Many groups worldwide keep an eye on NEOs, near-Earth objects, asteroids and some comets that might come dangerously close to our planet. Surveys have discovered many of them, but a lot more are yet to be found – and some are hidden by the Sun's glare because they come from the inner solar system.

In a perspective published in the journal Science, astronomer Scott S. Sheppard from the Carnegie Institution for Science discusses the known and many unknown asteroids whose orbit is completely within Earth’s own.

Asteroids that never go further than Earth are known as Atiras (sometimes called Apohele). Asteroids with an orbit completely within Venus’s own are known as Vatiras, the first known was 2020 AV2, discovered two years ago. Examples of Atiras are 2019 AQ3 and also 2021 PH27, the fastest orbiting asteroid in the Solar System.

If these objects were original in the Main Asteroid Belt and then moved orbit, there can only be a small fraction of them, as their orbit will not stay stable for long. Atiras would make 1.2 percent and Vatiras only 0.3 percent of the total NEO population. However, these small groups might be hiding some dangerous objects.

“From NEO formation models and the current NEO survey efficiencies, more than 90% of planet-killer NEOs have been found (those larger than 1 km), although only about half of the city-killer NEOs are known (those larger than 140 meters). The last few unknown 1-km NEOs likely have orbits close to the Sun or high inclinations, which keep them away from the fields of the main NEO surveys,” Sheppard wrote in the perspective.

It is possible that there are stable asteroid reservoirs in the inner solar system which are yet to be discovered. Maybe between the orbit of Venus and Mercury, or maybe even closer to the Sun there could be Vulcanoids, completely within the orbit of Mercury. Finding these objects is very difficult but the so-called twilight surveys have found several and will continue to spot more.