LEWIS DARTNELL

TWO PI­O­NEER­ING SPACE­CRAFT ARE ABOUT TO EN­TER OR­BIT AROUND TWO DIF­FER­ENT ASTEROIDS. WHAT THEY FIND COULD SHED LIGHT ON HOW LIFE ON EARTH BE­GAN

Focus-Science and Technology - - WELCOME - WORDS: PROF LEWIS DARTNELL

Two mis­sions are about to visit two sep­a­rate asteroids. Astro­bi­ol­o­gist Lewis re­veals what we might dis­cover about the ori­gins of life.

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.

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.

TIME CAP­SULES

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.

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 2

“THESE KIND OF SPACE ROCKS ARE THOUGHT TO HAVE DE­LIV­ERED A LOT OF WA­TER TO PRI­MOR­DIAL EARTH”

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

“OSIRIS-REx IS LIKE GO­ING ON A GRAND FIELD TRIP, AND WHEN IT COMES BACK IT WILL BE A TRULY AMAZ­ING MO­MENT”

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 2

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.

IN­COM­ING!

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

“OSIRIS-REx AND HAYABUSA2 COULD HELP PRE­VENT A COS­MIC COL­LI­SION”

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 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.

ABOVE: OSIRIS-REx be­ing as­sem­bled prior to its launch back in 2016 RIGHT: Timeline of the OSIRIS-REx and Hayabusa2 mis­sions

ABOVE: Ex­tract­ing sam­ples from the first Hayabusa mis­sion

LEFT: Asteroids are time cap­sules from the So­lar Sys­tem’s for­ma­tion ABOVE LEFT: Re­cent im­age of tar­get asteroid from Hayabusa2

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