Event Horizon Telescope links eight observatories to image M87 black hole

September 4, 2026 at 05:00 PM3 min

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The Event Horizon Telescope collaboration synchronized eight radio observatories across four continents to function as a single Earth-sized instrument. The array recorded about 5 petabytes of data during an April 2017 observing run targeting the supermassive black hole in galaxy M87. Data from the South Pole Telescope remained in Antarctica for eight months because the station closes to flights from February through October.

Key Facts

The Event Horizon Telescope linked eight radio observatories on four continents using very-long-baseline interferometry, a technique first demonstrated in 1967.
The target was the supermassive black hole at the center of galaxy M87, about 55 million light years away and roughly 6.5 billion times the mass of the sun.
During the April 2017 observing run, each telescope recorded about 350 terabytes of raw data per day, totaling about 5 petabytes.
University of Arizona astronomer Dan Marrone described the data volume as 'more than half a ton of hard drives' and said no internet could compete with shipping the drives by plane.
Hard drives from the South Pole Telescope could not leave Antarctica until the next cargo flight because the station closes to all flights from February through October.

The Virtual Telescope

No single radio dish on Earth is large enough to resolve a black hole's shadow, so the Event Horizon Telescope collaboration linked eight existing observatories on four continents into one virtual instrument roughly the diameter of the planet. The technique, very-long-baseline interferometry, was first demonstrated in 1967 and treats widely separated radio dishes as a single giant telescope. The Event Horizon Telescope pushed that fifty-year-old method to its current limits by adding enough telescopes, at high enough frequencies, with clocks precise enough to resolve a target as small and distant as a black hole's shadow.

Target and Observing Conditions

The team chose galaxy M87, about 55 million light years away, whose central supermassive black hole is roughly 6.5 billion times the mass of the sun. A black hole of that mass has an event horizon large enough, relative to its distance from Earth, to be resolvable in theory if enough telescopes could be synchronized precisely enough to act as one. Radio telescopes at the needed frequencies are sensitive to atmospheric water vapor, so the collaboration required clear, dry conditions simultaneously at facilities in Hawaii, Arizona, Mexico, Chile, Spain, and Antarctica during a narrow observing window in April 2017. Eight observatories ultimately took part, each recording independently rather than transmitting live data.

Data Logistics

Each telescope logged roughly 350 terabytes of raw data per day, and by the end of the observing run the collaboration had recorded about 5 petabytes total. University of Arizona astronomer Dan Marrone, who helped manage the data recording, said the volume 'amounts to more than half a ton of hard drives.' Marrone added, 'There's no internet that can compete with 5 petabytes of data on a plane,' explaining why the drives were shipped rather than transmitted. Seven of the eight sites moved their hard drives quickly, but the South Pole Telescope's drives remained in cold storage because Antarctica's research station closes to all incoming and outgoing flights from February through October.