The Robert C. Byrd Green Bank Telescope listens at radio wavelengths - frequencies between 290 megahertz and 115 gigahertz, from VHF up into the millimeter-wave band. Its 100-meter-diameter dish has a collecting area of 2.3 acres - more than two football fields. The dish can be steered to point at any object in the sky above 5 degrees elevation, covering 85 percent of the local celestial hemisphere. Unlike most radio telescopes, the GBT's primary reflector is an off-axis segment of a paraboloid - the same offset design used in home satellite TV dishes, just enormously scaled up. The asymmetric reflector means the focal point and feed horn sit off to one side rather than in front of the dish, so no support structure blocks incoming radio waves. The result is an unobstructed aperture that captures the faintest possible signals.

The dish is not a single rigid piece. It is made of 2,004 individual aluminum panels, each polished to a surface accuracy better than 50 micrometers RMS - about half the thickness of a human hair. Underneath each panel, 2,209 actuators - small computer-controlled motors - constantly adjust the panel positions to compensate for the dish's tendency to sag under its own weight as it points in different directions. Without this so-called active surface, observations above 4 GHz would not be possible: the dish would deform too much to focus high-frequency signals. The actuators run continuously while the telescope is operating, making thousands of tiny adjustments per second. Sixteen million pounds of steel and aluminum bend, sag, and twist according to known physics, and the active surface system keeps the receiving geometry effectively rigid by counter-bending in real time.

The GBT began regular science operations in 2001 and has accumulated a remarkable discovery list. In 2002, astronomers detected three new millisecond pulsars in the globular cluster Messier 62. In 2006, the telescope mapped a coil-shaped magnetic field in the Orion molecular cloud and identified the Ophiuchus Superbubble - a giant hydrogen gas structure 23,000 light-years away. In 2019, the GBT helped detect PSR J0740+6620, at that time the most massive neutron star ever observed - just barely under the theoretical maximum mass before a neutron star would collapse into a black hole. Since 2004, 28 new complex molecules have been identified in the interstellar medium using the GBT - building blocks of organic chemistry floating in interstellar space, where most astronomers had not expected them. The telescope is used for about 6,500 hours of astronomy a year out of 8,760 hours total.

The GBT is one of the principal facilities of the Breakthrough Listen project, a 10-year, $100 million SETI effort funded by Russian billionaire Yuri Milner since 2015. Breakthrough Listen uses the GBT to scan nearby stars, galaxies, and other targets for any radio signals that might indicate the presence of an extraterrestrial technology. In late 2017, the project pointed the GBT at the interstellar object 'Oumuamua as it passed through the solar system - a cigar-shaped chunk of rock or ice that had clearly originated from outside our system, and that some scientists had speculated might be artificial. The scan turned up no signals. 'Oumuamua appears to be a natural object, however weird its trajectory. The Project Ozma legacy continues at Green Bank, just on the world's largest steerable dish instead of an 85-foot one.

The GBT nearly closed. In August 2012, an NSF portfolio review committee chaired by Daniel Eisenstein of Harvard recommended defunding the Green Bank Telescope over a five-year period as part of a broader reallocation of astronomy budgets. The recommendation set off a years-long political and scientific scramble to find alternative funding. In July 2014, Congress approved an NSF budget that did not include divestment that year, buying time. The facility began seeking partners to help cover its roughly $10 million annual operating costs. On October 1, 2016, the National Radio Astronomy Observatory at Green Bank formally separated from the NSF and reconstituted itself as the independent Green Bank Observatory, with mixed federal and private funding. The model has worked. The Great Big Thing is still pointing at the sky.