When a car drives through a mountain tunnel, the car radio might stop picking up broadcasts because the mountain on top of the tunnel obstructs radio waves. Once every year or so, the Cassini spacecraft has the same problem. But instead of a mere mountain in the way, Cassini’s obstacle is a yellow dwarf star — our sun.
Cassini has orbited Saturn since 2004 and by the time the spacecraft burns up in Saturn’s atmosphere in September 2017, it will have orbited Saturn for more than 12 Earth years. For just a couple days in each of those years, Earth is in a segment of its orbit in which our planet, the sun and Saturn line up, with the sun in the middle — an event called a conjunction.
Even when the sun doesn’t quite block Earth’s view of Saturn (or the Cassini spacecraft orbiting it), the sun’s churning plasma and solar wind degrade radio signals if they pass too near the sun en route to or from Saturn.
Because of the conjunction, no critical communications will travel between Earth and the spacecraft from Dec. 8 to Dec. 12 of 2016.
“The spacecraft is commanded to do nothing but stare at Earth,” said Dave Doody, leader of the Cassini mission’s real-time operations team at NASA’s Jet Propulsion Laboratory. “We will be sending ‘empty’ commands,” Doody said. The engineers basically tell the spacecraft to do nothing. “Just to see how many the spacecraft rejects,” he said. The results of that experiment can help future missions to better understand how the sun interferes with spacecraft communications.
But conjunction is the exception. Except when communication is blocked by the sun or a planet, NASA’s spacecraft throughout the solar system successfully communicate with Earth, no matter how distant they are, or in what direction. It’s an achievement made possible by balancing the small with the enormous in NASA’s Deep Space Network.