\n<\/p>\n
This observatory has probably been the most transformative astronomy project of the 21st century, but there\u2019s a good chance you\u2019ve never heard of it. Just last week, for instance, the Hayden Planetarium at the American Museum of Natural History (AMNH) in New York City debuted a new \u201cspace show\u201d called Encounters in the Milky Way\u2014and this often overlooked spacecraft is its scientific superstar. But you\u2019re more likely to know about actor Pedro Pascal\u2019s narration in the show than you are to be familiar with the single space mission that serves as the presentation\u2019s backbone.<\/p>\n
The observatory is called Gaia. And, like so many good things, you wouldn\u2019t really miss it until it\u2019s gone\u2014and now it is.<\/p>\n
Launched in 2013 by the European Space Agency (ESA), it ceased operations this past March, when it used what little fuel it had left to steer into a graveyard orbit around the sun. From its station in a quiescent region of deep space more than 1.6 million kilometers from Earth, Gaia\u2019s mission was, in essence, quite simple: it was designed to give us a better sense of where we are\u2014a celestial \u201creference frame\u201d on overlapping interplanetary, interstellar and intergalactic scales. To do that, it used twin sky-sweeping telescopes and three instruments, including a billion-pixel camera, to painstakingly measure the distances, positions, motions, and more of about two billion celestial objects, most of them stars in our own galaxy. It made some three trillion observations in all, producing (among many other things) the largest, most precise three-dimensional map of the Milky Way ever made.<\/p>\n
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\u201cGaia was our best galactic cartographer, and I sometimes say that Encounters in the Milky Way is my love letter to it,\u201d says Jackie Faherty, a senior research scientist at the AMNH, who curated the new space show and regularly works with Gaia data. \u201cIt turns out you can learn a lot by determining where and how far off the stars are from you\u2014and especially by how they are moving…. Gaia\u2019s creation of this map is something we all should celebrate because it\u2019s just as iconic and useful as the maps of Earth we all see in school or pull up on Google. Looking at it, you can find and explore all sorts of different things you want to know.\u201d<\/p>\n
From Gaia\u2019s map, more than 13,000 peer-reviewed studies have already emerged, and many have concerned the fundamental structure and deep history of the Milky Way. Thanks to Gaia, scientists now can better gauge the amount of dark matter within our galaxy and have been able to track the Milky Way\u2019s growth and evolution across eons via relic streams of stars strewn from ancient mergers with other, smaller galaxies.<\/p>\n
\u201cStars retain memories of their origins in their ages, motions and chemical compositions\u2014all of which Gaia measured,\u201d says Amina Helmi, an astronomer at the Kapteyn Astronomical Institute in the Netherlands. She and her colleagues used the mission\u2019s data to discover evidence of a major galactic merger that, some 10 billion years ago, shaped our home galaxy into the Milky Way we know today.<\/p>\n
\u201cWith all that information, it was like a veil being lifted,\u201d Helmi says. \u201cWe could suddenly perform what\u2019s sometimes called \u2018galactic archaeology,\u2019 reconstructing the Milky Way\u2019s history to see when and how this merger happened with another, smaller galaxy that was about a third to a quarter of our galaxy\u2019s mass…. Gaia allows us to look billions of years into the Milky Way\u2019s history\u2014before our solar system even formed\u2014to see what actually happened back then, which is absolutely amazing.\u201d<\/p>\n
Tracing perturbations from one more recent and ongoing merger, astronomers have even managed to reveal an apparent warp in the Milky Way\u2019s disk, offering a new twist\u2014literally\u2014on the classic image of our cosmic home. At smaller scales, the spacecraft has refined the orbits of more than 150,000 asteroids, surveilling hundreds of them to see if they have their own moons. It has spied hints of thousands of worlds and even a few black holes orbiting other stars. At larger scales, it has helped estimate the expansion rate of the universe, and it has also teased out the subtle tugging of the Milky Way\u2019s heart upon the solar system across tens of thousands of light-years.<\/p>\n
