What couldn’t be seen without the technology of more modern telescopes were the asteroids. These pieces of rock of diverse composition were orbiting our Sun just like Earth. They include:
The composition of asteroids appears to be determined largely by their location related to the Sun. Silicaceous asteroids can be found in orbits closer to Mars. Metallic asteroids occupy the middle ground beyond Mars and approaching Jupiter. Carbonaceous asteroids are closer to Jupiter and are most influenced in their distribution and location by that planet’s immense gravity.
When an asteroid exploded over the city of Chelyabinsk in 2013, it was unknown. It came from a direction not usually associated with orbiting objects. It raised a big red flag regarding planetary defence. Scientists and engineers started asking how many other Chelyabinsk-sized objects could be on future paths to intercept Earth and cause considerable damage.
The estimated size of the Chelyabinsk object was 19 metres (62 feet) in length. It entered the atmosphere at 17.7 kilometres per second (11 miles per second) and exploded 22.5 kilometres (14 miles) above the surface. The energy it released equalled 440,000 tons of TNT producing a shock wave that blew out windows in homes and buildings over a 518 square kilometre (200 square mile) area. Buildings were damaged and 1,600 were injured most by flying glass. Could there be another near-Earth object (NEO) ready to strike in our future?
In 2002, eleven years before Chelyabinsk, Dutch astrophysicist Piet Hut, planetary scientist Clark Chapman, and Apollo astronaut Rusty Schweickart established the B612 Foundation to study NEOs and develop the technology to alter their flight paths and safely steer them away from colliding with Earth.
The Foundation established the Asteroid Institute to bring scientists and engineers together to develop the tools and technologies to map and navigate the Solar System. The Institute uses a computer system called ADAM (Asteroid Discovery Analysis and Mapping) running an algorithm called THOR (Trackletless Heliocentric Orbit Recovery) program and Google Cloud. With these tools, it announced this year the identification of 27,500 previously unknown asteroids.
In the Institute’s latest report, it classifies asteroids by size:
In pursuit of understanding their nature, several robotic spacecraft missions have visited asteroids. These include:
Two more missions have recently got underway. They are:
Future missions in development include two:
According to NASA, the current asteroid count is more than 1.1 million with the vast majority located in the Asteroid Belt. Of these, 220 are larger than 100 kilometres (62 miles) in diameter. The largest, Ceres, was recently reclassified as a dwarf planet. At 939 kilometres (584 miles) in diameter, it is the only spherical asteroid having attained sufficient mass to establish hydrostatic equilibrium and accounts for nearly 40% of the total mass within the Asteroid Belt.
Nearer to Earth are the NEOs which exceed 28,700 in numbers. Of these, 878 have diameters of one kilometre (0.62 miles) or larger. The impact of a one-kilometre NEO would be global, similar to the one that struck 65 million years ago leading to the demise of the dinosaurs. It would cause shock waves, extreme winds, global temperature changes, earthquakes, tsunamis, and life-ending disruptions to plant photosynthesis.
Besides the B612 Foundation, in 2013, the International Asteroid Warning Network (IAWN) was founded to track NEOs larger than 10 metres (33 feet) and map 90% of those larger than 1 kilometre (0.62 miles).
]]>Leviticus Lewis, in charge of FEMA’s Response Operations Division, stated: “What emergency managers want to know is when, where and how an asteroid would impact, and the type and extent of damage that could occur.”
In June 2018, a U.S. interagency working group published the “National Near-Earth Object Preparedness Strategy and Action Plan,” a 10-year plan to locate and track Near-Earth asteroids (NEOs). The plan had five goals:
The exercise this week isn’t the first time NASA has been practicing to combat a potential NEO threat, but it is the first time that ESA has joined in planning and development of threat of or real impact strategies.
