The authors needed at least two more observations to pin down Chelby’s orbit. Fortunately, videos from the city of Korkino (~35 kilometers south of Chelyabinsk) show Chelby passing through local zenith (straight up) when it exploded. Also, people found a hole in an ice-covered lake ~85 kilometers southwest of Chelyabinsk that wasn’t there before Chelby arrived. No other fragments of Chelby had been recovered and verified, so the authors assumed that this hole represents the intersection of Chelby’s trajectory with Earth’s surface. This is not certain! While Chelby’s center of mass should have maintained its trajectory after fragmentation, individual pieces of Chelby could have been quite dispersed.In this study, the authors used these three observations to constrain the location (height, latitude, and longitude) of Chelby at the brightening and fragmentation points. Using the timestamp on the security camera footage, they calculated Chelby’s average velocity, which they assumed is equal to its orbital velocity near Earth. They assumed that atmospheric friction is negligible because Chelby was moving so fast, although that’s just a naive approximation. With the position and velocity of Chelby at Earth fixed, the authors switched to numerical computations involving widely used software routines. Taking into account the gravity from Earth, Moon, and the other seven planets, they integrated Chelby’s orbit backwards for four years. Because each of the measurements used to reconstruct Chelby’s trajectory had some uncertainty, the authors computed a range of possible orbits. Specifically, they performed a suite of Monte Carlo simulations, in which they started their numerical integration of Chelby’s orbit from many different initial conditions, sampled randomly from the distribution of possible values.Preliminary Orbital ParametersScientists categorize near-Earth objects into many different groups. For example, the Atiras asteroids have orbital semi-major axes <1.0 astronomical units (AU), placing them entirely within Earth’s orbit. (Earth orbits the Sun at a distance of roughly 1 AU. We talk about semi-major axes because planetary orbits are elliptical.) The Amors group of asteroids, in contrast, orbit entirely between Earth and Mars. Two groups of asteroids regularly cross Earth’s orbit: the Atens and the Apollos. Asteroids in the Atens group have shorter semi-major axes than Earth, but they have eccentric orbits and hence pose a threat.
Chelby’s parent body, according to this study, was part of the Apollos—Earth-crossing asteroids with longer semi-major axes than Earth. Asteroids don’t necessarily stay in the same groups forever. For example, the near-Earth asteroid 2012 DA14, which passed by Earth as Chelby impacted, is currently part of the Apollo group, but gravitational interactions with other solar system objects will shift its orbit over the next decade or two, eventually pushing it into the Atens.
The semi-major axis of Chelby’s parent body is highly unconstrained because of the uncertainty in Chelby’s velocity. Incorporating data from more observations, along with detailed modeling of the effects of Earth’s atmosphere on Chelby’s trajectory, may allow a more precise determination of these orbital parameters. However, there is little doubt that Chelby was an Apollo, at least until recently.What About Next Time?Reconstructing Chelby’s orbit is a great detective story. Who doesn’t love using real science to interpret amateur video of giant explosions? But all of this is less than satisfying. After all, we’d like to know about giant killer rocks from space before they impact Earth. When the Big One comes, à la Armageddon, we won’t be able to rely on footage from security cameras and paranoid drivers, because said cameras and drivers will have perished in a conflagration. Fortunately, many people are very concerned about this problem and working to find realistic solutions. But that, dear readers, is a topic for a different post.