The Latest Head Count Of Solar System Asteroids

Astronomers, religious leaders, soothsayers, astrologists and people like me have viewed the night skies for millennia and before telescopes were invented. They saw the wandering planets changing position as the nights passed. They saw the Moon go through its phases every 28 days giving us our 7-day weeks. Occasionally, a comet would appear leading to portentous omens.

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:

• Carbonaceous asteroids – make up 75% of those discovered to date. They are composed of water ice, carbon compounds, organics, clay and silicate rocks. They are dark in appearance and difficult to detect.
• Silicaceous asteroids – are the second most common and consist of nickel-iron, olivine, pyroxene, and silicate rocks. They are easier to detect because they reflect 20% of the sunlight that falls on them.
• Metallic asteroids – are the least common and are made up of iron, nickel, gold, silver and platinum. They reflect from 10 to 18% of sunlight. They are highly prized by future Solar System miners.

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.

Near-Earth Asteroids Pose A Threat

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:

• Chelyabinsk-sized – are asteroids that measure between 19 and 44 metres (62 to 144 feet). There are an estimated 2.5 million of them with 0.1% having known whereabouts.
• Tunguska-sized – are asteroids named after the one that crashed into a Siberian forest in 1908. It was estimated to be 45 metres (148 feet) in length. Tunguska asteroids go from 44 to 140 metres (144 to 459 feet) in size and have been labelled “city killers.” There are an estimated 500,000 of these with only 1% known and tracked.
• NASA-targeted – are asteroids that vary from 140 metres to 1 kilometre (459 to 3,280 feet) in size. There are 25,000 of them with 30% known and tracked.
• Big asteroids – are over a kilometre in size which the Institute calls “civilization enders.” There are 1,000 of them with 93% known and tracked.

Visiting Asteroids Rather Than They Visiting Us

In pursuit of understanding their nature, several robotic spacecraft missions have visited asteroids. These include:

• NASA’s Dawn – a mission launched in 2007 that visited two of the largest Asteroids in the Belt: potato-shaped Vesta in 2011 and spherical Ceres in 2015.
• Japanese Space Agency (JAXA) Hayabusa2 – a mission that visited the asteroid Ryugu and returned samples from its surface to Earth in 2020.
• NASA’s OSIRIS-REx – a mission to the asteroid Bennu in 2020 that returned even more surface samples to Earth in 2023. With OSIRIS-REx completed, the spacecraft was redirected to rendezvous with another asteroid in 2029. That asteroid is named Apophis and the new mission is called OSIRIS-APEX.
• NASA’s DART – DART stands for Double Asteroid Redirection Test. The spacecraft rendezvoused with the dual asteroids Didymos and Dimorphos in 2022 and then crashed into the latter to see if the impact could alter its trajectory. It appears to have had some effect lending credence to a planetary defence strategy that could be used in the future.

Two more missions have recently got underway. They are:

• European Space Agency (ESA) Hera – a mission launched last month to do a close-up study of the results from the DART redirect mission. Hera will rendezvous with Didymos and Dimorphos in 2026.
• NASA’s Psyche – a mission launched last month to rendezvous and study a metallic asteroid, 16 Psyche beginning in 2029.

Future missions in development include two:

• United Arab Emirates – a planned 2028 visit to the Asteroid Belt rendezvousing with seven rocks and landing on 269 Justitia by 2034.
• JAXA’s MMX – an oft-postponed mission that was originally planned for launch this year but likely won’t happen earlier than 2026 or 2028. MMX plans to visit the two Martian moons, Phobos and Deimos. Both appear to be asteroids captured by Mars’ gravity. MMX would collect a surface sample from Phobos to return it to Earth by the mid-2030s.

The Current Asteroid Head Count

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).