The best friend mission is rather simple and stands alone in its group in Solar 2. Your best friend asteroid buddy requires your help in becoming a Small Planet. You, also as an asteroid, will need to push the target asteroid around into other rocks to make it grow larger while protecting it from hazards.
Tips for the Best Friend level:
Patience is important here in maintaining relative control of your buddy asteroid by keeping it traveling only fast enough to consume more mass. If you start rocketing it across the map, the chances a Nomad or physical object will destroy it are very high.
Remember, just like your asteroid, it can be destroyed relatively easily and only takes one stray shot to cause a mission restart. To best avoid other hazards, stick to a specific asteroid field as long as possible, avoiding traveling significant distances.