'Potentially hazardous' asteroid about to make pass close to Earth

Monday, 8 November 2021

Nasa's OSIRIS-REx spacecraft was launched in 2016. It travelled to the near-Earth asteroid Bennu. It collected a sample and is due to return it to Earth in 2023.

An asteroid as wide as three football fields will come within 3.9 million kilometres of Earth in December.

It is one of four close Earth approaches this century by the 330-metre diameter asteroid 4660 Nereus, which was discovered in 1982, and orbits the Sun every 664 days.

Coincidentally, between now and 4660 Nereus’ next close pass of Earth, Nasa will be launching what it describes as the world’s first planetary defence test mission.

The asteroid Bennu as seen from the OSIRIS-REx spacecraft.

That mission, called DART (Double Asteroid Redirection Test) will involve crashing a spacecraft into a small asteroid to try to slightly change its orbit.

* NASA's plan to stop a killer asteroid: Shoot it with a rocket

* Nasa spacecraft arrives at ancient asteroid Bennu, its first visitor in billions of years

Near-Earth object researcher Alan Gilmore said a close pass of Earth by an asteroid the size of 4660 Nereus was significant.

As the asteroid had been discovered in 1982, it would have been observed passing many times, and the accuracy for the projected timing and distance from the Earth of the coming close approach would be “high”, Gilmore said.

This image provided by the Southwest Research Institute shows an impression of the Lucy spacecraft that was launched last month on a voyage to explore the Jupiter Trojan asteroids.

The asteroid is due to be closest to Earth on December 12, according to Nasa.

Nereus would get within 3.9 million kilometres of Earth, which is a bit more than 10 times the average distance between the Earth and the Moon. It is classed as a “potentially hazardous asteroid”.

A simulated image of the DART mission which shows the impact on the moonlet of asteroid (65803) Didymos. Post-impact observations from Earth-based optical telescopes and planetary radar would, in turn, measure the change in the moonlet’s orbit about the parent body.

It also made a close pass in 2002, and will do so again in 2060 and 2071. In the 2060 event, Nereus is projected to pass about 1.2 million kilometres from Earth, a bit more than three times the average distance between the Earth and Moon.

The closest large asteroid to pass Earth in 2021 was 2001 FO32, which is about 1km in diameter and got to within about 2 million kilometres of Earth – a bit more than five times the Earth-Moon distance – in March.

As well as being large and close, 2001 FO32 is one of the fastest space rocks known to travel past Earth, moving at nearly 124,000kmh relative to Earth. In comparison, Nereus is travelling at under 24,000kmh.

The asteroid 99942 Apophis, which has a similar diameter to Nereus, is expected to pass 32,000km from Earth in April 2029. In March this year, Nasa published a statement assuring the world that its calculations “don’t show any impact risk [from Apophis] for at least the next 100 years”.

That has not stopped the agency doing some work on ways to defend the Earth in the event the planet does end up on a collision course with a large object in space.

The spacecraft it plans to crash into a small asteroid in the DART mission is on a launch site at the Vandenberg Space Force Base, California, and is due to launch on November 23 (local time) or soon after.

The spacecraft will head for the small moonlet asteroid Dimorphos, which orbits a larger companion asteroid called Didymos, neither of which pose a threat to Earth.

The aim is to prove a spacecraft can autonomously navigate to a target asteroid and “kinetically impact it”.

“Then, using Earth-based telescopes to measure the effects of the impact on the asteroid system, the mission will enhance modelling and predictive capabilities to help us better prepare for an actual asteroid threat should one ever be discovered,” Nasa said.