The current test involves an asteroid discovered 8 years ago that tracks around the Sun in a high inclination orbit compared to that of Earth’s orbital plane. The orbit is also egg-shaped. The combination of these two makes intercepting the asteroid to deflect or destroy it very difficult. This fictitious space rock in the scenario being tested is initially seen to have a 10% likelihood of striking the planet. But tracking it soon shows those likelihood to strike percentages rising. The decision to deflect requires looking at a number of options from the use of nuclear weapons to destroy it, or sending a spacecraft to deflect it.
The exercise also looked at what if the entire asteroid or a fragment struck the Earth after a deflection or destruction attempt. What kinds of emergency response would be needed to deal with the challenges that would present whether the impact occurred as an air burst, or a strike over land or water?
ESA, the European Space Agency, as its contribution to combatting a NEO strike, is developing the Asteroid Impact and Deflection Assessment (AIDA) Mission with a planned near-Earth encounter in 2022 with the binary asteroid Didymos. The plan is to send two spacecraft to Didymos, the first an asteroid rendezvous mission, the second an impactor developed by NASA and John Hopkins’ Applied Physics Laboratory. The launch of the rendezvous spacecraft would occur in 2020. The NASA-designed impactor would be launched later to arrive in 2022 and impact with the secondary Didymos asteroid at a speed of 6 kilometers (3.7 miles per second). The rendezvous mission will record the results of the impact and relay them to Earth stations for analysis. If successful the impactor will deflect the asteroid’s trajectory and allow us to track and measure the consequences of the test.
In making the call for a larger investment May points out “we are currently aware of less than one percent of objects comparable to the one that impacted at Tunguska, and nobody knows when the next big one will hit.” Tunguska refers to the explosion of an object over Siberia in 1908 some 7 to 10 kilometers above the Earth. The force of that blast flattened forests in an area over 2,000 square kilometers (800 square miles). The Tunguska object is estimated to have been only 50 meters (160 feet) in diameter.
Last year an estimated 17-meter (55-foot) object exploded several kilometers above the city of Chelyabinsk in Russia. The shockwave from the 500 kiloton blast injured 1,500 and caused extensive damage to buildings in the city. No one saw that meteor coming and it weighed 7,000 tons and exceeded 64,000 kilometers (40,000 miles) per hour in its descent into the atmosphere.
May and fellow scientists like Martin Rees, royal astronomer, point out that the problem is objects only a few meters across can cause extensive damage and those as big as 50 meters could flatten an entire city. Rees states that we must make it “our mission to find asteroids before they find us.”
NASA has been cataloging NEOs for the last decade-and-a-half in a program called Sentry. The NASA focus is more on large NEOs, the upper two categories seen in the census image below. These big ones (500 meters or greater) if they strike the Earth could cause an extinction-level event. But it is more than likely that we would see such large objects well ahead of time and launch an intercept to explode or deflect them. The little ones, the NEOs in the bottom category of which we have identified very few, will be as big as small buildings, and will virtually go undetected. These are the types of NEOs that conceivably hit Earth once in a century. Tunguska was the 20th century event. Chelyabinsk, much smaller, is probably not the only one that will strike us in the 21st century.
Today NASA’s Sentry uses ground-based telescopes. A privately funded space telescope called Sentinel is in development. It will be launched into a Venus-like orbit in 2019 and in a 6-1/2 year mission will use its onboard infrared cameras to catalog 90% of NEOs larger than 140 meters (approximately 450 feet) as well as smaller asteroids with diameters of 30 meters (100 feet). If you are interested you can donate to this project at the Sentinel Mission site.
One of the leaders in the group urging for increased investment in NEO detection is astronaut Edward Lu, CEO and Co-Founder of the Sentinel Mission. Lu describes what would happen if a 150-meter (approximately 500-fo0t) NEO were to impact with Earth. It would be equivalent to all the bombs dropped in World War 2 including the two atomic bombs dropped on Japan. And if we only detect an object that size a few weeks before impact it would be far too late for an intercept to deflect or destroy it. Detection of NEOs of that size has to happen decades before impact. Only then states Lu we can succeed in destroying them.
