Asteroid Flyby Yields New Thinking

It was refreshing to see the news media show general restraint when asteroid 2011 MD zipped 7,600 miles from Earth on June 27th.

I didn’t spot any over-the-top headlines or crazy reporting about potential collisions with Earth. Instead, this rogue rock passed by uneventfully and put on a pretty good show for amateur astronomers equipped with good scopes and blessed with dark skies.

The close pass of 2011 MD changed the asteroid’s aphelion distance from about 1.05 to 1.10 astronomical unit (about 5 million miles) and altered its orbital inclination.

Even though 2011 MD never got brighter than about 11th magnitude, its close flyby did trigger some interesting changes.

First, the asteroid’s orbit was yanked around quite a bit. Not only did it pass very close to Earth — well inside Earth’s ring of geosynchronous satellites on its outgoing leg — but the asteroid also sped by relatively slowly. This put it within our planet’s gravitational grip long enough to bend its trajectory significantly, causing the orbit to expand outward, as shown at right.

Steven Chesley, a member of the Jet Propulsion Laboratory’s team of solar-system dynamicists, calculates that 2011 MD’s trajectory was bent by 130°. “I don’t recall ever seeing such a large turning angle for any other object,” notes JPL’s Paul Chodas. The close pass also reoriented the orbit’s tilt by more than 5°, according to Andrea Milani, a near-Earth asteroid (NEA) specialist at the University of Pisa.

But a second consequence of the close pass has more to do with how Chesley, Chodas, and Milani do their computations — and showed that a little tweaking was in order.

During 2011 MD’s close flyby in late June, Earth’s gravity bent the little asteroid’s trajectory by about 130°. As a consequence, the inclination, or tilt, of 2011 MD’s orbit changed by several degrees.

Soon after the flyby, as the asteroid receded into the depths of space, observers noticed that 2011 MD wasn’t exactly following its calculated escape route. In some cases the positional mismatch was as great as 20 arcseconds — shockingly bad, given the all the precise positional data reported by professional and amateur observers worldwide.

It didn’t take long to track down the error’s cause. “The passage of 2011 MD was such a close approach that the orbit was significantly affected by the shape of the Earth,” Milani explains. Our planet isn’t a perfect sphere but instead is slightly oblate — squashed pole to pole by about 26½ miles (42½ km) relative to its equator, about one part in 300. This slight out-of-roundness causes, in turn, slight deviations from a perfectly spherical gravitational field, which geophysicists adjust for using a fudge factor known as J2.

Once dynamicists recalculated 2011 MD’s trajectory with J2 included, the positional errors reported by observers largely disappeared. So why weren’t the calculations done this way to begin with? “The answer is that it is an very insignificant term for almost all objects,” Chodas explains, “and yet it would add somewhat to the computational load. The object has to make an extremely close approach to the Earth for this term to make a difference, say, within 10 Earth radii,” or about 40,000 miles.

“Never before 2011 MD has an asteroid passed at a few Earth radii and been observed both before and after the encounter,” Milani points out.

So even though June’s interloper never posed a threat to Earth (nor will it in the foreseeable future, according to both JPL and NEODyS), its visit taught the world’s asteroid watchers a useful lesson that will pay dividends during future close calls.

As a consequence, the NEODyS asteroid-tracking system maintained by Milani and others has been tweaked. “We have implemented a model of Earth’s gravity field including oblateness,” he reports, “which kicks in only when the distance from the geocenter is less than 0.001 astronomical unit,” or about 90,000 miles. The JPL modelers will likewise invoke J2 as needed.

“Our work with NEA orbits and impact monitoring is research work, not routine, even though we have been doing it for more than a decade,” comments Milani. “These cases in which we have to upgrade the software, although not frequent, keep happening — and we do not expect they will stop, because we are certainly still in the learning phase.”

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