Solar conjunction is the period when Earth and Mars, in their eternal march around the Sun, are obscured from each other by the fiery orb of the Sun itself. Like dancers on either side of a huge bonfire, the two planets are temporarily invisible to each other.
During this period the daily communication between antennas here on Earth and those on spacecraft at Mars stops for a few weeks. Sun emits hot, ionized gas from its corona, which stretches out far into space. During solar conjunction, this gas can meddle with radio signals when engineers attempt to speak with spacecraft at Mars, undermining directions and bringing about unforeseen conduct from our deep space explorers.
To be safe, engineers hold off on sending commands when Mars disappears far enough behind the Sun’s corona that there’s increased risk of radio interference.
Solar conjunction occurs every two years. This year, the solar conjunction moratorium on commanding all Mars spacecraft is between Aug. 28 and Sept. 7, 2019, when Mars is within 2 degrees of the Sun.