According to Newton's Universal Law of Gravitation, every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Even in 'gravity-free space' (implying negligible external gravitational fields from planets or stars), the two astronauts possess mass and therefore exert a mutual gravitational force of attraction on each other. Since there are no external forces to counteract this, they will accelerate towards one another.