Tuesday 12 March 2019

Looking for asteroids in 2018 LA-like orbits.

Asteroid 2018 LA was first observed by Richard Kowalski at the Mount Lemmon Survey in Arizona on 2 June 2018 (the designation 2018 LA implies the first object – object A – discovered in the first half of June 2018 – period 2018 L), and was quickly determined to be on a collision course with Earth. A subsequent fireball meteor (shooting star brighter than the planet Venus) was observed over South Africa and Botswana, on a course that matched that predicted for the asteroid, and fragments of meteorite recovered in the Central Kalahari Game Reserve in Botswana. This was only the third asteroid ever detected which subsequently impacted the Earth; the first, 2008 TC3 fell in South Sudan in 2008, a few hours after being discovered, while the second, 2014 AA, fell into the Atlantic Ocean in January 2014. All three of these bodies were small Apollo Group Asteroids (asteroids on Earth-orbit crossing trajectories which spend the majority of their time further from the Sun than the Earth), and were potentially fragments of larger bodies that might produce other Earth-impacting bodies.

Scientists from Botswana and South Africa at the location where a fragment of Asteroid 2018 LA was discovered this week. Botswana International University of Science and Technology.

In a paper published on the arXiv database at Cornell University on 28 November 2018, and in the journal Astronomy and Astrophysics on 10 January 2019, Carlos de la Fuente Marcos and Raúl de la Fuente Marcos of the Universidad Complutense de Madrid, publish an analysis of the orbit of 2018 LA, and show that it appears to be part of a distinct family of asteroids with similar Earth-crossing orbits and presumably a common origin.

2018 LA is estimated to have been on an orbit inclined at an angle of 4.30° to the plane of the Solar System, which took it in to a perihelion (closest point to on its orbit to the Sun) of 0.78 AU (i.e. 78% of the distance at which the Earth orbits the Sun) and an aphelion (furthest point on its orbit from the Sun) of 1.97 AU, with a semi-major axis (average distance from the Sun) of 1.38 AU. A search of the Small Body Database at NASA’s Jet Propulsion Laboratory revealed seventeen objects with similar orbits, of which de la Fuente Marcos and de la Fuente Marcos concentrate on five which have a particularly close match.

The calculated orbit of 2018 LA. Minor Planet Center.

The first of these is 2016 LR, which has an orbit tilted at an angle of 2.54° to the plane of the Solar System, a perihelion distance of 0.78 AU, an aphelion distance of 1.98 AU, and a semi-major axis of 1.38 AU. The second is 2018 BA5, with an orbit tilted at an angle of 4.54° to the plane of the Solar System, a perihelion distance of 0.78 AU, an aphelion distance of 1.96 AU, and a semi-major axis of 1.37 AU. The third is 2018 TU, with an orbit tilted at an angle of 4.07° to the plane of the Solar System, a perihelion distance of 0.78 AU, an aphelion distance of 1.98 AU, and a semi-major axis of 1.38 AU.

Finally, and most significantly, there is (454 100) 2013 BO73, a Potentially Hazardous Asteroid with an estimated equivalent diameter of 550 m (i.e. it is estimated that if it was a perfect sphere it would have a diameter of 550 m), which has an orbit tilted at an angle of 4.54° to the plane of the Solar System, a perihelion distance of 0.77 AU, an aphelion distance of 1.89 AU, and a semi-major axis of 1.33 AU.

The calculated orbit of (454 100) 2013 BO73. JPL Small Body Database.

Asteroids with common orbital paths are thought likely to share a common origin, typically deriving from the break-up of a larger parent body. This can happen in several ways. The most obvious of these is a collision with another object, though such events are not thought to be particularly frequent, due to the small size of asteroids compared to the volume of the Solar System. A more common cause of asteroid break-ups is thought to be tidal forces, which can tear small bodies apart when they come close to planets.

