Over 30,000 near-Earth asteroids have been found by astronomers, and the number is growing at an exponential rate. Here is how they are found and how researchers determine if they pose a hazard to Earth.
An asteroid must pass within 1.3 Astronomical Units (AU) of the Sun to be designated as a near-Earth object (NEA). The separation between the Sun and our planet is one AU. Over 30,000 NEAs have been found so far, with the majority of those discoveries occurring in the previous ten years, according to the European Space Agency (ESA).
The majority of the one million or so NEA asteroids that we have so far found in the solar system—roughly one-third of them—live in the asteroid belt between Jupiter and Mars. Since the first asteroid, Ceres, was found by Giuseppe Piazzi in 1801, astronomers have been cataloguing asteroids. The first NEA was found on August 13, 1898, about a century later, as 433 Eros.
Since larger asteroids are considerably easier to observe, it makes sense that they were the first to be discovered. However, with to recent developments in telescope technology, we are discovering more asteroids at a far faster rate, some of which are tens of metres in size.
Every week, new asteroids are found by ground-based survey telescopes, which are built to sweep broad areas of the sky. They accomplish this by searching for new moving objects against a background of mostly stationary stars. The Catalina Sky Survey in Arizona is a prime instance.
More “focused,” bigger telescopes, like the Very Large Telescope in Chile, can be utilised to make additional studies when an asteroid is found. This provides information on the asteroid’s course, size, and even hints about its composition.
However, not all of these asteroids are close to the Earth. The majority of them have trajectories that do not bring them close to Earth, and the majority of those that do are so small that they will burn up as soon as they approach the atmosphere of our planet. However, NASA classifies asteroids larger than 140 metres in size and with a probability of approaching Earth within 0.05 AUs as potentially hazardous asteroids, or PHAs.
This is where initiatives like Gaia may be beneficial. One billion stars in the galaxy are being catalogued by the ESA’s Gaia project. Gaia has helped scientists learn more about the stars that surround asteroids. In turn, this makes it simpler to determine the locations of asteroids relative to these background stars.
Fortunately, for the next one hundred years, none of the near-Earth asteroids that have been found poses a threat. Even though some tiny objects will and occasionally do reach our planet, their impact is little to nonexistent. The worst they can do is burn up upon entering our atmosphere, leaving behind trails of “shooting stars.”
The bulk of the largest and most destructive asteroids that are larger than one kilometre have been found, and none of them poses a threat for the next century, according to the ESA. It is hoped that projects like NASA’s DART will pave the way for a time when, even if asteroids threaten Earth, we will have the technology to divert them away from it.