与地球共轨的小行星（英文）
本帖最后由 deepgreen 于 2009-2-1 04:47 编辑1. 2009 BD
http://astroprofspage.com/wp-content/uploads/2009/01/2009-bd.gif
Earlier today, a small asteroid designated 2009 BD passed Earth. It is estimated to be about 10 meters in diameter.2009 BD was recently discovered by the Mt. Lemmon Survey. An object of this size, if it had hit Earth, would likely have created an explosion similar in sized to that from an atomic bomb (depending upon composition and speed at impact).Earth gets hit by this sized body every now and then.That it is out there and that it is passing near Earth are not unusual.It is one of a large number of such bodies in the Solar System.
What makes 2009 BD unusual is that its orbit is very nearly the same as that of Earth.It is what we’d call a co-orbital asteroid.Literally, the term co-orbital means same orbit.Really, it isn’t the same orbit.2009 BD has almost the same orbit, but it is close enough that its orbit is still called co-orbital.2009 BD’s orbit is just a tiny shade larger than Earth’s, with a semi-major axis of 1.007 AU, compared with 1.000 AU for Earth (be definition, an Astronomical Unit, or AU, is the semi-major axis of Earth’s orbit).That means that 2009 BD takes just a shade longer to orbit the Sun than Earth.2009 BD’s orbit is also just a shade more elliptical than Earth’s orbit.It has an orbital eccentricity of 0.0278, versus Earth’s eccentricity of 0.0167.This means that 2009 BD’s orbit sometimes carries it a little closer to the Sun than Earth, and sometimes a little farther away from the Sun.It is currently passing us from being a little farther from the Sun to a little closer.However, as 2009 BD gets closer to the Sun, it speeds up.It is going to pull away from Earth, leading us around the Sun.However, its elliptical orbit will again carry it farther from the Sun than Earth.When that happens it will slow down.Remember that its orbit on the whole takes a little longer to go around the Sun than Earth.So, eventually it will be moving slow enough that Earth will pass it up.That will happen in October of this year.It won’t be passing us nearly as closely then as it is now.Earth will move in front of it, but 2009 BD’s orbit will once again carry it closer to the Sun, and it will speed up and pass Earth again.That will happen in middle of next year.That will be another very close approach (though I don’t think it will as close as this one).During all of that time, it will never drift too terribly far from Earth.Eventually, the orbits will separate, but one day they will intersect close again.The orbit passes inside of Earth’s Hill Sphere (the region of space where Earth’s gravity dominates), so its orbit around the Sun won’t be stable in the long term.Eventually, it will likely either run into Earth (or the Moon) or it will be tossed into a new orbit.However, it can keep up this dance with Earth for a very long time before that happens.It is extremely difficult (even impossible) to accurately compute orbits such as 2009 BD’s orbit too many iterations into the future, so we can’t really say with certainly what will happen with it in the end.
This asteroid is not the only co-orbital or nearly co-orbital asteroid with Earth, and it most certainly is not the only asteroid to cross Earth’s orbit.There are, at least, thousands of asteroids whose orbits cross Earth’s, with over 1000 computed as passing close enough to be called “Potentially Hazardous Asteroids.”Not all of these are co-orbitals, but they do have orbits that cross Earth’s orbit.
