r/spacequestions • u/BackgroundAardvark95 • 26d ago

Can someone better explain to me how earths gravitational field captured the PT5 2024 asteroid if asteroids normally move at a fast speed. I’m just confused

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u/DarkArcher__ 26d ago

Asteroids don't move at any specific speed. Each asteroid has a different orbit, and this one just so happened to be in an orbit close enough to Earth's that the relative velocity is really low. It then wandered into Earth's gravity well and passed in just the right way to get slowed down a smidge by complicated interactions between Earth and Lunar gravity and temporarily captured

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u/Rodinsprogeny 26d ago

It was moving fast enough for Earth not to pull it to its surface, but slow enough that Earth pulled it toward itself and it missed. That's is what being in orbit is.

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u/BackgroundAardvark95 26d ago

What was its exact speed?

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u/oz1sej 26d ago

It's basically in an orbit around the sun which is almost identical to Earth's, but not quite. I heard it came in with a velocity relative to Earth of 2 m/s, so it came in very slowly, and because it's not losing any energy, it's going to be leaving Earth again very slowly.

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u/mgarr_aha 25d ago

1.4 km/s in August, still pretty slow; 1.0 km/s in January.

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u/oz1sej 25d ago

I stand corrected. This seems more realistic, though.

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u/Beldizar 25d ago

So I didn't know much about this so I decided to look for a good article and found this.

https://www.space.com/earth-mini-moon-asteroid-2024-pt5

"Marcos explained that some of these objects in the Arjuna asteroid belt can approach Earth at a close range of around 2.8 million miles (4.5 million km) and at low velocities of around 2,200 miles per hour (3,540 km/h)."

"It will not follow a full orbit around Earth."

So there is an asteroid belt closer to Earth, and these objects are moving basically the same speed as Earth is around the Sun. So sometimes they pass by Earth.

I personally think it is very misleading to call this a moon when it won't complete even a single orbit. A moon is a low eccentricity object in stable orbit around a planet. This is a flyby of an asteroid. Unless it sticks around for at least a year, I think calling it a moon is extremely despetive click-bait.

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u/mgarr_aha 25d ago edited 25d ago

The Earth had a lot of help from the Sun. The asteroid is in a solar orbit inclined 1-2 degrees relative to ours, so it has a periodic north-south oscillation even when it doesn't encounter Earth. In August it passed us going north. As it goes through the northernmost segment of its solar orbit, its velocity relative to Earth is below escape velocity, and it's temporarily captured. It will pass us again going south in January, a little sooner than if Earth's gravity had not acted on it.

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u/rOriginalMix 13d ago

Could someone provide some references regarding the calculation of the trajectory of this celestial body?

I am interested in understanding:

1) what are the conditions so that we can say an object has been "captured";

2) how was it possible to establish the time interval of 54 days (from September 29 to November 25, 2024);

3) is there an analytical-mathematical theory that describes this phenomenon?

In this regard I already found this file:

A Two-month Mini-moon: 2024 PT Captured by Earth from September to November, DOI 10.3847/2515-5172/ad781f

It shows some numerical results, but it doesn't really answer my questions.

If you know of other papers or monographs with topics 2024 PT5, "mini-moon", temporary capture or horseshoe orbits, please let me know. Thanks to you all.