It’s going to be far simpler to explore the Solar System with humans (and with robots) by starting from the Moon.
What is in fact at present the minimum requirement to reach orbit?
On Earth: Atlas LV-3B / Mercury (the one used in the John Glenn’s launch below)
Total Mass: 116,100 kg (255,900 lb)
Diameter: 3.05 m (10.00 ft)
Length: 25.00 m (82.00 ft)
On the Moon: Apollo Lunar Module Ascent Stage
Mass: 4,670 kg (10,300 lb)
Diameter: 4.2 m (13.78 ft)
Length: 3.76 m (12.34 ft)