Finally, there is the Yarkovsky–O’Keefe–Radzievskii–Paddack, or YORP, mechanism, in which asteroids are torn apart by their own spin. This last seems somewhat unlikely, as it requires asteroids to spin faster and faster until they are ripped apart by centrifugal forces, but is probably one of the more common causes of asteroid disintegration. This is because asteroids (and everything else in the Solar System) are constantly buffeted by a stream of photons emitting from the Sun, photons which exert a very small, but not non-existent, pressure. However, this pressure is not equal on all surfaces, with darker surfaces tending to absorb photons, while lighter surfaces reflect them, with the upshot that lighter surfaces are pushed a little bit harder than darker ones. Since asteroids do not typically have homogenous surfaces, this means that some areas of the asteroid will be pushed slightly harder than others, causing them to spin, and since this difference is constant, over very long periods of time the asteroid will spin faster and faster, and since asteroids are often poorly consolidated piles of rubble, rather than single solid objects, this spin can often eventually tear them apart.

Since (454 100) 2013 BO73 was the largest body found in the survey, it seems possible that is could be the largest chunk of the body from which 2018 LA and the other bodies are derived. With this in mind, de la Fuente Marcos and de la Fuente Marcos tracked the orbital paths of (454 100) 2013 BO73 and 2018 LA backwards in time, looking for points at which they appeared to intersect (or at least come very close). This revealed numerous calculated close encounters between the two bodies between 500 and 1500 years ago, when the two bodies passed one another at their mutual perihelions, and close to the planet Venus, with relative velocities of about 2.6 km per second.

De la Fuente Marcos and de la Fuente Marcos then calculated the past orbit of 2016 LR, finding that it also had numerous close encounters with 2018 LA at perihelion between 500 and 1500 years ago. The orbit of 2018 TU was also found to follow this pattern, though with one notable difference, in that it would pass (454 100) 2013 BO73 with a relative speed of 21.9 km per second, a speed at which two bodies colliding might result in catastrophic fragmentation of one or both bodies.

If these bodies do all result from the break-up of a larger object, then there is likely to be a large amount of smaller debris as well, debris that would manifest itself as a periodic meteor shower were it to cross the Earth’s orbit (which it is predicted to do). De la Fuente Marcos and de la Fuente Marcos therefore looked for records of meteor showers with trajectories similar to 2018 LA, noting that the asteroid fell at the beginning of June and appeared to originate from the constellation of Scorpius. This search produced a single likely candidate, the χ-Scorpiids (Chi-Scorpiids), which fall between 28 May and 5 June. Tracking the orbital path of this meteor shower suggests that it too has a very similar trajectory to (454 100) 2013 BO73, with an orbit tilted at an angle of 6.8° to the plane of the Solar System a perihelion distance of 6.4 AU, and a semi major axis of 1.94 AU, suggesting that it might represent a stream of debris originating from that body.

De la Fuente Marcos and de la Fuente Marcos also note that 2018 UA, a near Earth object which came within 0.04 Lunar distances of the Earth on 19 October 2018 (making it the fourth-closest non-collisional close encounter ever recorded) also has an orbital path very similar to both that of (454 100) 2013 BO73 and of the χ-Scorpiid meteors, with a pre-encounter orbit (the path of the asteroid was altered by its encounter with the Earth.) tilted at an angle of 6.4°, and a semi-major axis of 1.93 AU, making it one of nine bodies with (454 100) 2013 BO73-like orbits.

The passage of 2018 UA past the Earth on 20 October 2018. Tom Ruen/Wikimedia Commons.

Finally de la Fuente Marcos and de la Fuente Marcos compared the number of observed objects on 2018 LA-like orbits to the number of such bodies predicted by the Near-Earth Object Population Observation Program, a computer model that can be used to generate random Near-Earth Orbits, finding that there were significantly more bodies on such orbits than would be predicted, after running the program 10 000 times. This statistical anomaly is present even though such bodies are on an Earth-crossing orbit that would be expected to deplete their numbers, both due to asteroids colliding with the Earth, and due to their being shifted onto new orbital trajectories by the Earth’s gravity.

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