Comment by daveslash | Hacker News Reader

Comment by daveslash | Hacker News Reader

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This is bit of an off-topic question, and I realize that it might sound a bit "out there", but I'm compelled to ask and I'll take the down-votes that I deserve:

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.

When casually stargazing, I don'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's not always reliable - so a more reliable way is to note how quickly the "perceived" 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.

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'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.

I hate to ask such far-fetched "I saw..." questions here, but it's been driving me nuts since 1992. All attempts at research have been fruitless. I thought maybe someone here could help?

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't far enough apart to discern depth beyond a certain distance.

That object could appear to turn "90 degrees" in _two dimensions_ when actually it changes course by a smaller angle in 3D space.

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 "straight down" from your perspective, but actually, it could be moving towards you at high speed.

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's moving entirely rightwards, from your perspective, but it is still moving towards you.

This is hard to explain without visualisation, but completely explains all of these anecdotes about sudden direction changes.

This is exactly what I thought about when reading about the UFO sightings of the Roswell era. Less than 30 years after the biplane era, there were secret jet engine aircraft projects screaming around the Southwest desert at near Mach speeds and aircraft in production well over 500 mph. See one of those flying low almost directly toward you at night would look appear as if it is floating or barely drifting across the sky. If it then had a moderate course change left or right, it would appear to you to go from floating to extreme speeds with almost instant acceleration.

Right. I try to apply Occam's Razor in these scenarios. Either aliens fly around in the sky regularly enough that I've witnessed it multiple times, or there's something about perspective that I just don't fully understand (not meaning to throw shade on OP here).

First thought is two separate satellites whose orbits appeared to intersect at the same time they faded/became visible respectively.

Any abrupt change in heading is indeed pretty much impossible. A 90 degree plane change would require a staggering amount of delta-v.

Just to add to this, when a satellite goes into the Earth's shadow it disappears like turning off a light, and vice versa for a satellite going in the other direction coming out of the Earth's shadow.

I suspect this is it - you can't "see" where the shadow is but satellites go in all sorts of directions - and your eyes see a light moving and then another moving another direction and assume they're the same thing.

Even if somehow that delta-v can be reached without burning so much fuel/energy that it makes any mid- to longterm mission fail, the forces on the satellite equipment which need to be withstood are insane. At orbital speeds it would just break the satellite/the craft unless there are some very special designs out there (?).

The typical pop-culture UFO shape, a saucer or a sphere, would be pretty good at distributing these forces across the "fuselage" I would guess, but internal systems are still affected. Anything with wings or solar panels would be toast, no?

That was my first thought as well. It's not unusual to see several satellites overhead at the same time, and they can appear and disappear suddenly as they cross the earth's shadow boundary. Also they're quite faint and a brief discontinuity in their visibility is hard to spot reliably, or distinguish between blinking, glancing away for a moment, etc.

Have you ever noticed how sometimes when you look at your watch, the second hand seems to linger for slightly more than a second? This is due to saccadic masking. When your eyes move quickly, your brain cannot process the visual data. So it just backtracks and fills it in backwards with whatever it ends up seeing.

If the satellite passed into the shadow of the earth and dissappeared, your eyes could move to track another one without you seeing any perceptual gap at all.

https://en.wikipedia.org/wiki/Saccadic_masking

Next time remember the exact date, time and location. You can look up satellites overhead.

The easiest explanation is two satellites with 90 degree between their inclination and lucky situation with the shadow that hides one at the approximate time second satellite shows up. It's not common but it someone will see it with enough time.

Just like with enough time and observation you can see a meteor that is heading straight toward you (it looks like a bright dot that dissappears).

This is a fairly common phenomenon in aviation. After seeing planes do the same thing you're describing I looked into it and this post covers what's happening accurately [1]. The phenomenon is called Foreshortening [2].

[1] https://aviation.stackexchange.com/questions/12762/what-airc...

[2] https://en.wikipedia.org/wiki/Perspective_(graphical)#Foresh...

