There is no clear boundary because the atmosphere gradually thins the higher you go.
The lowest artificial satellites can orbit at about 160km (100 miles) above the Earth - but even at this height traces of the atmosphere we breathe are enough to slow them down.
- Space shuttle - 300 km (185 miles)
- International Space Station 350 km (220 miles)
- Passenger aircraft - 9 km (6 miles)