Based on Gaia\u2019s data, this artist\u2019s impression shows our Milky Way galaxy from its side, highlighting an apparent warp in the galaxy\u2019s starry disk.<\/p>\n
Gaia\u2019s sprawling cosmic reckoning is now a cornerstone for most state-of-the-art Earth- and space-based telescopes, which rely on the mission\u2019s target-dense celestial map to orient and calibrate their own observations and operations. Whether it\u2019s NASA\u2019s James Webb Space Telescope, ESA\u2019s Euclid mission, the ground-based, U.S.-built Vera C. Rubin Observatory or Europe\u2019s under-construction Extremely Large Telescope, practically all of the world\u2019s most exciting starlight-gathering telescopes will, in some sense, be guided by Gaia.<\/p>\n
And stunningly, the best is yet to come. More than two thirds of the mission\u2019s treasure trove of data is still under wraps. It is being prepared in a time-consuming process for two major upcoming milestones: about half of Gaia\u2019s total data are targeted for release next year, and the mission\u2019s full data are set to arrive no earlier than 2030.<\/p>\n
But because it didn\u2019t beam back images ready-made for lush wall posters and desktop backgrounds, Gaia was destined from the start to be \u201ccriminally under-recognized outside astronomy,\u201d says Mark McCaughrean, an astronomer and former senior adviser to ESA. \u201cAnd because Gaia provided utterly essential, if mundane, information such as precise stellar distances, it\u2019s been doomed with this curse of simultaneous ubiquity and obscurity as many people use its data but take it for granted as just \u2018coming from a catalog.\u2019\u201d<\/p>\n
Anthony Brown, an astronomer at Leiden University in the Netherlands, who leads the mission\u2019s data processing and analysis group, puts it most succinctly: \u201cFor astronomers, Gaia has become almost like the air you breathe,\u201d he says.<\/p>\n
At the heart of Gaia\u2019s mapmaking is a technique called astrometry, the measurement of celestial positions and motions in the plane of the sky. Paired with a phenomenon called parallax\u2014the apparent shift of an object\u2019s position when viewed from two vantage points\u2014astronomers can use Gaia for determining distances, too. You can see the parallax effect with your own two eyes: hold your thumb out at arm\u2019s length and watch as it appears to jump around as you blink one eye and then the other. The closer the object is, the bigger its displacement will be. And the bigger your baseline is between two vantage points, the smaller the displacement will be that you can discern. Your eyes have a baseline of about six centimeters; Gaia\u2019s was 300 million kilometers, set by the opposite sides of Earth\u2019s orbit around the sun.<\/p>\n
A Gaia predecessor, ESA\u2019s Hipparcos mission, used that same gigantic baseline to survey the sky from 1989 until it ran out of fuel in 1993. But the technology of the time limited Hipparcos\u2019s astrometric reckoning to a precision of about one milliarcsecond, with high-quality measurements only for about 100,000 objects within about 200 parsecs (650 light-years) of the solar system. (A single arc second is a very small angular slice of the heavens, making Hipparcos\u2019s milliarcsecond precision all the more noteworthy. The moon, for instance, takes up about 1,800 arc seconds in Earth\u2019s sky.)<\/p>\n
As impressive as Hipparcos was, Gaia shattered the records set by its precursor\u2014although not without challenges, such as precision-threatening sprays of stray light that leaked around the edges of the spacecraft\u2019s sun shield and through a hole punched by an errant micrometeoroid. But ultimately, Brown says, Gaia\u2019s measurements achieved on the order of 100 times greater precision\u2014reaching about 10 microarcseconds. And within the Milky Way, the spacecraft\u2019s view encompassed 100 times more volume and included 10 times more targets.<\/p>\n
This map of the entire sky is based on Gaia\u2019s data for the positions, brightness and color of more than 1.8 billion stars.<\/p>\n
