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As Viking emerged from its conjunction with the sun, a team headed by Physicist Irwin Shapiro of Massachusetts Institute of Technology measured the time it took for radio signals to make the round trip between the earth and the Viking orbiters, 400 million kilometers (248.5 million miles) away. The scientists were making the most accurate tests yet of one of the tenets of Einstein's theory of relativity, which holds that radio waves passing close to a massive body like the sun should be slowed down by its gravitational field. The signals to and from the Viking orbiters have further strengthened Einstein's case. Their round trip required 200 microseconds (200 millionths of a second) longer than they would have taken had they not been affected by the sun's gravity.