The Roswellian Aliens in 1945,and many other Aliens that have visited the Earth, and or crash landed on our planet have acheived Hyperspace travel.
That is the only way that they could possibly visit Earth,given the many
light years that their Solar systems are from our solar system.Humans
here on Earth have not yet,acquired the technology,or we do not have the
minerals here on earth to construct such a vehicle or propulsion system.
Remember the Rosewell indestructable metal,you could bend it crush it , and it still would regain its original form!.Yes, hyperspace is possible.