Hunting for life's cosmic origin
Two pioneering spacecraft are about to enter orbit around two different asteroids. Prof Lewis Dartnell explains how their findings could shed light on how life on Earth began.
OSIRIS-REx. Hayabusa2. Make a note of these two names: you’re going to be hearing a lot about them over the coming months and years. These spacecraft – one operated by NASA and the other by the Japan Aerospace Exploration Agency, JAXA – will this summer each enter into orbit around a target asteroid. They promise to teach us a great deal about the origins of the Solar System, how we might deflect an asteroid on a collision course with the Earth, and even the molecular origins of earthly life.
Both NASA’s OSIRIS-REx and JAXA’s Hayabusa2 are sample-return missions, which means that not only will they touch gently onto their asteroid’s surface to collect a scoop of its ancient material, but they will then return it safely back to eagerly waiting scientists on Earth. This sort of return trip mission within deep space is fabulously complex, and both probes are marvels of engineering. The Japanese probe is a follow-up to their earlier asteroid mission, Hayabusa, which returned a small sample from the asteroid Itokawa in 2010. Despite suffering numerous glitches, Hayabusa racked up a string of accomplishments, including being the first spacecraft designed to land and take off from an asteroid and the first to return an asteroid sample to Earth.
Hayabusa2 uses the same basic spacecraft structure as its predecessor, but incorporates more backup systems to improve reliability, along with some technological advances. As well as upgrades to the communication antenna and guidance systems, Hayabusa2’s ion engines are 25 per cent more powerful than its predecessor’s, and the probe will be able to autonomously control its own final descent to the remote asteroid’s surface. Hayabusa2 is also like a mothership in its own right – it will deploy a small lander and three rovers onto the asteroid’s surface for a closer look, which can hop around the landscape to different locations.
“OSIRIS-REx is like going on a grand field trip, and when it comes back it will be a truly amazing moment”
Meanwhile, NASA’s OSIRIS-Rex (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) will be the first ever US asteroid sample-return mission. This spacecraft is about twice the size of Hayabusa2, and rather than using ion engines will fire standard rocket thrusters to accelerate on its trajectory to its target asteroid. When they arrive this summer, both missions will survey their target asteroids for about a year and a half, mapping the surfaces and remotely detecting minerals using spectroscopy. Scientists will then use these results to help them decide the best spot on their asteroids for Hayabusa2 and OSIRIS-REx to descend to collect their samples.
Asteroids are important because they represent primordial material left over from the making of the planets. They are like time capsules from before the creation of the Earth, preserving matter since the beginning of the Solar System. By studying them up-close, scientists hope to be able to answer some pretty fundamental questions about the formation and development of the Solar System. Specifically, it will help us to understand how planets like the Earth were born, by allowing us to observe the material from which rocky planets form.
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Even more excitingly, both OSIRIS-REx and Hayabusa2 are being sent to carbonaceous asteroids. These kind of space rocks have a high percentage of carbon compounds as well as water-containing minerals, and are thought to have delivered a lot of water to the primordial Earth to fill our oceans, along with organic chemicals like amino acids. As Dr Yuichi Tsuda, project manager for Hayabusa2, puts it, “The primary reason we chose our target asteroid is that it is a C-type [carbon-rich]. Telescope observations suggest that it should contain lots of carbon as well as water-related minerals, and so give us important clues as to how life on Earth became possible. We’ve never explored or sampled this type of asteroid before, so these missions are really exciting”.
Organic chemistry forms the building blocks of all life on Earth. Cells of organisms are made up of certain molecules joined together into long chains: amino acids that build our proteins; nucleotide bases that make DNA and RNA; and the long, oily chains that make up the outer membranes of cells. We know that many of these chemical building blocks are formed in the cosmos – through what is known as ‘astrochemistry’– in the cold gas clouds floating through space, as well as the warmer regions around old, dying stars. When this material pulls together under gravity as a new solar system forms, the organic molecules become incorporated into asteroids and comets. So while asteroids don’t deliver fully-formed cells to young planets, they may have provided many building blocks for the origin of life – and finding organic molecules on these asteroids would offer support for this idea.
Organic molecules like amino acids have previously been found in meteorites that have landed on the Earth, but these missions will be the first time that scientists will be able to get their hands on carbonaceous material directly from an asteroid. Although meteorites naturally deliver us lumps of primordial space rock, as soon as they land they’re susceptible to contamination from the Earth’s environment. And that’s why sample-return missions are so important to researchers – material is collected from the source and hurried back via a robotic courier.
