What part of a space rock survives to the ground?

Newswise – August 8, 2022, Mountain View, CA – When a small asteroid enters Earth’s atmosphere from space, its surface is brutally heated, causing it to melt and fragment. Therefore, why the rocks near the surface survive as meteorites down to the ground has been a mystery. That mystery is solved in a new study of the fiery entry of asteroid 2008 TC₃published online today in Meteorology and planetary science.

“Most of our meteorites fall from grapefruit-sized rocks onto small cars,” says lead author and meteorastronomer Peter Jenniskens of the SETI Institute and NASA Ames Research Center. “Rocks that large don’t spin fast enough to disperse heat during the short meteor phase, and we now have evidence that the backside survives to the ground.”

In 2008, a 6 meter asteroid named 2008 TC₃ was spotted in space and tracked for over 20 hours before impacting Earth’s atmosphere and producing a bright meteor that dissipated over the Nubian Desert in Sudan. The explosion scattered a shower of meteorites over an area 7 x 30 km. Jenniskens worked with Muawia Shaddad, a professor at the University of Khartoum, and his students to recover these meteorites.

“In a series of targeted search campaigns, our students recovered over 600 meteorites, some the size of a fist but most no bigger than a thumbnail,” says Shaddad. “For each meteorite we recorded the location.”

Conducting grid searches perpendicular to the asteroid’s orbit, the researchers were surprised to find that the larger, fist-sized meteorites were more widely distributed than the smaller meteorites. Working with NASA’s Asteroid Threat Assessment Project (ATAP) at the Ames Research Center, they decided to investigate.

“As the asteroid neared Earth, its brightness flickered as it twisted and tumbled,” says ATAP theoretical astronomer Darrel Robertson. “For this reason, Asteroid 2008 TC₃ is unique in that we know the shape and orientation of the asteroid as it entered Earth’s atmosphere.”

Robertson created a hydrodynamic model of the 2008 TC inflow₃ into Earth’s atmosphere, which showed the asteroid melting and breaking up. The observed heights of meteor brightness and dust clouds were used to calibrate the height of the phenomena detected in the model.

“Because of the high velocity that was coming in, we found that the asteroid almost slammed a vacuum wave into the atmosphere,” says Robertson. “The first fragments came from the sides of the asteroid and tended to travel into that wake, where they mixed and fell to the ground at low relative speeds.”

As they fell to the ground, the smallest meteorites were soon stopped by friction with the atmosphere and fell close to the point of departure, while larger meteorites were harder to stop and fell further down. As a result, most of the recovered meteorites were found along a narrow 1 km wide strip in the asteroid’s path.

“The asteroid melted more and more at the front until the surviving portion at the back and bottom of the asteroid reached a point where it suddenly collapsed and broke into many pieces,” Robertson said. “The fact that the lower defender survived so long was due to the shape of the asteroid.”

No longer caught in the shock of the asteroid itself, the shocks of each piece now repelled them, sending these final fragments outward at much higher relative speeds.

“The largest meteorites of 2008 TC₃ were more widely dispersed than the small ones, meaning they stem from that final collapse,” Jenniskens said. “Based on where they were found, we concluded that these pieces remained relatively large down to the ground.”

The position of the large meteorites on the ground still reflects their position in the back and lower back of the original asteroid.

“This asteroid was an eclectic mix of rocks,” said co-author Cyrena Goodrich of the Lunar and Planetary Institute (USRA). Goodrich led a team of meteorologists who determined the meteorite type of each recovered fragment in the large mass area.

The researchers found that the different meteorite types were randomly distributed on the ground and therefore also randomly distributed in the original asteroid.

“This is consistent with the fact that other meteorites of this type, albeit on a much smaller scale, also contain random mixtures,” Goodrich said.

These results can also help to understand other meteorite falls. Asteroids are exposed to cosmic rays in space, creating low levels of radioactivity and more near the surface.

“Based on this radioactivity, we often find that the meteorites did not come from the better-shielded interior,” Jenniskens said. “We now know they came from the surface on the back side of the asteroid.”

More information
Peter Jenniskens, Darrel Robertson, Cyrena A Goodrich, Muawia H Shaddad, Ayman Kudoda, Anna M Fioretti, Michael E Zolensky (2022) Bolide fragmentation: What parts of asteroid 2008 TC₃ survived to the ground? Meteorology and planetary science:

https://onlinelibrary.wiley.com/doi/10.1111/maps.13892

About the SETI Institute
Founded in 1984, the SETI Institute is a nonprofit, multidisciplinary research and education organization whose mission is to lead humanity’s quest to understand the origins and distribution of life and intelligence in the universe, and to share that knowledge with the world. Our research spans the physical and biological sciences and leverages data analysis, machine learning and advanced signal detection technologies. The SETI Institute is a respected research partner to industry, academia, and government agencies, including NASA and the National Science Foundation.