<blockquote><font class="small">In reply to:</font><hr /><p>ISS reboosts do not change its velocity.<p><hr /></p></p></blockquote><br /><br />Yes they do. To raise the orbit of a spacecraft in circular Earth orbit, you need to increase its velocity.<br /><br />Obital mechanics 101:<br /><br />Let's say that ISS is at an altitude of 300 km above Earth's surface. If you increase velocity (you accelerate prograde, which is along the direction of travel) at a point in your orbit (let's call this Point A), this extra speed will cause you to arrive at a higher altitude and slower speed at the point that is on the exact opposide side of Earth (Point B) than you would if you hadn't done the speed change.<br /><br />If you accelerated by just the right amout of speed this will cause you to arrive at point B at, say, 350 km altitude. If you don't do anything more, you will stay in that orbit, with a low point of 300 km and a high point of 350 km. At point A you move faster than Earth's gravity can pull you in (and faster than what you did when you were in a circular 300 km orbit), so you slip away. As your altitude increases and you get closer to point B, your speed goes down. At point B you don't have enough speed to stay at that altitude, so you start falling back in towards Earth.<br /><br />So at point B, you need to do another prograde burn to increase speed again, so that you raise the altitude in point A to 350 km. You then have a circular orbit that is higher than the one you had.<br /><br />Funny thing is that you end up in a higher orbit, which has <i>less forward speed</i> than you had to begin with, despite the fact that you did 2 prograde speed increases. But your <i>total orbital energy</i> has increased since you're now at a higher altitude.<br /><br /><br />This leads to some counter-intuitive effects when you're in orbit. Imagine that you're in the same orbit as ISS, but you're 200 km behind it in its orbit (which at orbital speeds mean that ISS arrives at any point about 25 seconds be <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>