Earth has caught a 'second moon' | Hacker News Reader

Earth has caught a 'second moon'

space.com

41 pointsdivbzero37 comments

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I'd hardly call a 37 foot object that wont even make a complete orbit a "second moon."

Obi-Wan Kenobi couldn't have said it better!

That's no moon!

https://www.youtube.com/watch?v=eT4shwU4Yc4

Third at best!

FTA: "You may say that if a true satellite is like a customer buying goods inside a store, objects like 2024 PT5 are window shoppers."

The article agrees with you. It's always those pesky headlines.

How can it be called a 'capture' when it's already known when and how it will leave. That's like calling a resort vacation stay a kidnapping.

Technically The Moon is slowly leaving earth's orbit too, to the tune of 38mm per year[0]. Everything's temporary...

[0] https://en.wikipedia.org/wiki/Orbit_of_the_Moon

Any impact on tidal activity?

Like measurable in our lifetime? The average distance to the moon will be 3.8 meters further away in a hundred years. That's insignificant as far as tides goes.

Despite what another commenter says, the moon will not leave earths captive field. It will recede until it gets tidally locked with earth, and both the tide cycles and the moons recension will halt. That is, if there still is water in a few billion years and it hasn't been replaced with Brawndo

Is the moon not already tidally locked? The same hemisphere of it always faces the earth. Is there a different kind of tidal locking or orbital resonance it will settle into?

It goes on until an Earth's day is as long as a month. Both will face each other in a fixed way. But it takes many billions of years and the Sun will burn both to a cinder much before that.

That's like calling a lifetime lived within just one village 'temporary'. On the scale of human events it's nearly as permanent as anything else for which we commonly ascribe permanence and reliability within lifetime scales.

It's fair to describe objects in relation to the lifespans of those objects, not of those who observe. "On the scale of human events" is not the only relevant timeframe for discussing celestial bodies.

the way I remember that fact is it is also average fingernail growth per year

Goodbye Moon would be a great name for a children’s book focused on space and time!

Because during that time from its frame of reference it will be falling around the earth until it reaches escape velocity and starts falling around the sun again.

I had the same confusion so here's my layman's understanding. They are defining capture/orbit as having negative "geocentric energy" which is a term of art no one else seems to use but I think is just a mathematical representation of the following:

> a temporary satellite is any body that enters the Hill sphere of a planet at a sufficiently low velocity such that it becomes gravitationally bound to the planet for some period of time. [1]

again stretching my understanding too far, I think this basically means that in the absence of other celestial bodies the satellite would be in a stable orbit, but that in reality after some time it gets far enough away that the sun's gravitational pull dominates and stops it from making a full orbit.

[1] https://en.wikipedia.org/wiki/Temporary_satellite

> in the absence of other celestial bodies the satellite would be in a stable orbit

Presumably entering such an orbit is only possible due to forces from other celestial bodies in the first place, since otherwise if you reversed time it would spontaneously leave its orbit. In other words, the act of the earth "capturing" the object is ultimately performed by external forces?

To be fair, it's still kidnapping in most states even if the duration is very short.

> Marcos explained, "Asteroid 2024 PT5 will not describe a full orbit around Earth. You may say that if a true satellite is like a customer buying goods inside a store, objects like 2024 PT5 are window shoppers."

Is it a NASA thing to deploy outrageously absurd analogies for no apparent reason? Is there a checkout desk for space objects?

It's not absurd, you're reading too far into it.

Yes, I'm not sure why they chose that analogy. It just sounds cutesy and doesn't really add anything.

"There’s a Moon in the Sky (Called 2024 PT5)" just doesn't hit the same.

https://genius.com/The-b-52s-theres-a-moon-in-the-sky-called...

I wish spaceflight was sufficiently commoditized that sending a satellite there to get some pictures would be trivial

Which website/software/app is good for tracking the location of 2024 PT5, so that it can be found in the local night sky?

Being 100ft across, it won’t be visible except for long exposure [1].

[1] https://theskylive.com/where-is-2024pt5

> While the moon is an estimated 2,159 miles (3,475 km) in diameter [...]

Is the article implying that we don't know the Moon's (I assume they're referring to the capital-M one) diameter to at least kilometer-precision...?

It’s an irregularly shaped body so there’s not one single radius. Mean radius is always going to be an approximation (even for Earth); the mean radius of the Moon is 1,737.4 km.

But it's still an approximation, not an estimate, right? (The fact that you can list the mean radius with sub-decimal precision suggests as much.)

To me, an estimate suggests that there's error bars; an approximation suggests that there's variance that we can quantify (or at least we're very confident about our error bars).

There are error bars on absolutely everything that we ever measure.

Sorry, missed a "large". It would sound pretty strange to me to claim that the distance between New York and London is "an estimated x kilometers" (with single-kilometer precision), even though there is tectonic movement etc.

Didn’t know we were referring to asteroids as moons now

The Jovian satellites would like to have a word with you.

Also, Earth already has a second moon: 3753 Cruithne.

https://www.youtube.com/watch?v=CIqOsM6_3Dw

so even the Green Bank telescope could only get two pixels?

https://petapixel.com/2023/01/26/these-are-the-highest-resol...

Now that it's here, can it be mined?

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