Prof Sara Russell is a planetary scientist at London’s Natural History Museum, and will run some of the preliminary studies on the material returned by OSIRIS-REx. “I’ve worked on meteorites my whole career, but we’re never really sure what sort of asteroid, or where in the Solar System, they’ve come from,” she explains. “OSIRIS-REx is like going on a grand field trip to pick our own sample, and when it comes back to Earth in 2023 it will be a truly amazing moment – a meteoriticist’s dream come true!”
Both NASA and JAXA chose their target asteroids because they offer pristine carbonaceous material for researchers to study. But they also needed asteroids that are roughly the right size (with enough gravity for their probes to orbit), that aren’t spinning too quickly (so that the probes can touch down safely), and that are in a near-Earth orbit that the probes can actually reach. “Asteroids that fit all these criteria are actually quite rare,” says Prof Hitoshi Kuninaka, who has been leading the development of Hayabusa2’s ion engines. The NASA scientists picked asteroid 101955 Bennu for their mission, whereas JAXA’s Hayabusa2 is heading towards 162173 Ryugu. And they will collect their samples in a spectacularly audacious way.
After it manoeuvres into orbit around Bennu, OSIRIS-REx will slowly lower itself towards the asteroid’s surface, without actually landing, and fold its solar panels upwards to protect them. Here, it will extend a robotic arm and puff a sharp burst of nitrogen gas to blow up particles into its collection head. After just five seconds, the sample collector will close and OSIRIS-REx will automatically begin to back away from the surface. With anything between 60g and 2kg gathered, this precious cargo will be sealed into a re-entry capsule and fired back towards the Earth, where it’ll parachute safely to the ground in Utah to be picked up.
Hayabusa2 is even more innovative. It carries a device called the Small Carry-on Impactor (SCI), consisting of a 2.5kg copper projectile and a shaped charge of plastic explosive. This explosive will fire the copper impactor into Ryugu’s surface at over 7,000km/h, blasting out a crater while Hayabusa2 flies around the far side of the asteroid to protect itself from the flying shrapnel. A camera released by the probe will watch the impact, transmitting the images to Hayabusa2 before the probe returns to the crater to collect its sample. This will enable Hayabusa2 to analyse the interior structure of the asteroid, and to gather material that has not been exposed to ultraviolet radiation and the solar wind.
Beyond teaching us about the origins of the Earth and the conditions for life, OSIRIS-REx and Hayabusa2 have another critical aim: to help prevent a cataclysmic cosmic collision. As Bennu and Ryugu orbit the Sun close to the Earth, they are also exactly the sort of asteroids that present a potential hazard to our planet. Asteroids have been slamming into the Earth throughout our planet’s history, and the mass extinction 66 million years ago that saw the demise of the dinosaurs, along with around three-quarters of all animal and plant species, coincided with a massive impact crater in the Gulf of Mexico. Bennu and Ryugu are much smaller than the asteroid that may have triggered that extinction event – they’re both less than a kilometre across – but the results would still be catastrophic if they were to hit a populated area. The orbit of Bennu, for example, brings it close to the Earth every six years, and it’s been calculated that there’s a 1 in 1,410 chance that it might hit us between the years 2169 and 2199.
“OSIRIS-REx and Hayabusa2 could help prevent a cosmic collision”
OSIRIS-REx will help us understand how the orbit of asteroids like Bennu might change over time through a process known as the Yarkovsky effect. This is a tiny force caused by the emission of infrared radiation from the Sun-warmed surface of an asteroid, but over long periods it can significantly nudge an object’s orbit. OSIRIS-REx will study this effect and what it means for the probability of Bennu impacting the Earth in the future. The probe will also measure the asteroid’s physical properties. “It’s important to understand the interior structure of asteroids like Bennu,” explains Dr Kerri Donaldson Hanna, a researcher at the University of Oxford who will be helping with OSIRIS-REx’s surface investigations. “Is it a single body, or perhaps composed of multiple large boulders held together only loosely? We’d need to know this before we could decide on the best technique to attempt to deflect away an asteroid if it were on a collision course.”
These missions are spectacular not just for their audacity, but also for the sheer breadth of their vision. From the origins of life on our planet to protecting the life that now clings to survival here, OSIRIS-REx and Hayabusa2 promise to offer new insights into our place in the cosmos.
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