<story><title>LightSail 2 Spacecraft Successfully Demonstrates Flight by Light</title><url>http://www.planetary.org/blogs/jason-davis/lightsail-2-successful-flight-by-light.html</url></story><parent_chain></parent_chain><comment><author>mabbo</author><text>My understanding of what&amp;#x27;s going on here is that the light sail isn&amp;#x27;t aiming or changing at all. It&amp;#x27;s just a big sail, in orbit.&lt;p&gt;When it&amp;#x27;s moving away from the sun, the sail is being pushed &amp;quot;forward&amp;quot;, which adds to it&amp;#x27;s velocity- this increases the apogee on the opposite side of the orbit. When it&amp;#x27;s moving towards the sun, the sail is being pushed &amp;quot;backwards&amp;quot;, lowering it&amp;#x27;s current velocity and reducing the perigee. As the perigee comes lower, it spends less time in that end of the orbit, where it&amp;#x27;s gaining velocity, and when it&amp;#x27;s apogee is high it spends more time in that end of the orbit, where it&amp;#x27;s losing velocity. It&amp;#x27;s not a winning strategy I&amp;#x27;m afraid.&lt;p&gt;But imagine! Imagine if we had a slightly different solar sail. One that could retract and expand at will, via solar powered electric motors. Then we could have it open it&amp;#x27;s sails when the &amp;quot;wind&amp;quot; is favorable, then close them when it&amp;#x27;s not. It could slowly but surely escape earth&amp;#x27;s gravity well, and gracefully travel around the solar system. A big enough one might even be able to accelerate right out of the solar system.</text></comment>
<story><title>LightSail 2 Spacecraft Successfully Demonstrates Flight by Light</title><url>http://www.planetary.org/blogs/jason-davis/lightsail-2-successful-flight-by-light.html</url></story><parent_chain></parent_chain><comment><author>adrianpike</author><text>Can someone more familiar with orbital mechanics help me understand why it looks like the perigree is coming down in opposite to the apogee?&lt;p&gt;I&amp;#x27;m but a mere KSP player, but I&amp;#x27;ve always thought the perigee comes down in a fairly linear fashion based on drag, which is mostly a function of altitude - I would assume they&amp;#x27;d be using the sail to just &amp;quot;burn prograde&amp;quot; whenever the solar angle is possible, raising both the apogee and the perigee, but the apogee coming up way faster?</text></comment>