Passage: The engineering problems associated with leaving Earth and developing space propulsion systems have been examined for over a century, and millions of hours of research have been spent on them. In recent years there has been an increase in research on the issue of how humans can survive and work in space for extended and possibly indefinite periods of time. This question requires input from the physical and biological sciences and has now become the greatest challenge (other than funding) facing human space exploration. A fundamental step in overcoming this challenge is trying to understand the effects and impact of long-term space travel on the human body.

Question: How many hours of research has been conducted?
millions of hours