Hacker News Reader

Comment by nntwozz | Hacker News Reader

The most common metallic meteors are iron-nickel, so green is a common color. This glow tends to be brightest when meteors hit the atmosphere at high speed.

View on HN

Do meteors ever hit the atmosphere at low speed?

Yes, relatively speaking.

First likely hit I found:

"Meteors enter the atmosphere at speeds ranging from 11 km/sec...to 72 km/sec ... The wide range in meteoroid speeds is caused partly by the fact that the Earth itself is traveling at about 30 km/sec (67,000 mph) as it revolves around the sun. On the evening side, or trailing edge of the Earth, meteoroids must catch up to the earth’s atmosphere to cause a meteor, and tend to be slow. On the morning side, or leading edge of the earth, meteoroids can collide head-on with the atmosphere and tend to be fast."

https://www.amsmeteors.org/meteor-showers/meteor-faq/

The "relatively slow" 11 km/sec is still 39.600 km/h, or around Mach 30 (although that depends on air density and temperature), so still pretty fast I would say...

There's no authoritatively correct reference for what "slow" or "fast" should mean in extraterrestrial contexts, however, I think it makes sense to look at it like this:

Suppose you were watching a (CGI) movie or a video of a meteor hitting Earth, with the whole globe on screen.

The diameter of the Earth is almost 13,000 km. So a meteor(oid) going even 70 km/s is going to take 3 minutes to hit from one diameter away.

It should look like it's barely moving on a planetary scale.

The scale of things in astronomy is humbling to say the least. "Close" can be 1 light-year away... That's 9.4607e+12 km. "Slow" is Mach 30. We're nothing at that scale.

I wonder, if a meteor was moving very slowly (relatively) and basically just gently falling to earth, if we'd notice.

Yes. They are the largest source of extraterrestrial mass. https://www.sciencealert.com/5-200-tons-of-micrometeorites-a...

And one has been caught on film (we think) https://www.universetoday.com/110963/norwegian-skydiver-almo....

There’s also a nice coffee table book about them:

https://www.amazon.com/Search-Stardust-Micrometeorites-Terre...

> one has been caught on film

apparently it was just a rock[0]

[0]https://www.universetoday.com/111076/follow-up-on-skydiving-...

Damn nearly getting cut in half by a meteor while skydiving!

"Ann Elizabeth Fowler Hodges...was an American woman known for being the first documented individual not only to be struck by a meteorite, but also to live through the encounter."

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

Neat! I hadn’t thought that through, but it makes sense.

So this can't be a meteor then. That's not 11 km/s unless the video has been drastically slowed down.

I just read a page on meteors that said they tend to blow up starting around 300,000 to 200,000 feet or so.

If I make a wild-ass guess that the fireball was visible over a trajectory of about 100,000 feet, that's about 30 km.

At 11 km/s, then, it should appear for around 3 seconds.

I rewatched the video, which is 5 seconds in all, and it looks to me like the light is pretty close to three seconds long.

Aside from that, sure, it could've been slowed down, and could be at an angle, so it may all be a coincidence.

I'm sure you've seen a plane traveling 400 mph but seemingly hovering in one spot in the sky. It's just not moving perpendicular to your vision.

Not sure what you're getting at but 400 mph is 0.178816 km/s. Off by two orders of magnitude from 11 km/s.

I am sure everyone has seen a shooting star. Planes don't move like shooting starts or whatever is in this vide.

I think it's safe to conclude that this is not a plane nor a meteor.

Here's another factor: When something is far away from you, it appears to travel slower. For example, the sun is ~150M kilometers away. As the earth rotates through a day, from our perspective it travels ~942M kilometers, meaning it appears to be traveling at 40 million kph through the sky. However, it's just sitting there in one spot.

Velocity is relative to a frame of reference. My understanding is that if an object has a low velocity compared to Earth it will more likely drift away due to earth-moon gravity interactions. An example is Apollo 10 third stage and lunar module ascent stage which are in solar orbits to this day.

https://en.wikipedia.org/wiki/Apollo_10

While the odds of what I'm about to describe are astronomically (pun intended) low, I can see the following occurring:

- earth moves in its orbit

- meteor is moving in a straight line

- that line is perfectly parallel to a point on the earth's orbit

- the earth and the meteor basically link up and the speeds match so precisely that the meteor just goes into orbit

You haven't accounted for the massive gravity well that is the sun. The difference in potential energy from infinity to the radius of Earths orbit is GMm/r. Set that equal to momentum (.5mv^2) and you get sqrt(2GM/r). Plug in the numbers and you'll see you can't really have a meteor that stumbled in from outside the solar system with a similar velocity to Earths orbit by the time it came down to Earths orbit. Put another way, if it was moving slow enough for the speeds to basically match, it would also be in an orbit around the sun similar to our own, and you'd have to account for the missing potential energy it had before entering our stars gravity well. If you want to argue it came from within the solar system, it would have to have already been in an Earth like orbit this whole time, so why don't we see it?

Near-earth asteroids periodically wander into temporary orbits around the Earth and then wander off.

https://earthsky.org/space/2020-so-mini-moon-asteroid-or-spa...