ESA\/Gaia\/DPAC; CC BY-SA 3.0 IGO; Acknowledgement: A. Moitinho (CC BY-SA 3.0 IGO)<\/p>\n
The numbers underpinning Gaia are so alien to everyday experience that they border on nonsensical, says Michael Perryman, a former ESA researcher, who has served as project scientist for Hipparcos and Gaia and played a crucial developmental role for both missions. He likens Hipparcos\u2019s precision to discerning a second\u2019s worth of growth of a human hair from a distance of one meter. Gaia\u2019s 100-times-better view, he says, is more like measuring the width of a single hydrogen atom from the same distance.<\/p>\n
Another comparison involves the size of the two missions\u2019 datasets. When the Hipparcos team printed out its complete catalog, Perryman recalls, it comprised five thick volumes\u2014almost enough to fill a single shelf of a bookcase. Printing out the full Gaia catalog with the same density of information per page, he says, would require about 10 kilometers of shelf space.<\/p>\n
\u201cThe mind boggles,\u201d he says. \u201cIt\u2019s almost incomprehensible; these are numbers and dimensions we\u2019re simply not equipped to visualize, so even the analogies are very difficult to grasp.\u201d<\/p>\n
The best example of the heights such precision can reach may be Gaia\u2019s tour de force determination of the solar system\u2019s acceleration with respect to a vast, sky-encompassing field of quasars. Quasars are the conspicuously bright cores of remote galaxies that harbor actively feeding supermassive black holes. As such, quasars are among the most powerful beacons astronomers can use to probe distant regions of the universe. Gaia pinpointed the positions of more than one and a half million of them to establish a fixed backdrop of sorts, against which various minuscule motions of our solar system or other nearby celestial objects could be seen.<\/p>\n
One motion Gaia managed to measure was an astonishingly small acceleration of just 0.232 nanometer per second squared\u2014a continuous atom-scale deflection in the solar system\u2019s 220-kilometer-per-second trajectory through the Milky Way, attributed to the gravitational pull from our galaxy\u2019s center some 26,000 light-years away. Writ large, the displacement adds up to less than a meter per day\u2014and essentially reflects the real-time sculpting of our galactic orbit as the solar system carves a path through the Milky Way\u2019s gravitational field.<\/p>\n
\u201cIt\u2019s an almost circular motion around the galactic center, and it\u2019s directed toward the supermassive black hole there,\u201d says astronomer Sergei Klioner of Germany\u2019s Dresden University of Technology, who led much of the work behind the measurement. \u201cNo other observational data could come anywhere close to competing with Gaia here…. You often hear the term \u2018astronomical\u2019 in the sense of something being very large\u2014but this is an example where Gaia has shown us something that\u2019s astronomically small.\u201d<\/p>\n
Now that Gaia has gone dark, there\u2019s already talk of what comes next. \u201cDo we really need another astrometry mission?\u201d asks Brown, who first began working on Gaia in 1997. \u201cWell, not immediately, but the extremely precise stellar reference frame it gave us\u2014upon which many other observatories depend\u2014will eventually deteriorate because all the stars are moving, right?\u201d ESA is envisioning a follow-on mission, which would potential launching in the 2040s. This time that mission would be optimized for infrared observations to allow astronomers to see through the dust that otherwise clouds their view of the Milky Way\u2019s star-packed disk and galactic center.<\/p>\n
\u201cIt\u2019s, in a way, wonderful but also a bit sad that people take Gaia for granted because, my God, it was a tough mission,\u201d Perryman reflects. \u201cI don\u2019t feel sadness that it\u2019s gone; I\u2019m just delighted and relieved it lasted so long, and I\u2019m very conscious of how remarkable it is that we live in a time when society is willing to pool its resources to support such things, and we have the technology in place to do them. I hope this period continues\u2014but I worry we\u2019ve been taking that for granted, too.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"
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