The researchers, piecing through satellite photos of the area around Shahrud, found, in a crater a few kilometers away, what they say were two telltale ground scars. They were larger than those at General Moghaddam’s publicly known facility.

The scars were recent. One appeared in 2016, the other in June 2017.

The researchers scrutinized the test stands. Such structures typically weigh between four and six times the thrust of the engine being tested. And they are concrete, allowing their weight to be inferred from their dimensions.

The researchers say Shahrud’s 2017 test used a stand estimated to be 370 tons, suggesting the engine powered between 62 and 93 tons of thrust — enough for an intercontinental ballistic missile. Two as-yet-unused test stands are even larger.

Hidden Activity

There were other hints. Shahrud appears to house three pits of the sort used for casting or curing rocket components, the researchers say. One pit, at 5.5 meters in diameter, is far larger than those used for Iran’s medium-range missiles.

The researchers confirmed that the facility remains active by using a new type of satellite imagery known as synthetic-aperture radar. By firing radio waves and measuring their echo, the satellite reveals greater detail than a photograph. Because of how it stores data, it can track minute changes between two sets of images, such as dirt kicked up by someone walking between buildings.

“We can see human traffic, human activity that isn’t visible on your traditional satellite,” said David Schmerler, one of the California-based researchers. “They’ve been driving all over the crater where the engine tests are done.”