Earth Must Have Another Moon, Say Astronomers

MIT Technology Review Physics Blog ^ | 12/20/2011 | kfc

[Lonesome in Massachussets]

Back in 2006, the Catalina Sky Survey in Arizona noticed that a mysterious body had begun orbiting the Earth. This object had a spectrum that was remarkably similar to the titanium white paint used on Saturn V rocket stages and, indeed, a number of rocket stages are known to orbit the Sun close to Earth.

But this was not an object of ours. Instead, 2006 RH120, as it became known, turned out to be a tiny asteroid just a few metres across--a natural satellite like the Moon. It was captured by Earth's gravity in September 2006 and orbited us until June 2007 when it wandered off into the Solar System in search of a more interesting neighbour.

[Lonesome in Massachussets]

I don't understand the plot in inertial coordinates, or why it moves in that more or less triangular orbit. I assume the LD is [mean] lunar distance, about 248,000 miles and that the X-Y inertial plane is the plane containing the earth's equator, with positive X in the direction of the Vernal Equinox point. (The intersection of the ecliptic plane and equatorial plane, with positive X pointing in the direction of the sun in March.) They also do not give any time ticks, which is somewhat fustrating.

I downloaded the linked article in PDF. Maybe when I get done chewing on that I'll report back.

[JohnG45]

... this was not an object of ours. Instead, 2006 RH120, as it became known, turned out to be a tiny asteroid just a few metres across--a natural satellite like the Moon. It was captured by Earth's gravity in September 2006 and orbited us until June 2007 when it wandered off into the Solar System in search of a more interesting neighbour.

Captured and released? How exactly did it get "released" to wander off in search of a more interesting neighbor?

[Tallguy]

bump for later.

[bigheadfred]

It didn’t exactly get released. It just left in disgust.

[JohnG45]

I think I see it now. It only completed two orbits before moving on - yes?

[Larry Lucido]

Did it “wander” off or was it slingshotted? Sounds like it was out for a Sunday stroll and then just moseyed off.

[Larry Lucido]

orbited us until June 2007 when it wandered off

FR's resident Rapturists are slapping their foreheads asking "How could we have missed THAT sign of the imminent End-Of-The-World?"

[coloradan]

It’s all the moon’s fault. Without the moon, something not already in Earth orbit can only come by parabolically or hyperbolically (some which crash into the Earth). But a deflection by the moon can put it into a stable orbit, at least until the moon kicks it out again. Which it is likely to do, given how many other moons we have besides the main one, accumulated over billions of years of moon collection time.

[US Navy Vet]

http://www.freerepublic.com/focus/f-chat/2823802/posts

[Lonesome in Massachussets]

It was only ever weakly gravitationally bound to earth. An asteroid orbiting the sun in the same direction as the earth and the other planets can be captured by a planet when it's orbit crosses near the orbit of the planet in front of the direction in which the planet is orbiting the sun. The gravitational attraction drags the asteroid towards the planet (and the planet slightly towards the asteroid). The orbital velocity of the asteroid decreases and it can be kept in orbit around the planet. The amount of deceleration experienced by the asteroid determines how tightly it is bound to the planet.

The any object orbiting a planet is perturbed by the gravity of every other particle in the solar system. For objects orbiting the earth, the significant perturbing bodies are, more or less in order: the moon, the sun, Venus and Jupiter. Depending on their relative locations, they can provide just enough of a push to free a weakly bound object from the grasp of earth's gravity and continue in orbit around the sun.

Jupiter, of course is the king of capture, remember Shoemaker-Levy? It was captured in 1968, but not observed until 1994, shortly before its perturbed orbit caused it to collide with Jupiter.

The opposite of capture occurs, too. An asteroid approaching a planet's orbit from behind can be given a boost that increases its velocity enough to hurl it out of the solar system. Numerous solar system missions take advantage of the gravitational boost available from Jupiter and the other planets.

[JohnG45]

Thanks for the nice, succinct explanation.

[blueunicorn6]

Damn.....even the asteroids want our welfare money. Go over to Mars! They have free healthcare!

[Lonesome in Massachussets]

PS: That also explains why the earth and the inner planets do not have significant ring structures and all the outter gas giants do. The relative strength of the perturbing forces, especially the sun, are so much stronger.

Both of Mars’ moons are believed to have been recently (like the past few million years) captured. Phobos is in retrograde orbit, opposite the direction of Mars’ rotation. The effect of tidal drag continually reduces the radius of its orbit and it is expected to collide with mars in a few million more years. (Tides actually increase the radius of our moon’s orbit by about one centimeter per year increase the length of the earth’s day by about 0.0015 seconds per century. The transfer of energy from earth’s rotation to the moon’s orbit is about one gigawatt.)

[blasater1960]

Too bad it didnt beam up Hussein before it left...

[calex59]

Captured and released? How exactly did it get "released" to wander off in search of a more interesting neighbor?

My thoughts exactly. Does it have it's own source of power, if not how did it move out of our gravity field. If it does, how did it get that source of power and is it man made?

[Lonesome in Massachussets]

Planets can capture objects that cross their orbits. This thing, obviously, was orbiting the earth-moon system, but it came within one lunar distance of something, either the earth or the baricenter. The most likely perturbation that released it from orbit around the earth-moon barycenter was probably due to the moon.

[COBOL2Java]

Captured and released? How exactly did it get "released" to wander off in search of a more interesting neighbor?

It must have crossed the border from Mexico.

[JohnG45]

Okay, now that we have all of that orbital eccentricity stuff explained, how about explaining that face on Mars!?

[HiJinx]

how about explaining that face on Mars!?

Intelligent Design?

[dr_lew]

any object orbiting a planet is perturbed by the gravity of every other particle in the solar system. For objects orbiting the earth, the significant perturbing bodies are, more or less in order: the moon, the sun, Venus and Jupiter.

If you look for accurate ephemerides on the web, you will find instead of earth, a mysterious object called EMB. This is the earth-moon barycenter, which must be regarded as the object orbiting the sun as a single mass.

To speak of 2006 RH120 as being "perturbed by the moon" is like saying that Apollo 8, e.g., was orbiting the earth and "perturbed by the moon". I notice that the cited article is ambivalent on this point, which is amusing in light of the flap over the planetary status of Pluto. The embarrassing fact is that the earth-moon system is a double planet.