View from Mars Hill: Detecting an intruder from outer space
VIEW FROM MARS HILL

View from Mars Hill: Detecting an intruder from outer space

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Meteorites

Twelve of the recovered Dishchii'bikoh meteorites found in June 2016.

Like a detective fixated on deciphering the sequence of events in a crime, astronomers use every available tool in their arsenal to build an understanding of the universe. Such is the case with Lowell Observatory’s Nick Moskovitz and fellow celestial sleuths as they looked to profile an intruder from outer space. Incorporating a variety of analytical techniques, applied to both direct observations of the event and forensic evidence collected afterwards, they’ve been able to characterize a 2016 meteorite fall in eastern Arizona.

The story starts on June 2, 2016, when at 3:56 in the morning, a super-bright meteor — commonly known as a fireball — streaked through the skies of eastern Arizona. More than 400 people, who for whatever reason were awake at that time, witnessed the event and reported it to the American Meteor Society, a non-profit scientific organization that collects data and encourages research on such meteoritic phenomena.

Several of the witnesses photographed the streaking light, and the event was also captured on sky-surveying cameras set up in Flagstaff, Payson, and Albuquerque. The Flagstaff camera system is run by Moskovitz and goes by the acronym LO-CAMS, for “Lowell Observatory Cameras for All-sky Meteor Surveillance.” It is based off a similar concept developed by Peter Jenniskens of the SETI Institute in Mountain View, California and consists of arrays of 16 off-the-shelf security cameras to gather what Moskovitz calls a “fly’s eye approach to recording the night sky.”

Each of the cameras records a different part of the sky and the observations are then stitched together. This accurately records — at 30 frames per second — trajectories for hundreds of meteors per night, which allows scientists to determine where they came from in the solar system.

Moskovitz currently helps to operate four LO-CAMS stations, but at the time of the 2016 event only two were yet in operation — at Lowell Observatory’s center of operations on Mars Hill in Flagstaff and at the Lowell Discovery Telescope site 40 miles outside of town. Both captured images of this fireball meteor. In fact, while the system had imaged scads of meteors up to that point, this was the first time it captured an actual meteorite fall, when pieces of the intruding debris survive entering Earth’s atmosphere and crash onto the planet.

Observations from the LO-CAMS arrays, as well as from the Payson and Albuquerque cameras, allowed Moskovitz and a collaboration led by Jenniskens to determine that the intruding rock was traveling at a speed of 16.6 kilometers (10 miles) per second when it entered Earth’s atmosphere. From this number they initially estimated the meteoroid’s mass to be around 15,000 kilograms (33,000 pounds), with a diameter of about 2 meters (6 ½ feet; though later estimates reduced the probable size to about 80 centimeters/31 inches).

As the meteoroid nosedived toward Earth, it was spinning more than twice per second and ultimately broke into many small fragments. Mimicking hail, they reflected Doppler weather radar signals. This allowed scientists to estimate where the pieces actually hit Earth — within the confines of the Fort Apache Indian Reservation. Before the month was out, an Arizona State University-based search party had formed, acquired permission from the White Mountain Apache Tribe to enter the lands, and recovered 23 meteorites from the fall — a testament to the precision of the scientists’ observations and calculations.

Following the suggestion of White Mountain Apache Tribe members, scientists christened the meteorite material Dishchii’bikoh Ts’iłsǫǫsé Tsee, which in the Apache language means, “Cibecue Star Stone” (Dishchii’bikoh = Cibecue, a community near the meteorite recovery zone; Ts’iłsǫǫsé Tsee = star stone). In scientific discussions and publications, the name has been shortened to Dishchii’bikoh.

Analysis of the meteoritic material resulted in Moskovitz and his team characterizing the parent meteoroid as an “LL chondrite”, a type of stony (as opposed to iron) meteorite. A little less than 10% of all meteorite falls are of this LL chondrite type. They originate from a parent body located in the asteroid belt — between the orbits of Mars and Jupiter — that broke up into a collection of smaller asteroids. Over time some of them inevitably collide, throwing rocks towards the inner part of the solar system and, as in the case of Dishchii’bikoh, toward Earth.

The most famous example of an LL chondrite is the 20-meter-diameter (66-foot) Chelyabinsk meteor that exploded over the skies of Russia in 2013. Evidence collected of these two LL chondrites suggest that Dishchii’bikoh and Chelyabinsk may very well have originated from the same parent body, probably toward the inner region of the asteroid belt.

Dishchii’bikoh is just the first meteorite fall detected by LO-CAMS. Nick Moskovitz and his fellow celestial sleuths look forward to more down the road in their quest to decipher the mysteries of the universe.

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