13 September 2017
Stuart Gary
Stargazers will have a chance to get a close-up look at NASA’s OSIRIS-REx spacecraft when the three-metre-wide probe sweeps through Earth’s skies for a close approach above Australia during the early morning hours of September 23, 2017.
The spacecraft is undertaking a gravity-assist slingshot manoeuvre, designed to build up speed and modify its trajectory to eventually intercept 101955 Bennu, an asteroid half a kilometre across which has a 1-in-2700 chance of hitting the Earth in the next 200 years.
That’s the third-highest rating on the Palermo Technical Impact Hazard Scale – a logarithmic rate used by astronomers to determine the potential impact hazard of a near-Earth object (or NEO) based on the probability of an impact and the estimated damage likely to be caused.
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OSIRIS-REx will inspect Bennu in great detail, examining its orbit and composition, and collecting samples for return to Earth.
A key focus of the mission will involve studying the “Yarkovsky effect” caused by sunlight heating the surface of the asteroid.
That heat is eventually radiated away as the asteroid rotates, and in the process it generates a tiny but measurable propulsive force which can influence the asteroid’s movement through space.
Bennu’s orbit around the Sun takes 1.2 years and brings it close to Earth every six years. To properly predict potential collisions with Earth, scientists need to know how the Yarkovsky effect influences that orbit.
Bennu is a carbonaceous member of the Apollo asteroid group – a collection of more than 8000 near-Earth asteroids with orbits that intersect Earth’s.
The asteroid was first detected on September 11, 1999 by the Lincoln Near-Earth Asteroid Research (LINEAR) project developed by NASA, the United States Air Force, and the Massachusetts Institute of Technology (MIT).
Bennu is thought to be composed of ancient carbonaceous material dating back to the formation of the solar system 4.6 billion years ago, and to pre-solar system material blown into space by dying stars known as red giants, and by supernovae – such as the one that triggered our solar system’s birth.
Earth-based observations indicate Bennu has a relatively smooth surface with a well-defined equatorial ridge line, and at least one prominent boulder – up to 20 metres wide – on its surface.
Scientists think the ridge line was created by fine-grained surface regolith particles accumulating in this area, due to the asteroid’s low gravity and rapid four-hour rotational period.
Spectroscopic observations suggest asteroids like Bennu contain anhydrous silicates, hydrated clay minerals, organic polymers, magnetite, and sulfides.
The OSIRIS-REx spacecraft (Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer) was launched on an Atlas V 411 rocket from the Cape Canaveral Air Force Base in Florida on September 8, 2016 on a seven-year mission to collect and bring home samples.
The probe will approach over the Pacific Ocean close to the equator before passing 16,000 kilometres over the city of Rockhampton in Queensland, Australia at 00:22 am AEST (10:22 am EDT, September 22) travelling south-southwest, reaching the skies over Adelaide in South Australia at 00:53 am local time, before zooming over the Great Southern Ocean and the Antarctic continent and heading back toward deep space.
While the best spots for watching are in Australia (and Antarctica), observers with telescopes will be able to see the craft from a much larger area. The website of the OSIRIS-REx mission has detailed instructions for how to spot it.
The gravity-assist Earth flyby will give astronomers and sky watchers with high-end cameras a chance to view this rare encounter.
During the flyby manoeuvre Professor Phil Bland and colleagues from Curtin University will use the meteor-hunting Desert Fireball Network to develop a 3-D triangulated track of the spacecraft’s flight path to test the network’s capabilities.
Bland says teams equipped with high-end DSLR cameras will be stationed at key vantage points across the country, working together to track the flight path of OSIRIS-REx as it speeds over the continent.
The Desert Fireball Network project is designed to help NASA unlock the mysteries of the universe by studying meteorites, fireballs and their pre-Earth orbits.
Bland says together with NASA, the Desert Fireball Network is expanding to form a Global Fireball Observatory through the Solar System.
“We know very little about how the planets came together and why the Earth has the composition that it does,” says Bland.
“Projects like this will help fill in the gaps and provide a clearer picture of our origins.”
The gravity assist will fling the 2110-kilogram OSIRIS-REx spacecraft towards its August 2018 rendezvous with Bennu, and the start of its remote-sensing science mission.
OSIRIS-REx will study the asteroid in minute detail – analysing its chemical composition and mineralogy, characterizing its geologic and dynamic history, and mapping its surface to identify potential landing sites.
Eventually the spacecraft will slowly descend to the surface and use nitrogen gas to puff more than 60 grams of regolith into the sampler head fitted to the spacecraft’s robotic arm.
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Meanwhile, contact pads on the sampler head will collect fine surface dust particles during the operation.
The landing manoeuvre will be repeated up to three times to gather enough samples from the asteroid’s surface.
The samples will be placed in a special capsule for the mission’s return to Earth in 2023.
The sample return capsule – which is equipped with a heat shield and parachute – will touch down in the Utah desert on September 24, 2023.
OSIRIS-REx isn’t the first asteroid sample-return mission.
In May 2003 the Japanese Aerospace Exploration Agency (JAXA) launched their Hayabusa sample return mission to the near-Earth asteroid 25143 Itokawa. It arrived at the asteroid in mid September 2005.
Hayabusa spent two months studying the asteroid’s shape, spin, topography, colour, composition, density, and history, before landing on the surface in November and collecting tiny grains of material.
These were returned to Earth, and jettisoned in a sample return capsule which successfully parachuted into the Woomera Rocket Range in outback South Australia in 13 June 2010.