Angular size of the earth should be about 2 degrees. That is about nothing compared to the beam width of a 15 dB antenna. So space noise is definitely a factor

Angular size of the earth should be about 2 degrees. That is about nothing compared to the beam width of a 15 dB antenna. So space noise is definitely a factor here. However they made it work, and could have used an antenna with much more gain (think voyager dish) if necessary. Probably the idea behind the experiment is to track rovers on the surface where the antenna is only roughly pointed at earth - without the complexity and energy budget coming with a rotator.