Asteroids

Although the sky falling on our heads is not of immediate concern, fragments of comets and asteroids continuously bombard our planet. As of today, there are over 24 000 Near-Earth asteroids. Of those, less than 10 000 are larger than 140 meters in size and 891 are larger than 1 kilometer in diameter. On its Earth Impact Monitoring page, NASA lists the impact probability for some known objects that may pose a threat. In the near future, 2009 JF1’s trajectory is predicted to be the closest to Earth in May 2022. It is a small asteroid about 13 meters in diameter whose odds to hit Earth are calculated at 0.026%. While Bennu poses a threat 150 years from now, its diameter is half a kilometer, and its chance of Earth impact is higher at 0.037%. In the goal to demonstrate asteroid deflection for the first time, the binary near-Earth asteroid (65803) Didymos is the target of the DART spacecraft. The Double Asteroid Redirection Test (DART) mission will be launched in the summer of 2021 and will deliberately crash into the binary’s moonlet Dimorphos a year later to alter its speed. As a result, it is expected to change the binary’s trajectory.

NEODyS is an ESA-sponsored site with a risk list of 1128 Near-Earth asteroids. Just in the next 30 days, 20 known asteroids will move closer to the Earth. The Jet Propulsion Laboratory (JPL), a national research center managed for NASA by Caltech, provides orbit diagrams for their trajectory. Most objects today burn up in our atmosphere. Some, however, particularly those larger than a few meters, could pose a threat not only to the Earth but to other planets of the solar system. Every two years, asteroid experts from across the globe come together to simulate a fictional but plausible imminent asteroid impact on Earth.

While some disintegrate before our eyes, there are still objects unaccounted for that may be outside the range of observation, the limit of detection or the capabilities for identification, such as 2014 AA that hit the Earth’s atmosphere over the Atlantic Ocean about 20 hours after its discovery. While four, including 2014 AA, have reached the Earth’s surface over the past ten years, the asteroid named 2020 HS7 came close to the geostationary ring. Since the 14th of November 2020, another small asteroid, about 10-meter long, now designated 2020 VT4, holds the record of the closest non-impacting asteroid. Its closest approach happened about 370 km above the Pacific Ocean to the East of French Polynesia.

I have previously mentioned that asteroids are an incredible source of information in regard to any elemental and molecular signatures of life. Terrestrial impacts are believed to occur before the onset of significant environmental and biological changes. Most asteroids have irregular shapes. Scientists evaluate their brightness, geometric form, diameter and rotation period. The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) conducts an all-sky infrared survey to gather data for the past seven years. Some objects, though, have dynamical properties that fall outside the window detectable to NEOWISE. A manual search through over five years of NEOWISE survey data has resulted in the discovery of 299 more Near-Earth objects.

Evidence of cataclysmic events on the Earth fades away because of weathering and erosion. A massive global-scale erosion event coincident with major episodes of globally extensive “Snowball Earth” glaciation resulted in the almost complete absence of terrestrial craters older than 650 million years. Since the Earth-Moon system has been co-evolving over 4.5 billion years, asteroid showers have occurred not only on the Earth but also on the Moon. Traces of meteorites and lunar craters are evidence of those past events. Asteroid evolution models suggest that the contribution of kilometer-sized impactors from a large parent-body disruption would have reached their new level within a few tens of millions of years of the breakup events, with the wave of bodies perhaps receding after hundreds of millions of years. Disrupted asteroids within families provide opportunities to study disruption processes for which real-time observations are relatively lacking.

Since March 2017, ESA’s Near-Earth object Lunar Impacts and Optical TrAnsients (NELIOTA) has detected over one hundred flashes of light produced when asteroids collide with the lunar surface, where they are easiest to observe -- on the dark side of the Moon not illuminated by the Sun. A 2020 report studied 59 lunar craters whose production stretches over 3 billion years based on two models. The Eulalia family, considered to be the parent body of asteroids Bennu and Ryugu27, could have produced an impact spike between 830 and 800 million years ago, resulting in the formation of small lunar craters as well as on other terrestrial planets and satellites inside the asteroid belt. The paper concludes that an object 30 to 60 times greater in mass than the Chicxulub asteroid impactor -- that changed the course of our planet history 66 million years ago -- must have collided on the Earth around 800 million years ago. However, the report raises the possibility that some lunar craters, including Copernicus named after the famous astronomer, were actually formed around 660 million years ago when sporadic meteorite bombardment occurred across the whole Moon, possibly due to the disruption of asteroids.

Another report published last year suggested that the impact rate went up within the past 500 million years with an increase of the Earth and Moon impact flux by a factor of 2.6 about 290 million years ago. In comparison, there was a lower impact flux between 300 million and 650 million years ago. Besides the Eulalia family mentioned above, there is among many other groups of asteroids the Flora family, a primary source of present-day Near-Earth asteroids and Earth and Moon impactors. A 2017 paper found that 700-950 and 35-47 kilometer-sized asteroids struck the Earth and Moon, respectively, most within the first 300 million years after family formation, which happened at least one billion years ago. 

Not all end up passing by or burning up in the atmosphere. A tiny fraction of asteroids sticks around as they become temporarily captured by the gravity of Earth-Moon system. The capture is always temporary due to interactions with the gravity of the Sun and other massive solar system bodies.  An asteroid --known as Minimoon 2020 CD3 -- was captured by the gravitational field for two years until it appeared to have left the Earth-Moon system in mid-2020. It had a longer duration than the 9-month capture duration averaged over the minimoon population. Determining its origin remains unclear. 2020 CD3 may have been the product of the fragmentation of a larger parent asteroid from the inner Main Belt.