Asteroids and meteors were created when the Solar System was being formed, and travel its vast emptiness to this day. In its early history, a large number of them showered down upon Earth’s surface, a period known as the late heavy bombardment. They are thought to have brought along with them large quantities of water – water that we still see on Earth today – and carbon-based molecules.
The impact of one particular asteroid also most likely eliminated the mighty dinosaurs. Could such an event occur again? Should we prepare to deal with what the planet might look like after? The answer to these questions could be hidden in asteroids floating around near Earth today, and studying them could provide a snapshot of the Solar System’s early history. They could also yield clues about the early development of complex ingredients that served as the basis for life on Earth.
Finally, if humanity has to spread its footprints to elsewhere in the Solar System, asteroids could serve as potential sources of water and other minerals on our journey.
These are in fact some of the goals of one of NASA’s latest exploration missions: Origins, Spectral Interpretation, Resource Identification, Security – Regolith Explorer (OSIRIS-REx). Its total cost is $800 million (Rs 5,545 crore).
After travelling over 3.21 million km, the OSIRIS-REx spacecraft arrived at an asteroid named Bennu late last year. Since then, it has been in orbit around Bennu, taking photographs, collecting spectroscopic data and performing reconnaissance for a suitable place to touchdown. After that, it will collect a sample of its surface next summer.
The satellite also carries a number of camera systems to image and document the sample acquisition process, provide tracking and photometry data, and map the surface. Its thermal, visible and infrared spectrometers will measure the albedo, thermal flux and spectral properties necessary to identify its material composition.
OSIRIS-REx’s primary objective is to return about a kilogram of pristine sample from the asteroid to Earth for analysis. It will also endeavour to complete multiple secondary objectives: provide spectral interpretations to ground telescopic observations, measure the deviation of a potentially hazardous asteroid orbit due to the Yarkovsky effect, map the chemistry and mineralogy of a primitive carbonaceous asteroid for future resource identification, etc.
Bennu is an astonishingly dark object. It has an average surface albedo – the amount of light reflected – of 3.5%, darker than a freshly laid asphalt road. Discovered in 1999, Bennu is almost spheroidal with an equatorial ‘belt’ about 490 m wide – about two-thirds the size of Kolkata’s Howrah bridge.
It may completely wipe out a large city if it were to fall near one. In fact, among all known near-Earth objects (NEOs), Bennu has the highest impact probability in the next two centuries.
For any NEO to have a realistic chance of hitting Earth in the near future, it has to go through certain ‘impact keyholes’ in space during its orbit around the Sun. These are imaginary but well-defined regions in space selected by the mass of the object and its Keplerian orbit. Whether Bennu will pass through one of those keyholes – and so have a higher chance of hitting Earth in future – will depend on an interesting physical phenomenon called the Yarkovsky effect.
An asteroid is exposed to sunlight in its orbit around the Sun. It absorbs some of that radiation, reflects some and re-radiates some more after absorption. This heat released by Bennu is highest in the late-afternoon part – the part still exposed to the Sun but rotating away from it. And this re-radiation creates a photon pressure (by Newton’s third law) that slowly nudges an asteroid away from its orbit, especially if it is small. This is the Yarkovsky effect. Because of it, Bennu has deviated from its gravity-dictated orbit by 12 km since 1999.
On Bennu, a day-night cycle lasts about 4 hours and 18 minutes. The difference in temperature between day and night sides is greater than 200º C. This means its surface material undergoes extreme and periodic heating-and-cooling every four hours. As a result, the material on the surface – and maybe even in the subsurface – is thought to have been highly ‘processed’ by the effects of temperature, radiation, microgravity and centrifugal forces, among others.
As a B-type asteroid, Bennu is expected to be hydrated, microporous, rich in volatile substances and to possess carbonaceous molecules on its surface. Its average density is 1,190 kg/m3 (the density of loose gravel on Earth is about 1,500 kg/m3). Its low thermal inertia and radar polarisation ratio – calculated using computer models – suggest it has small rocks in its soil.
However, cameras on OSIRIS-REx found large boulders up to tens of meters long spread around the entire surface. There were also loose rocks (too small to damage the spacecraft) flying off the surface once every two weeks on average.
A shape model based on other initial results from OSIRIS-REx spacecraft implies that it is indeed microporous. There is also evidence that suggests Bennu may have been formed by accumulation, and that it may have gone through episodes of rapid rotation. It is not completely dry. In fact, infrared observations suggests the presence of substantial hydrated minerals, magnetite and clay minerals – which means the asteroid could have contained water ice in the past. Its rotation rate is falling by a tiny amount every day due to the Yarkovsky and other effects.
The data confirms its interior is stiff and its surface is littered with large boulders prone to mass loss. Early results also indicate that our existing models can’t adequately predict the sizes of particles from ground-based observations.
Right now, the OSIRIS-REx team is focused on finding a suitable ‘landing’ site. After rendezvous, it will extend a 3.3-meter-long sampling arm to perform a touch-and-go sample acquisition manoeuver. Because of its small size, Bennu exerts a negligible gravitational pull, which implies a proper landing won’t be possible. Even a small amount of thrust would be enough to push the spacecraft away from the surface.
Bennu’s surface was initially thought to be much smoother, with broad swathes of sampling region up to 25 m wide. However, the surprising consistency with which its surface is littered with boulders makes the job of scooping material a hazardous task. Scientists had to develop new sampling strategies for a very congested area a couple meters wide.
The current plan is, after rendezvous, on-board canisters filled with ultra-pure nitrogen gas will fire to agitate the regolith into a fluid, which will flow into the collection chamber. (By comparison, Hayabusa 2, a mission of the Japan Aerospace Exploration Agency to the asteroid Ryugu, will perform a controlled explosion on the surface, followed by collection. This mission is expected to bring back a smaller sample.)
These measurements will help map Bennu, and better identify its composition, create mitigation strategies in case of a future impact and help characterise the NEOs in general. OSIRIS-REx will return from Bennu in 2023 with the sample module, which will crash-land in the salt flats of Utah. From there, scientists will collect the sample and laboratory analysis will begin.
Partha Pratim Bera is an Indian scientist at the space science and astrobiology division of NASA Ames Research Center, California. His work focuses on quantum chemistry, high-resolution spectroscopy and interstellar reaction modelling.