The difficulty is how hard the game is by setting the scale of the planets to a larger, more realistic scale. There are three difficulty options in the game, and one of them is exclusive to the PC version. The first difficulty is the normal difficulty, which is the solar system with a 1:20 scale. That means the planets are 20 times smaller than in real life. Most players play in this difficulty because it is easier for their rockets to reach other celestial bodies quickly. The second difficulty is the hard difficulty, where the solar system is twice as large as the normal mode scale, or at 1:10 scale. This scale is similar to the one used in the game Kerbal Space Program. Finally, there is the realistic difficulty, where the planets are in their realistic size as in real life. This difficulty is only exclusive to PC players. For planet atmospheres, the scale is increased by a factor of 1.67× in hard difficulty, and 3.33× in realistic difficulty, in default. You can change how large the planets appear in different difficulties in the planet editor.
The table of sizes of planets in each difficulty is listed below.
Planet radius in each difficulty (in km)
Planet
Normal
Hard
Realistic
Sun
34785
69570
695700
Mercury
122
244
2440
Venus
302.6
605.2
6051.8
Earth
318.6
637.1
6371
Moon
86.9
173.7
1737.4
Captured Asteroid
0.1
0.1
0.1
Mars
169.5
339
3389.5
Phobos
1.1
2.3
11.3
Deimos
0.6
1.2
6.2
Jupiter
~3500
6991.1
69911
Io
91.1
182.2
1821.6
Europa
78
156.1
1560.8
Ganymede
131.7
263.4
2634.1
Callisto
120.5
241
2410.3
Planet semi-major axis in each difficulty (in Gm)
Planet
Normal
Hard
Realistic
Sun
—
—
—
Mercury
2.895
4.629
46.297
Venus
5.41
10.821
108.209
Earth
7.48
14.96
149.598
Moon
0.0192
0.0384
0.384
Captured Asteroid
0.0372
0.0744
0.744
Mars
11.4
22.8
228
Phobos
0.000469
0.000938
0.00938
Deimos
0.00117
0.00232
0.0232
Jupiter
38.9
77.8
778
Io
0.0168
0.0422
0.422
Europa
0.0268
0.0671
0.671
Ganymede
0.0428
0.107
1.07
Callisto
0.0753
0.188
1.883
Planet atmospheric height in each difficulty (in km)