<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>cnity</author><text>An object moving through the sky at a far distance from you is moving through 3D space. It appears to you to move in 2D space because of the fact that the image is being projected onto the back of your eyes, and our eyes aren&amp;#x27;t far enough apart to discern depth beyond a certain distance.&lt;p&gt;That object could appear to turn &amp;quot;90 degrees&amp;quot; in _two dimensions_ when actually it changes course by a smaller angle in 3D space.&lt;p&gt;Imagine a satellite is heading straight for you. It would appear to not be moving at all. Now imagine it is moving toward but slightly below you. It will now appear to be moving slowly &amp;quot;straight down&amp;quot; from your perspective, but actually, it could be moving towards you at high speed.&lt;p&gt;Now imagine there is a small impact with the satellite that causes it to bump up and right slightly from its current trajectory. It might now suddenly look like it&amp;#x27;s moving entirely rightwards, from your perspective, but it is still moving towards you.&lt;p&gt;This is hard to explain without visualisation, but completely explains all of these anecdotes about sudden direction changes.</text><parent_chain><item><author>daveslash</author><text>This is bit of an off-topic question, and I realize that it might sound a bit &lt;i&gt;&amp;quot;out there&amp;quot;&lt;/i&gt;, but I&amp;#x27;m compelled to ask and I&amp;#x27;ll take the down-votes that I deserve:&lt;p&gt;Twice in my life I have seen what I believed to be a satellite make a sudden and abrupt (near instantaneous) 90-degree turn. I realize this is impossible [probably?], which is why I ask. Once was in 1992 - and I spent nearly 30 years telling myself I mis-remembered. And the second time was in 2018 while backpacking with my wife - she was a second witness and confirmed that we saw the same thing. Both times the turn occurred when directly over-head.&lt;p&gt;When casually stargazing, I don&amp;#x27;t spot the satellites when they first pop up on the horizon. I usually spot them around 30-45 degrees above the horizon. One way to differentiate between an aircraft and a satellite is the blinking, but that&amp;#x27;s not always reliable - so a more reliable way is to note how quickly the &amp;quot;perceived&amp;quot; speed changes as it moves from 30-degrees towards the azimuth. The larger the change in perceived speed, the lower the altitude, while constant speed indicates orbit.&lt;p&gt;In both cases, I was casually looking at the sky - I had no measurement or recording tools. In both cases I took mental note of the speed when I first spotted it and how it changed (or didn&amp;#x27;t) as it moved. I mentally noted that that both appeared to be a satellite. Obviously, a 90 degree turn is impossible (?), which leads me to guess that I may have seen an extremely high altitude, high speed, aircraft make a near 90 degree turn while reflecting sunlight shortly after sunset. This also seems to be a stretch.&lt;p&gt;I hate to ask such far-fetched &amp;quot;I saw...&amp;quot; questions here, but it&amp;#x27;s been driving me nuts since 1992. All attempts at research have been fruitless. I thought maybe someone here could help?</text></item><item><author>modeless</author><text>Many people don&amp;#x27;t know that you can actually see some of these satellites in the sky as they pass overhead, without a telescope or anything. I made a site to help catch them, which has been on HN before: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;james.darpinian.com&amp;#x2F;satellites&amp;#x2F;&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;james.darpinian.com&amp;#x2F;satellites&amp;#x2F;&lt;/a&gt;</text></item></parent_chain></comment><story><title>Low Earth Orbit Visualization</title><url>https://platform.leolabs.space/visualization</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>andbberger</author><text>First thought is two separate satellites whose orbits appeared to intersect at the same time they faded&amp;#x2F;became visible respectively.&lt;p&gt;Any abrupt change in heading is indeed pretty much impossible. A 90 degree plane change would require a staggering amount of delta-v.</text><parent_chain><item><author>daveslash</author><text>This is bit of an off-topic question, and I realize that it might sound a bit &lt;i&gt;&amp;quot;out there&amp;quot;&lt;/i&gt;, but I&amp;#x27;m compelled to ask and I&amp;#x27;ll take the down-votes that I deserve:&lt;p&gt;Twice in my life I have seen what I believed to be a satellite make a sudden and abrupt (near instantaneous) 90-degree turn. I realize this is impossible [probably?], which is why I ask. Once was in 1992 - and I spent nearly 30 years telling myself I mis-remembered. And the second time was in 2018 while backpacking with my wife - she was a second witness and confirmed that we saw the same thing. Both times the turn occurred when directly over-head.&lt;p&gt;When casually stargazing, I don&amp;#x27;t spot the satellites when they first pop up on the horizon. I usually spot them around 30-45 degrees above the horizon. One way to differentiate between an aircraft and a satellite is the blinking, but that&amp;#x27;s not always reliable - so a more reliable way is to note how quickly the &amp;quot;perceived&amp;quot; speed changes as it moves from 30-degrees towards the azimuth. The larger the change in perceived speed, the lower the altitude, while constant speed indicates orbit.&lt;p&gt;In both cases, I was casually looking at the sky - I had no measurement or recording tools. In both cases I took mental note of the speed when I first spotted it and how it changed (or didn&amp;#x27;t) as it moved. I mentally noted that that both appeared to be a satellite. Obviously, a 90 degree turn is impossible (?), which leads me to guess that I may have seen an extremely high altitude, high speed, aircraft make a near 90 degree turn while reflecting sunlight shortly after sunset. This also seems to be a stretch.&lt;p&gt;I hate to ask such far-fetched &amp;quot;I saw...&amp;quot; questions here, but it&amp;#x27;s been driving me nuts since 1992. All attempts at research have been fruitless. I thought maybe someone here could help?</text></item><item><author>modeless</author><text>Many people don&amp;#x27;t know that you can actually see some of these satellites in the sky as they pass overhead, without a telescope or anything. I made a site to help catch them, which has been on HN before: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;james.darpinian.com&amp;#x2F;satellites&amp;#x2F;&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;james.darpinian.com&amp;#x2F;satellites&amp;#x2F;&lt;/a&gt;</text></item></parent_chain></comment><story><title>Low Earth Orbit Visualization</title><url>https://platform.leolabs.space/visualization</url></story>