Co-orbital and nearly co-orbital asteroids of Earth have been known for years.There are also a host of asteroids in special orbits with Jupiter (60 degrees in front and in back of Jupiter as it orbits the Sun).These Trojan Asteroids, as the ones that share Jupiter’s orbit are known, have been known for over a century.This is one of the things that makes the 2006 IAU definition of a planet quite controversial.One of the requirements of being a planet is that a body have sufficient mass to “clear its orbit.”Unfortunately, the term “clear its orbit” is not well defined.Pluto has insufficient mass to clear its orbit.There are plenty of things in the vicinity of Pluto’s orbit.But, has Earth cleared its orbit?Has Jupiter?Just what does this mean?Generally, it is taken to mean that a body has cleared its orbit of anything similar in mass.Certainly Earth and Jupiter have done that (but not Pluto).Also, the co-orbital asteroids of Earth are in unstable orbits.So, they are only temporary.Pluto is small enough that plenty of things can continue to share its part of the Solar System.Those other Kuiper Belt bodies are not going to be kicked out of their orbit by Pluto.But, Jupiter’s Trojan Asteroids are in pretty stable orbits.The stay in position due to the interaction of Jupiter and the Sun’s gravity.So, you could argue that Jupiter’s gravity is “clearing” its immediate neighborhood and keeping these bodies near their LaGrange points 60 degrees in front and behind Jupiter.But, it would have been very nice if the IAU had come up with a definition that was a lot cleaner and didn’t require such explanation.But, as I had written at the time that all that was going on, the definition was put together rather quickly after the general assembly rejected the unpopular definition that had been set forth by the committee that had been working on a planetary definition for some the past year.Before anyone goes off on wild postings about how you really like Pluto and want it to stay on the list, if that earlier definition had been kept, recent findings would not leave us with 14 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Haumea, Makemake, and Eris.And, there’s a list of about ten other bodies that possibly would be on the list once a bit more data about them is available.If one of these additional bodies (Pluto) is a planet, then they all are.
来源：Astroprof 2. Two More Co-Orbitals of Earth
In my last post, I wrote about the asteroid 2009 BD, which has an orbit very nearly that of Earth’s. But, it is not Earth’s only orbital companion. I know of more than a half dozen asteroids whose orbits would be considered co-orbital with Earth. The two that I want to talk about today have even stranger orbital properties. These are the asteroids 2002 AA29 and 2003 YN107.
http://astroprofspage.com/wp-content/uploads/2009/01/2002-aa29.gif
http://astroprofspage.com/wp-content/uploads/2009/01/2003-yn107.gif
As with 2009 BD, these asteroids have an orbit almost the same as that of Earth, but not quite. Both have orbits that are a bit inclined with respect to Earth’s orbit, as you can see in the accompanying orbital diagrams. These diagrams, courtesy JPL’s Small Body Database, show an oblique view of the Solar System, so you can see how the orbits drift above and below Earth’s orbit. They also have orbits nearly as elliptical as Earth’s, but the long axes of their orbits is not aligned with that of Earth’s orbit. Thus, they drift closer and farther from the Sun that Earth. This drifting closer and farther while going up and down (compared with the plane of Earth’s orbit) makes their orbit appear almost helical if you were to look at the orbit from Earth’s perspective instead of from the Sun’s point of view.Paul Wiegert has a nice explanation of this sort of orbit, and I’m using a couple of his images here, including this one:
http://astroprofspage.com/wp-content/uploads/2009/01/aa29_horseshoe.gif
This is a typical orbit for this sort of asteroid, as seen from Earth.Furthermore, it is also what we call a horseshoe orbit.The name comes from the fact that typically, if the asteroid is moving a little faster than Earth, when it approaches from behind in Earth’s orbit, Earth’s gravity pulls on it a bit, it speeds up, slides to an orbit just a shade farther out than Earth’s, slows down, and Earth pulls away from it.Effectively, it would look like the asteroid got close to Earth in its helical orbit and then started going back the other way.Eventually, Earth catches up with it again, and the reverse happens.So, the asteroid simply goes back and forth along paths that loop in and out of Earth’s orbit, and whenever it gets close to Earth, the orbit of the asteroid shifts ever so slightly so that it continues to loop in and out of Earth’s orbit for years, decades, or centuries until it comes close to Earth again.One of the first asteroids that I heard about that does this sort of thing was 3753 Cruithne, discovered in 1986. Clearly, these orbits are of bodies about the Sun, but they are greatly influenced by the presence of Earth and Earth’s gravity.But, there are plenty of other bodies in the Solar System influencing these bodies, and in order for them to behave as they do, they have to have an orbit that is unaltered by these other bodies.That isn’t going to happen.Thus, these orbits, while pretty stable for a long time, are ultimately unstable.For most unstable orbits, that means that they can be shifted into an orbit that either comes too close to Earth at some point, resulting in their being thrown out of this neat orbital relationship with Earth.They can even wind up on a path that would ultimate collide with our planet, though that is far less likely than their being tossed into another orbit.
