NASA test planetary defence spacecraft to divert asteroid's path
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Paper author and astronomer Dr Scott Sheppard of the University of Hawaii said: “Asteroid surveys generally operate at night.” This, he explained, means they are “mostly finding objects beyond Earth’s orbit. This creates a blind spot because many near-Earth objects could be lurking in the sunlight, interior to Earth’s orbit.”
Near-Earth Objects can be classified based on the planetary orbits whose paths they cross.
For example, those bodies that approach the Earth but do not cross its orbit are known as the “Amors”, while the “Apollos” and “Atens” cross Earth’s orbit to varying degrees.
“Atiras”, meanwhile, have orbits that are contained completely within Earth’s orbit, and “Vatiras” have orbits that are completely interior to Venus.
Astronomers have also assigned the name “Vulcanoids” to the hypothetical asteroids whose orbits are entirely contained within that of the planet Mercury — however, these have yet to actually be observed.
According to Dr Sheppard, the technology does now exist to look for and identify near-Earth objects and calculate if their trajectories might pose a threat to life on Earth.
He said: “New telescopic surveys are braving the Sun’s glare and searching for asteroids toward the Sun during twilight.”
These recent discoveries include the first-ever–reported Vatiras which was discovered by the Zwicky Transient Facility, a wide-field sky survey undertaken from the Palomar Observatory in California, in the January of 2020.
This asteroid — which is estimated to be around 0.6 miles in diameter — has been named ꞌAylóꞌchaxnim, which means “Venus girl” in the Luiseño language of Southern California.
Another body, the Atira 2021 PH27, has the shortest-known orbital period around the Sun and was discovered by Dr Sheppard himself using Chile’s Cerro Tololo Inter-American Observatory, which has a “Dark Energy Camera” that can be pointed closer to the Sun.
Dr Sheppard explained that near-Earth objects have dynamically unstable orbits that last for around 10 million years before, for example, they disintegrate near the Sun or end up being ejected from the inner Solar System.
Despite this, however, the number of these bodies are believed to have remained stable for the past few billion years.
Dr Sheppard said: “A reservoir must exist that replenishes the near-Earth objects.
“Most near-Earth objects are likely dislodged objects from the main belt of asteroids between Mars and Jupiter.”
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Near-Earth objects get rarer the closer they get to the Sun, Dr Sheppard explained, “because it becomes harder and harder for an object to move inward past Earth’s and then Venus’ orbit.”
Atiras should account for only 1.2 percent of all near-Earth objects originating in the asteroid belt — and Vatiras only 0.3 percent.
Dr Sheppard said: “From near-Earth object formation models and the current […] survey efficiencies, more than 90 percent of planet-killer near-Earth objects have been found — those larger than 1km (0.6 miles).”
He added: “although only about half of the city-killer near-Earth objects are known — those larger than 140 metres (5,511 feet).
“The last few unknown 1-km near-Earth objects likely have orbits close to the Sun or high inclinations, which keep them away from the fields of the main near-Earth object surveys.”
The full findings of the study were published in the journal Science.
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