Imagine solar-powered ion-propelled aircraft. Ion propulsion allows flight at high altitudes like 40-50 km. The air is rarefied at high altitudes, that means the aircraft should have high velocity like 1-2 km/s to maintain the altitude. Solar energy allows unlimited range. So, such an aircraft can be long-range and high-speed.
For high velocity such an aircraft should have enough power like 0.2-0.4 watts per gramm of mass.
If it has some 5 watts per gramm, it might be able to reach the space.