Thanks for the link.

"Strange space object 2020 SO was discovered on September 17, 2020, on approach to Earth. On November 8, it slowly drifted into Earth’s sphere of gravitational dominance, to become a new mini-moon. It’ll escape back into a new orbit around the sun in March 2021. During that time, it’ll make 2 large loops around our planet."

Why would it leave orbit if it already does 2 circles?

They're orbits, but not stable ones. Not even really ellipses. By the time it's made one orbit-ish motion around the planet, the moon has moved relative to it. The second go around it gets affected by the new position of the moon. Eventually the combined effects of the moon and the sun's changing relative positions will nudge it out of orbit.

You all in the comments are rediscovering Lagrange points the hard way. L4 and L5 are stable, BTW, and we have a relatively famous space telescope at L2 which requires active stability control to stay there.

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

Thanks. I wonder if something similar is responsible for the gaps in atomic orbitals.

https://upload.wikimedia.org/wikipedia/commons/d/d0/Lagrangi...

Think of them as increasingly elongated elipses and you can imagine the instability building...

This made me think how the moon's orbit remains stable after so many small objects fly around. I guess they are too small to be significant enough (obviously good).

Orbital mechanics is considerably more complex than this because there are several over massive planets orbiting the Sun. Granted, the odds are very low of getting kicked into a trajectory that will just barely kiss Earth's orbit (at the right time), but it can happen. Not everything orbits in a conic. Also, extrasolar meteors are probably a lot more rare than the above comment would suggest.

That is so unlikely if it happened I would seriously consider the possibility of an intentional gravity maneuver by an alien race.

It has happened. Such orbits are not stable, due to the moon and sun's influence.

https://earthsky.org/space/2020-so-mini-moon-asteroid-or-spa...

There's also the very low possibility of something just skipping along the upper atmosphere and ricocheting back into space.

Well presumably meteors travel at different speeds in outer space, and some meteors could be regarded as high speeds and others at low speeds. I don't think you necessarily have to be in synchronization with Earth's orbit to be considered low speed relative to some reasonable perspective.

For this to happen, it must have a low-enough potential energy to begin with. Otherwise, it will just shoot out the other side of the orbit.

In that case, most likely a meteor was already in orbit, and we just didn't know about it.

11.2 km/sec at a minimum. (Earth's escape velocity!)

It's not immediately obvious to me – why would escape velocity also be the minimum speed at which an object could enter the atmosphere?

Even if an object is moving in a parallel orbit to Earth's around the sun, if you "zoom in" to the point where it enters the Earth's sphere of influence, the influence of the sun is relatively small. It's roughly as if Earth and the object are free-falling together in empty space, with a small relative velocity. And Earth's escape velocity is the same velocity that would be reached by a stationary object "dropped" from infinitely far away. So relative to the Earth it will accelerate and hit the atmosphere with at least escape velocity.

(This is only an approximation, but it's a pretty good one because there's such a large difference between Earth's mass and the sun's.)

Another way to look at this is to simply observe that Newtonian physics is time-symmetric (neglecting effects like friction that dissipate energy). If you have a trajectory that arrives at the edge of Earth's atmosphere with a certain velocity, you can reverse it to get a trajectory that leaves with the opposite velocity (i.e. equal speed).

It's not the minimum possible, but it is the minimum typical case.

Escape velocity is the upwards velocity needed at the surface to reach an unbounded height with velocity zero. Run that in the other direction and you see an object that starts from infinity will reach Earth at escape velocity.

For that not to happen, you'd need something to perturb the object, slowing its fall or deflecting its velocity in some direction other than down. This can happen from the Moon, but it's not common given its small mass and gravitational field relative to Earth.

Have to hold multiple concepts in head at the same time.

If you're "far away" with gravitational potential energy and get "close" to the atmosphere that potential will have to go somewhere and it'll go into speed. Its like how, without a parachute on earth, falling 40 M means about 100 KM/h impact speed. And parachutes don't work outside the atmosphere so its retrorockets or hit pretty fast. Going from up there to down here means a certain velocity on the fall in.

Conceptually there is a calculatable difference between the speed "from infinity if the earth were not orbiting the sun" vs "from entering the earth's sphere of influence where the earth gravity matters more than the sun's gravity" but it turns out not to matter much. You're probably going to be coming in very fast from falling into the sun's gravity well. But even if the stars align and you hit the earth's influence at zero m/s (LOL good luck) you'll still hit the atmosphere "around escape velocity" after some rounding.

Indeed. What if the object was traveling at the same speed as Earth as it revolves around the Sun, and their paths intersect at a very acute angle? That is to say, they met at a traffic 'zipper' ... Could an object collide with earth at a lower speed?

No - because as they approach, Earth's gravitational field accelerates it.

Escape velocity is the velocity needed to reach an unbounded height from the surface, so reverse that timewise and any object free-falling towards Earth will have escape velocity upon reaching it.

Yes, but it is rare because most objects in such orbits tend to either be gobbled by Earth right away, settle into L2, L3, or L4, or enter a high orbit.

relatively...yes

Reply on news.ycombinator.com