NASA and other space agencies are preparing for some down time next week as communications with the planet Mars are cut off or seriously degraded. On July 27 at 01:17 GMT, the Red Planet will be in conjunction with the Sun. This means that between July 22 to August 1 there will be so much radio interference from the solar corona that it will not be possible to safely transmit orders to any of the Mars spacecraft.
There are currently eight missions from the United States, Europe, and India operating in orbit or on the surface of Mars and they keep in touch with their various mission controls through tightly beamed radio signals designed to convey commands and data, as well as a tracking signal for the spacecrafts' antennae to lock onto.
Maintaining communications between Earth and Mars over distances of hundreds of millions of miles is hard enough, but when the orbital positions of the two planets places them on opposite sides of the Sun, things get very dicey.
The Sun is by far the most powerful radio source in the Solar System as well as being extremely hot and bright. This means that there is a danger that a spacecraft will lock onto the Sun during the conjunction instead of the tracking signal, causing communications to be lost entirely. Even if they are maintained, it's a bit like two people on opposite sides of giant arc lamp trying to talk to each other by flashing Morse code from a couple of pen lights.
Even though Mars is only in conjunction for a brief time, the Sun's corona extends so far into space that its hot ionized plasma can still seriously interfere with signals for days before and after the event. The problem for NASA and the other space agencies is how to handle this period of interference. Since this is a situation that has occurred every 26 months since Mariner 9 went into orbit around Mars in 1971, the standard procedure is to partly suspend communications from Earth.
During this time, the orbiters and landers are put into autonomous mode, carrying out only basic duties while sending telemetry data and status reports back to Earth. Meanwhile, mission control plays it very safe by not sending any commands that might be misinterpreted by the spacecraft computers and cause it to do something stupid like drive off a cliff or shut down its power systems.
"The vehicles will stay active, carrying out commands sent in advance," says Mars Program Chief Engineer Hoppy Price of the Jet Propulsion Laboratory (JPL) in Pasadena, California. "Orbiters will be making their science observations and transmitting data. The rovers won't be driving, but observations and measurements will continue."
The video below explains the Mars solar conjunction.