Near-Earth objects are Asteroid and comets with orbits that bring them to within 120 million miles (195 million kilometers) of the Sun, which means they can circulate through the Earth’s orbital neighborhood. Most near-Earth objects are asteroids that range in size from about 10 feet (a few meters) to nearly 25 miles (40 kilometers) across.
Asteroid impacts can be catastrophic for Earth and everything that calls it home – just ask the dinosaurs. That doomsday rock is believed to have been around 10 km (6.2 miles) wide, but even a much smaller asteroid could do some serious damage. The meteor that exploded over Chelyabinsk, Russia in 2013 was estimated to be just 20 m (66 ft) wide, and it still injured almost 1,500 people. A rock measuring a few hundred meters could wipe a city off the map with an explosion many times greater than the bomb dropped on Hiroshima.
The majority of near-Earth objects have orbits that don’t bring them very close to Earth, and therefore pose no risk of impact, but a small fraction of them – called potentially hazardous asteroids – require more attention.
These objects are defined as asteroids that are more than about 460 feet (140 meters) in size with orbits that bring them as close as within 4.6 million miles (7.5 million kilometers) of Earth’s orbit around the Sun. CNEOS continuously monitors all known near-Earth objects to assess any impact risk they may pose.
Of course, to pull that off we would need to know about it in advance, and so the Center for Near Earth Object Studies (CNEOS) calculates the orbit of every known near-Earth asteroid (NEA), a number that’s approaching 28,000 objects. Then it’s determined whether they pose a threat to our home planet, using JPL software called Sentry.
“The first version of Sentry was a very capable system that was in operation for almost 20 years,” says automation engineer Javier Roa Vicens, now with SpaceX’s Starlink, who led the development of Sentry-II at NASA’s Jet Propulsion Laboratory (JPL).
“It was based on some very smart mathematics. In under an hour, you could reliably get the impact probability for a newly discovered asteroid over the next 100 years – an incredible feat.”
Sentry was very effective at calculating orbital paths based on how an asteroid is affected by the gravitational pull of the Sun and planets, but there were a few factors that it couldn’t account for. In the long run, these uncertainties can snowball into many possible orbits that may or may not impact Earth.
The orbital positions of near-Earth objects come from the databases of the Minor Planet Center, the internationally recognized clearinghouse for small-body position measurements. This data is collected by observatories around the world, including significant contributions from amateur observers.
Whenever a new near-Earth asteroid (NEA) is detected, astronomers get to work on figuring out its most likely orbit around the Sun, based on its position and velocity, as well as the gravitational effects of other objects in the Solar System.
The Yarkovsky effect, for instance, is where the Sun unevenly heats the surface of an asteroid as it spins, creating thermal forces between the “day” and “night” sides of the rock that can produce thrust. Other times, asteroids that swing past Earth very closely could be nudged into different orbits by the planet’s gravity, changing the paths of their eventual return.
The Center for Near-Earth Object Studies is home of the Sentry impact-monitoring system, which continuously performs long-term analyses of possible future orbits of hazardous asteroids. There is currently no known significant threat of impact for the next hundred years or more. The Center also maintains the Scout system that continually monitors brand-new potential near-Earth object detections, even before they have been confirmed as new discoveries, to see whether any of these generally very small asteroids might pose a threat of short-term (possibly imminent) impact.
The first Sentry system couldn’t incorporate either of these two factors, meaning that for special case asteroids like Bennu or Apophis, astronomers would have to manually analyze their orbits, which is a complex and time-consuming process.
But Sentry-II is designed to account for things like these. This latest version uses a different algorithm that models thousands of random points within the uncertainty space of an asteroid’s orbit, then figures out which ones have a chance of striking Earth in future. This, the team says, could help find scenarios that have very low probability of impact.
Even without the new and more powerful space observation equipment mentioned by Vicens, we’re already detecting around 3,000 NEAs every year, with a running total in the region of 28,000. That’s a lot of asteroids to try and keep track of.
Another improvement in the new system is a better method for tracking asteroids that pass very close to Earth. Our planet’s gravitational pull can cause a lot of uncertainty in terms of asteroid trajectories, but Sentry-II is better set up to factor in this gravity.