way, which could make it impossible for the spacecraft to directly discern the galaxy’s spiral structure, says Reid of the Center for Astrophysics. Nevertheless, astronomers expect that Gaia’s observations of the visible parts of the sky will reveal more about the galaxy’s structure and history, including the distribution of dark matter, the unseen stuff that helps bind the Milky Way together. In the course of scanning the sky, Gaia should also turn up new planets by finding stars that wobble due to the gravitational tug of orbiting bodies; more of the low-mass, “failed stars” called brown dwarfs that litter the galaxy; and new asteroids, including some that may potentially threaten Earth. The mission is also expected to have a big impact on cosmology by helping better calibrate the “cosmic distance ladder”—the interlinked strategies astrophysicists use to estimate the distance to astrophysical objects. Many of these strategies ultimately depend on parallax, which is the only direct way astronomers have to measure distances in space. One of Gaia’s biggest contributions to this ladder could come from observations of two types of pulsating stars, Cepheids and RR Lyraes. These stars are often called standard candles because the period of their pulsation closely indicates their intrinsic brightness. By comparing that intrinsic brightness with the apparent brightness as seen from this part of the galaxy, astronomers can work out how far away the candle is. But to get a firm calibration of these standard candles, astronomers need an absolute way to measure distance, one that’s independent of the physics of stars. In the last 10 years, a team led by Fritz Benedict of the McDonald Observatory, at the University of Texas at Austin, has used the Hubble’s fine guidance sensor to measure parallaxes for the 10 Cepheids and 5 RR Lyrae stars closest to Earth. Gaia, which will be able to see finer and more distant shifts in stellar position, should be capable of measuring distances to hundreds of Cepheids and thousands of RR Lyraes, Benedict says. That will give astrophysicists a much-improved stick by which to measure how fast the universe is expanding—a basic number that cosmologists need in order to calibrate the properties of distant objects and understand the composition and fate of the universe as a whole. A better measure of the universe’s expansion rate could, for example, help pin down the number of different types of neutrino species present in the universe, says Madore of the Carnegie Observatories. Analysis of radiation dating from just 400 000 years after the big bang, which depends on the expansion rate, suggests there may be four different flavors of the particles, but so far physicists have seen only three. Whatever Gaia’s impact, it will take a good three months for the spacecraft to travel to the Lagrange point and prepare for the start of science operations, and it will take lengthy number crunching to handle the data the craft returns during its five-year mission. Chances are, astronomers won’t get to see Gaia’s final results until 2021 or even later, more than 20 years after the mission was approved. But many astronomers say it will be worth the wait. “It’s been a century of second-guessing what the universe looks like and how bright things are and how far away things are,” says Madore. “This [mission] will put concrete on the ground where before we were just sort of floating on logs on a mass of quicksand.” This article originally appeared in print as “Mapping the Milky Way.”