But, one really interesting thing that can happen is if one of these asteroids approaches Earth at just the right spot in its orbit.Then, their orbit becomes even more tightly entwined with Earth’s.In this case, they orbit the Sun always in the vicinity of Earth.If you were looking at the asteroid from Earth, it would look almost as if the asteroid were spiraling around Earth.This would be a case of an asteroid whose orbit, instead of going back and forth in the helical horseshoe orbit above, would be trapped in the gap in the horseshoe shape shown.Such an orbit would look like the following (also from Wiegert):
http://astroprofspage.com/wp-content/uploads/2009/01/aa29_qs1.gif
For a while, Earth would be orbiting the Sun with a companion.It is important to note that, while the asteroid looks like it is looping around Earth, it is in fact still orbiting the Sun, not Earth.That means that it is not really a satellite, or moon, of Earth.It is still a sun-orbiting body, but its orbit looks almost like that of an Earth satellite body, so we sometimes call it a quasi-satellite or a quasi-moon.The asteroid 2003 YN107 got trapped in just such a situation a few years back.For about ten years, from 1996 to 2006, it made loops around Earth, taking one year to make one complete circuit.It is small and wasn’t discovered until 2003, having already been a temporary companion of Earth for ten years.In 2006, though, it managed to get a slight nudge to get back into its normal horseshoe orbit.
2002 AA29 does something similar.It orbits the Sun.But, its orbit looks like the horseshoe seen above.About once per century it approaches Earth, where it is turned back.Eventually, it approaches from the other side (one time it is gaining on Earth, and the next Earth gains on it).Again, it is turned away.It bounces back and forth, gaining on Earth, then losing ground until Earth catches it.It, then, pulls away once again.But, once in a while it gets caught into a temporary holding pattern, spiraling around Earth.That last happened perhaps nearly 1500 years ago, and it may happen again in about 600 years.There are other asteroids with this behavior, too.
So, Earth does not orbit the Sun alone.Besides our Moon, there are a variety of other bodies out there keeping us company.
来源：Astroprof 看不懂英文贴. 等我有空翻译一下。。。 会不会来个亲密接触啊 哪位同好翻译一下。 这不是和LW的一个贴一样的嘛~~::42::不过这个是英文版的,暂时先留着了~::070821_18.jpg:: 7# 一分半
链接呢？ 我发前搜了一下，有个有关系的帖子讲到了2009 BD，但是很简略。这两篇英文文章讲了3颗共轨小行星，做了较为详细的介绍，并且讨论了它们的存在对行星定义的影响等，还是挺有意思的。 看不懂英文贴.
liuyu 发表于 2009-1-29 01:16 http://www.astronomy.com.cn/bbs/images/common/back.gif对！看不懂。 8# deepgreen
就是这个原因所以把你的贴留着没合并哈~::42:: deepgreen不厚道，应该直接贴译文~~~ 11# positron
这个。。。还没译好呢。。。::02:: 心急的同好翻翻以前的《天文爱好者》吧，专门有一期说这个的。 译文在这里，欢迎拍砖：
http://www.astronomy.com.cn/bbs/viewthread.php?tid=104710 ::070821_09.jpg::感谢LZ的辛苦翻译~ 15# 一分半
与版主真是有共同语言啊。。。:handshake 由于这种小行星的轨道受日、地引力的共同影响，且地球引力的影响更大，所以也可以认为是地球捕获的卫星，虽然比较另类~
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