Grueling Weather Watch

Getting this far took years of planning and cooperation between international partners at observatories stretching from the tallest mountain in Hawaii to the frozen terrain of the South Pole. This electronically linked network of eight observatories created a virtual telescope dish as wide as the planet.

Astronomers observe these black holes in millimeter radio waves, the wavelength band at which light can penetrate the dense concentrations of gas and dust at the center of the galaxy and travel relatively unimpeded to Earth.

But water absorbs and emits radio waves, which means precipitation confounds observations.

To minimize this problem, radio telescopes are placed at high altitudes—the tops of mountains or high desert plateaus—but incoming clouds, rain, or snow can still take an observatory offline. Buffeting winds at these high altitudes can also shut down a telescope.

“The probability of having really good weather at every site is almost zero,” says Fish.

With only five nights available during the observing window, Fish and his colleagues met daily to make the nerve-wracking decision whether to activate the network, juggling information about current weather conditions at each site and how those conditions might change over the coming days. From the MIT facility, Fish was constantly monitoring the weather at each site on one screen and communicating with astronomers on another.

“It’s a heartbreaker if you fire off a night and [bad] weather closes in,” or if observations are cancelled on what turns out be a good night, says Shep Doeleman, director of the Event Horizon Telescope at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Hoping for Peanuts

Now that all five days’ worth of observations have been completed, astronomers have a long wait—and months of analysis—to find out if they’ve produced a black hole portrait.

Each observatory records so much data that it can’t be transmitted electronically. Instead, the information from all the telescopes—equivalent to the storage capacity of ten thousand laptops—has been recorded on 1,024 hard drives. The drives must be mailed to the Event Horizon Telescope’s processing centers at MIT Haystack and the Max Planck Institute for Radio Astronomy in Bonn, Germany.

The hard drives from the South Pole telescope can’t be flown out until the end of the winter season there at the end of October.

Once the data reach each processing center, a stack of servers will perform the all-important task of combining the time-stamped signals from the eight observatories. Comparing and combining the radio waves must be done with extraordinary care, so that critical information about the size and structure of the event horizon is not lost when they are added together.

The technique of combining radio waves, known as very long baseline interferometry, is common enough in radio astronomy. But usually the telescopes are not so numerous nor spread out over such a large area.