*resolved* thank you -- unsolvable problem

[MrPolite]

my friend gave this to me, I cant think of a possible way to solve this:

A space traveler named Rex is dropped off by his team at the bank of a river on a strange planet that has only one river. The river is completely straight and is very long. Rex begins to explore the planet in his dune buggy by driving in a straight line away from the river. But alas, his journey is abruptly ended when a tornado hits his dune buggy, spinning him around and knocking him unconscious so that he has forgotten which direction he was traveling.
Rex's dilemma: Rex does not know how to get back to the river so that his team can pick him up. What he does know is that he has traveled 1000 miles, he has enough gas in his tank to travel an additional 6400 miles. He has an accurate compass and an accurate odometer. He can travel in any direction he desires. What strategy should he employ to make sure that he can get back to the river?
(Any point on the river is OK)

Note: Rex is driving perpendicular to the river.

I though of a circle with radious 1000mi... so you can walk 1000mi and then drive in a circle with a radius of 1000mi, but that would be more than 6400mi

I cant solve this!!!
see if you can