Nasa probe to observe near-Earth asteroid's 2029 close encounter

About five-and-a-half years from now, astronomers predict an asteroid about as wide as the Empire State Building is tall will streak through space within 32,200km of Earth, the closest any celestial object of that size will have come to our planet in modern history.

When it does, a spacecraft launched by Nasa in 2016 is expected to be in position to provide a detailed examination of this rare close encounter.

The mission, directed by University of Arizona scientists, is expected to yield insights into planetary formation and knowledge that could inform efforts to build a defence system against possible doomsday asteroid collisions with Earth.

At the time of its 2004 discovery, the asteroid Apophis, named for a demon serpent embodying evil and chaos in ancient Egyptian mythology, appeared to pose a dire threat to Earth, with scientists forecasting a potential collision in 2029. Refined observations have since ruled out any impact risk for at least another century.

Still, its next approach in 2029 will bring the asteroid within a cosmic cat's whisker of Earth, less than a 10th the moon's distance from us and well within the orbits of some geosynchronous Earth satellites.

The spacecraft now headed for a rendezvous with Apophis is OSIRIS-REx, which made headlines plucking a soil sample from a different asteroid three years ago and sending it back to Earth in a capsule that made a parachute landing in Utah in September.

Rather than retire the spacecraft, Nasa has rebranded it as OSIRIS-APEX, short for APophis EXplorer, and fired its thrusters to put it on course for its next target.

The Apophis expedition was detailed in a mission overview published in the Planetary Science Journal.

Apophis, oblong and somewhat peanut-shaped, is a stony asteroid believed to consist mostly of silicate materials, with iron and nickel. Measuring about 340m across, it is due to pass by Earth's surface on April 13 2029, becoming visible to the naked eye for a few hours, said Michael Nolan, deputy principal investigator for the mission at the University of Arizona.

“It's not going to be this glorious show,” Nolan said, but it will appear as a point of reflected sunlight in the night sky over Africa and Europe.

An asteroid that large passing so near to Earth is estimated to occur about once every 7,500 years. The Apophis fly-by is the first such encounter predicted in advance.

The tidal pull of Earth's gravity is likely to cause measurable disturbances to the asteroid's surface and motion, changing its orbital path and rotational spin. Tidal forces could trigger landslides on Apophis and dislodge rocks and dust particles to create a comet-like tail.

The spacecraft is set to observe the asteroid's Earth fly-by as it nears and catches up with Apophis. These images and data would be combined with ground-based telescope measurements to detect and quantify how Apophis was altered as it passed Earth.

OSIRIS-APEX is scheduled to remain near Apophis for 18 months, orbiting, manoeuvring around it and even hovering just over its surface, using rocket thrusters to kick up loose material and reveal what lies beneath.

Like other asteroids, Apophis is a relic of the early solar system. Its mineralogy and chemistry are largely unchanged in more than 4.5-billion years, offering clues to the origin and development of rocky planets such as Earth.

Close examination of Apophis could give planetary defence experts valuable information about the structure and other properties of asteroids. The more scientists know about the composition, density and orbital behaviour of such celestial “rubble piles”, the greater the chances of devising effective asteroid-deflection strategies to mitigate impact threats.

Nasa deliberately crashed a spacecraft into a small asteroid last year in a planetary-defence test that nudged the rocky object from its normal path, marking the first time humankind altered the natural motion of a celestial body.

Apophis is substantially larger than that asteroid, but tiny compared with the one that struck Earth 66-million years ago, wiping out the dinosaurs.

While not big enough to pose an existential threat to life on Earth, an Apophis-sized asteroid striking the planet at hypersonic speed could devastate a major city or region, Nolan said, with ocean impact unleashing tsunamis.

“It wouldn't be globally catastrophic in the sense of mass extinctions,” but an impact “would definitely come under the category of bad.

“This thing is coming in at many miles per second if it hits. And at that speed, it doesn't [matter] whether it's made of gravel or ice or rocks or whatever. It's just a big, heavy thing moving fast,” Nolan added.

Reuters