I have also seen something similar at Surfside Beach, SC. I was in a hot tub, star gazing, and watched what I thought to be a satellite actually intersect another object and make a sharp 90 degree turn before disappearing. Still no idea what it was to this day!

Is it possible that you witnessed a collision? I don't recall having seen what appeared to be an intersection. But if it was a satellite, then it's possible that there was a second one up there that wasn't angled correctly to reflect light and be seen from the ground - maybe I witnessed a collision, in which only 1 of the 2 objects was angled/big-enough to be visible? Do others on HN know how often collisions occur?

Edit: Did both objects change direction, or just 1 of the 2?

An orbital collision would not cause an object to change direction 90 degrees. Satellites are not billiard balls. They are traveling at ten thousand miles per hour plus. Collisions at that speed result in two debris clouds that are largely traveling in approximately the same direction as before.

I would hazard a guess that you saw two objects that appeared to cross paths at the time one was disappearing and the other was appearing. Could be planes or satellites. Both can abruptly appear or disappear as they enter or exit shadow, or as they turn and reflect light in a different direction.

I saw a similar thing when I was a child, during daytime though.

A bright white spot moving over the blue sky and suddenly doing a 90 degree turn. It really scared me at the time.

I now believe we saw a high-altitude balloon that got into a layer of different winds at that moment.

If it was fast, it might be a meteor fracturing, and the fractured parts slowing down very quickly. It's hard to judge directions and speeds accurately against an empty blue sky, so a sudden change in velocity could easily be misinterpreted.

No clue about this, but I can imagine a highly elliptical orbit in perspective projection makes a seemingly sharp turn at the highest point, right?

Nope, Molnaya orbits appear to dwell for a very long time at their apogee. That's why they're popular for Russian spy satellites.

Yeah, that makes sense.

I got nada for a “true” 90 turn...

How about a smooth change in direction? This might be weird enough to be interesting in its own right. In that case picture a satellite in a highly elliptical orbit, where you catch it near apogee, and assuming conditions are such that you can see it (big enough shiny surface/close enough to earth for human eyes, sunlit) then the earth could “outrun” it for a bit while the sat. is at its slowest, and the craft might trace a loop out in the sky, or (presumably) some other curve depending out your relative line of sight, etc.

Edit: trying to remember if there was a more official name for this orbit. Perhaps a Molniya orbit could give the ground based observer a loop?

https://www.google.com/amp/s/www.researchgate.net/figure/A-3...

Same experience in the early 90s, Midwest summer evening starwatching with my mom. We were pretty well in the habit of discerning satellite from plane, and saw a satellite stop for a few seconds, go a different direction for a short bit, stop again for a second, then a different direction until the horizon.

This sounds like apparent retrograde motion (https://en.m.wikipedia.org/wiki/Apparent_retrograde_motion). Not sure how's possible to observe this effect in real time though

Do you know where and when it was?

[1] 1992. I was extremely young, so I can't recall anything other than the location. In my adulthood, I can lookup the exact GPS coordinates. For privacy reasons, I'll say "Downeast Maine". It was about 1.5 hrs after sunset and I wasn't in bed, but it was dark, so I'd like to guess either the spring or fall - neither winter nor summer. I was old enough to know that you should note the change in perceived speed. It's note-worthy that Downeast Maine was not terribly far from Loring Airforce Base, which was still operating in those days.

[2] March 2018, at approximately 8:30-9pm. Again, I don't recall the exact date or time, (I should have written it down immediately), but we were outside Borrego Springs, California approximately 100 miles east of San Diego. San Diego is also very close to a lot of Military bases.

Maybe https://space.stackexchange.com/ knows.

If it was just AFTER sunset you get a situation where the sun is set for you but it is NOT set for the satellite tens of miles above, as long as they're in the right part of the sky.

So one moves into the shadow of the earth, the other moves out - at a 90° angle.

Foo fighter

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