Yesterday, the media reported widely on the upcoming launch of OSIRIS-REx. While many of these reports were accurate and informative, a few of the headlines were a little misleading. For example:
Fortunately, as the news cycle progressed, these inaccuracies were corrected:
I thought it would be useful to summarize the effects of a Bennu impact. The Minor Planet Center classifies Bennu as a Potentially Hazardous Asteroid because it has a diameter larger than 150 m and a minimum orbit intersection distance (MOID) of < 0.05 Astronomical Units (abbreviated AU and equal to the average distance between the Earth and Sun) with the Earth. Bennu has a mean diameter of 492 meters and its MOID is 0.003 AU. The MOID will steadily decrease resulting in a potential Earth impact during the later decades of the 22nd century. The tabulation of potential Earth impacts results in a cumulative impact probability of approximately 1 in 2700 sometime in the 2175– 2196 time frame.
I used the Impact Earth! calculator provided by Purdue University and Imperial College London to assess the results of a Bennu impact. For these calculations I used the following input values:
Projectile Diameter: 500 meters
Density: 1,260 kilograms per cubic meter (kg/m3)
Angle of Impact: 45˚
Velocity: 13 kilometers per second (km/s)
Target Type: Crystalline Rock
I varied the “Distance from Impact” value between 5 and 500 km (3.1 – 310 miles) to determine the effects as a function of distance from the impact site. I also ran one calculation for an impact into 1000-m of water, to determine the size of the resulting tsunami.
Here are the results of the calculations:
During atmospheric entry, Bennu would begin to break up at an altitude of 71,900 meters (236,000 feet). The asteroid mass would hit the surface of the Earth at a velocity of 12.1 km/s (over 27,000 miles per hour). The impact would release energy equivalent to 1,450 megatons of TNT. For comparison, the fission bombs used in World War II had an energy release of roughly 20 kilotons of TNT each and the most powerful nuclear weapon ever detonated, the Russian “Tsar Bomba”, had a yield of 50 megatons. The total energy expended during all nuclear testing throughout history is estimated at 510 megatons of TNT. So, a Bennu impact would release 3x more energy than all nuclear weapons detonated throughout history. For reference, the average interval between impacts of this size somewhere on Earth during the last 4 billion years is once every hundred thousand years.
Effect on the Earth
The effect of a Bennu impact on Earth would be insignificant on a planetary scale. The Earth is not strongly disturbed by such an impact and loses negligible mass. The impact does not make a noticeable change in the tilt of Earth’s axis and it does not shift the Earth’s orbit noticeably. Local effects, however, would be much more noticeable.
A Bennu impact would create an initial transient crater with a diameter of 3.85 km (2.39 miles) and a depth of 1.36 km (0.846 miles). After everything had settled down, the final crater diameter would be 4.62 km (2.87 miles) with a depth of 469 meters (1,540 feet). For reference, Meteor Crater in Arizona is about 1,200 m (3,900 feet) in diameter and 170 m (560 feet) deep.
Unlike Meteor Crater, the crater formed by a Bennu impact would be a complex crater. This means that it would have an uplifted central peak, a broad flat shallow crater floor, and terraced walls. The volume of the target rock that is melted or vaporized is 0.0382 km3 (0.00916 miles3). Roughly half the melt remains in the crater, with an average thickness of 3.27 meters (10.7 feet).
Five Kilometers Away
At a distance of 5 km (3.1 miles) away, the ejecta would arrive approximately 32 seconds after impact. This location would be buried by impact ejecta to a depth of 15.7 meters (51.6 feet). The mean diameter of the falling fragments would by 80 meters across (262 feet). At this impact velocity, little vaporization occurs and no fireball is created. Therefore, there is no thermal radiation damage.
The impact would trigger an earthquake with a magnitude of 6.7 on the Richter Scale. At 5-km distance the major seismic shaking would arrive approximately one second after impact. From the earthquake alone, damage would be negligible in buildings of good design and construction, slight to moderate in well-built ordinary structures, and considerable in poorly built or badly designed structures.
An air blast would arrive approximately 15.2 seconds after impact. The maximum wind velocity would reach 1,660 m/s (3,720 miles per hour). The sound Intensity would exceed 133 dB, which is dangerously loud. The air blast would cause significant damage. Under these conditions, multistory wall-bearing buildings will collapse. Wood frame buildings will almost completely collapse. Multistory steel-framed office-type buildings will suffer extreme frame distortion and experience incipient collapse. Highway truss and girder bridges will collapse and glass windows will shatter. Cars and trucks will be knocked over and grossly distorted. Up to 90 percent of trees will be blown down with the remainder stripped of branches and leaves.
Fifty Kilometers Away
At a range of 50 km (31 miles) from the impact, the ejecta will arrive approximately 1.69 minutes after the impact. There would only be a fine dusting of ejecta with occasional larger fragments. The average ejecta thickness would reach 1.57 cm (0.62 inches) with a mean fragment diameter of 17.9 cm (7.04 inches). The major seismic shaking would arrive approximately 10 seconds after impact. At this range, building damage is negligible from the seismic shaking in buildings of good design and construction, slight to moderate in well-built ordinary structures, with considerable damage in poorly built or badly designed structures. The air blast would arrive approximately 2.53 minutes after impact with a maximum wind velocity of 69.9 m/s (156 mph) and a sound intensity of 91 dB (loud enough to cause ear pain). The air blast would cause wood frame buildings to almost completely collapse, glass windows to shatter, and up to 90 percent of trees to be blown down with the remainder stripped of branches and leaves.
If Bennu impacted into the ocean, a tsunami would be generated. At a distance of 50 km (31 miles) from the impact site, the impact-generated tsunami wave arrives approximately 8.75 minutes after impact. The tsunami wave amplitude is between 28.2 meters (92.4 feet) and 56.3 meters (185 feet).
Five-hundred Kilometers Away
Life gets a little better for those who witness the impact from a range of 500 km (310 miles) away. At this distance the ejecta would arrive approximately 5.57 minutes after the impact. There would only be a fine dusting of ejecta with occasional larger fragments. The major seismic shaking would arrive approximately 1.67 minutes after impact. The earthquake would be noticeable by people indoors, especially on upper floors of buildings but many people would not recognize it as an earthquake. The vibrations would be similar to that of a passing truck. The air blast would arrive approximately 25.3 minutes after impact with a wind velocity of only 3.13 m/s (7 miles per hour) and a sound Intensity of 63 dB, which is as loud as heavy traffic.
A Bennu impact would be a major natural disaster. The results would be felt for hundreds of miles with most of the damage concentrated within tens of miles of the impact site. The Earth would be largely unaffected, with the exception of a new geologic feature – the Bennu impact crater!