2009 BD
Published on Jan 25, 2009 at 4:36 pm.
12 Comments.
Filed under asteroids.
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.
-Astroprof
Image courtesy JPL Small-Body Database
Doonesbury asteroid glides along with us | Bad Astronomy | Discover Magazine on January 25, 2009 at 7:01 pm: 1
[…] on about resonances and horseshoe orbits and co-orbital debris, but you can read all about it at AstroProf’s page and Universe Today. And I still have nothing in my email inbox about it. […]
Laurel Kornfeld on January 25, 2009 at 7:10 pm: 2
Yes, they all are planets. What is the problem with that? Kids don’t need to memorize the names of the planets; they need to understand the different types of planets and their characteristics. We don’t limit the number of elements in the Periodic Table for convenience. Doing so with planets makes absolutely no sense. The biggest bungle by the IAU was defining dwarf planets as not being planets at all. This begs to be changed; dwarf planets should simply mean planets (objects that have enough self-gravity to pull themselves into a round shape) that are not big enough to be gravitationally dominant.
Because the current IAU planet definition is so vague and untenable, it is rightfully being ignored by many planetary scientists.
MattFunke on January 26, 2009 at 7:59 am: 3
I’d argue that any definition of “planet” is automatically going to be vague and untenable, especially as you get near the edges of the definition. Celestial objects seem to exist on a continuum, and trying to classify them into discrete bins is more of a convenience for our desire to break down large amounts of information than any useful description of these bodies’ inherent characteristics.
The comparison to elements in the periodic table does not apply. There is no element with half a proton in its nucleus. There is no sense of continual change. A boron atom is a boron atom, and (assuming atomic theory is correct) there is no threat that something will be found halfway between it and carbon. On the other hand, there is nothing in current theory to bar finding something with characteristics halfway between, say, Ceres and Mercury.
That said, though, I prefer the earlier (fourteen-planet) definition. It seemed more parsimonious to me, and raises the exciting possibility of finding new planets in the Solar System in the future.
Astroprof on January 27, 2009 at 4:32 pm: 4
In another day or two, I am going to revisit my discussion of the definition of planet that I had after the 2006 decision. As of you note, the definition is vague. I think that either they should all be planets, and let’s forget trying to memorize all gazillion of them, or else pick a definition that makes sense. The “dwarf planet” concept seems to be a bone thrown out there to keep people happy who want Pluto to remain a planet, only it made them mad instead of keeping them happy. Bad idea.
Week of January 26th, 2009 « Dad2059’s Webzine of Science Fiction, Science Fact and Esoterica on January 28, 2009 at 1:56 pm: 5
[…] 2009 BD […]
Laurel Kornfeld on January 31, 2009 at 1:14 am: 6
Astroprof, the “dwarf planet” concept would have been fine if the IAU had assigned dwarf planets as a subcategory of planets. That is consistent with astronomical terminology: dwarf stars are still stars, and dwarf galaxies are still galaxies. A huge part of the problem is the untenable position that dwarf planets are not planets at all. Change that, and a lot of the controversy will be resolved.
Astroprof on January 31, 2009 at 11:29 am: 7
Laurel,
I think that the whole definition of planet and dwarf planet that they came up with is flawed. I will address this in a few days, when I get a bit more time.
Boxorox on February 2, 2009 at 6:57 am: 8
Defining a planet is actually rather simple:
1. in primary orbit of a star
2. shape is nearly spherical, due to hyrdostatic equilibrium
3. a planet does not cross the orbit path of the next inner planet neighbor (disqualifying Pluto)
4. either or both must be met:
A - orbit eccentricity .lte. 0.1
B - orbit inclination .lte. 10 deg
5. a planet’s orbit is unique in that no other body has common epherides: period, node, a, Q, q, etc.
6. subjective: a planet should be expected, in accordance with its characteristics, to endure for the lifetime of the star it orbits.
All these standards considered, only 8 bodies in our solar system qualify.
Astroprof’s Page » Defining Planets (Part I) on February 2, 2009 at 4:49 pm: 9
[…] wrote about the matter at the time, but I now feel that I should revisit that discussion. Last week I wrote about an asteroid sharing Earth’s orbit and that this asteroid raised issues with the IAU definition of planet. […]
Astroprof’s Page » Defining Planets (Part IX) on March 3, 2009 at 4:40 pm: 10
[…] is what got me started on this whole defining the planet series in the first place. In January, I wrote about the asteroid 2009 BD, which has nearly the same orbit as Earth. I followed up that posting with one […]
dongshenghall » Blog Archive » ?????????? on April 22, 2009 at 8:58 pm: 11
[…] ???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????“??????????????”?“????????”????????“????—?????”??????(2009?)1?????????2009 BD????10???????40????644,000?????????????????????????????????????????“???????”???????????????????? ?????????????????????????????????????????? […]
anonymous on June 5, 2011 at 5:33 am: 12
>BOOM< and we are all dead