Up close and cosy with BENNU



FOR THE past two years, the Osiris-Rex space­craft has sailed across the so­lar sys­tem by the light of the stars. Like an­cient mariners and the Apollo as­tro­nauts, it needed the con­stancy of the con­stel­la­tions to nav­i­gate the dark un­known.

All that changed last week, when the Na­tional Aero­nau­tics and Space Ad­min­is­tra­tion (Nasa) probe fi­nally reached its tar­get, an Em­pire State Build­ing-sized as­teroid called Bennu.

Now Osiris-Rex faces a whole new kind of chal­lenge: ex­plor­ing the small­est ob­ject ever or­bited by a space­craft.

Sit­ting at mis­sion con­trol at the Den­ver of­fices of Lock­heed Mar­tin, which op­er­ates the space­craft for Nasa, en­gi­neer Javi Cerna waited for the sig­nal in­di­cat­ing Osiris-Rex had be­gun the burn needed to bring it close to its tar­get.

“Standby for Bennu ar­rival,” Cerna an­nounced.

He fid­geted in his chair, then stood. The room was ut­terly silent.

Then Cerna grinned and spread his arms out wide.

“We have ar­rived!” Osiris-Rex was within 20 kilo­me­tres of Bennu’s sur­face – about the dis­tance be­tween the White House and Nasa’s God­dard Space Flight Cen­tre, which man­ages the space­craft.

Soon an im­age of the as­teroid ap­peared on the mis­sion con­trol screens: a di­a­mond-shaped body with a rough, speck­led ex­te­rior. Osiris-Rex was fi­nally at the doorstep of its new home.

Bennu is a car­bona­ceous as­teroid – a prim­i­tive, car­bon-rich piece of de­bris left over from the process that formed the so­lar sys­tem 4.6 bil­lion years ago.

Osiris-Rex will spend the next 18 months there, sur­vey­ing the land­scape and prob­ing Bennu’s chem­i­cal makeup be­fore fi­nally se­lect­ing what piece of the as­teroid it wants to bring back home. In a kiss-like ma­noeu­vre, the space­craft’s robotic arm will col­lect some ma­te­rial from Bennu’s sur­face, then sling the sam­ple back to­ward Earth. It will be the largest plan­e­tary sam­ple re­trieved since the Apollo era, when as­tro­nauts brought rocks back from the moon.

Study­ing the sam­ple in ter­res­trial labs, sci­en­tists hope to un­cover clues about the birth of the plan­ets and the ori­gins of Earth’s wa­ter and life. They may also un­cover po­ten­tially use­ful nat­u­ral re­sources such as or­ganic mol­e­cules and pre­cious met­als. And since Bennu has a 1-in-2,700 chance of im­pact­ing Earth about 200 years from now, re­searchers fig­ure it would be good to glean some in­sights about the as­teroid’s fate – and how it might in­ter­sect with our own.

Bennu is so small, dark and dis­tant (about 75 mil­lion miles from Earth at the mo­ment) that sci­en­tists could only the­o­rise about what it might look like when they launched Osiris-Rex two years ago. To their de­light, newly ac­quired close-ups of the as­teroid closely match their pre­dic­tions.

But there’s still a lot to learn about the ob­ject, says Univer­sity of Ari­zona Plan­e­tary Sci­en­tist Bashar Rizk, who over­sees three of the probe’s cam­eras. In the com­ing weeks and months, his team aims to get de­tailed mea­sure­ments of the as­teroid’s shape, den­sity and grav­ity that will al­low sci­en­tists to fine-tune how they or­bit it.

Bennu is so small (about 0.05 per­cent of the mass of Mount Ever­est) that its grav­ity is nearly neg­li­gi­ble. If you stood at Bennu’s North Pole and jumped, you would achieve es­cape ve­loc­ity and go soar­ing off into the void.

That makes or­bit­ing – which re­lies on a del­i­cate bal­ance be­tween a space­craft’s ve­loc­ity and an ob­ject’s grav­ity – es­pe­cially hard.

“It will re­ally be record-break­ing in terms of the pre­ci­sion, the nav­i­ga­tion, com­pared to any­thing we’ve done be­fore,” says flight nav­i­ga­tor Co­ralie Adam, an en­gi­neer at aerospace com­pany KinetX.

With grav­ity so weak, other fac­tors could po­ten­tially knock Osiris-Rex off course. Even the faint pres­sure of sun­light warm­ing the space­craft can cre­ate suf­fi­cient thrust to warp its or­bit.

To coun­ter­act the in­flu­ence of the sun, Adam and her col­leagues will fly over Bennu’s “ter­mi­na­tor” line, where day turns to night on the as­teroid’s sur­face. This en­sures that the so­lar ra­di­a­tion pres­sure re­mains con­stant, so en­gi­neers can make sure they con­tin­u­ously coun­ter­act it.

Yet Bennu’s small size also makes it pos­si­ble for Osiris-Rex to per­form a se­ries of care­fully chore­ographed hair­pin ma­noeu­vres around the as­teroid. En­gi­neers will up­link new or­bital in­struc­tions to the space­craft ev­ery day (a typ­i­cal plan­e­tary mis­sion might only up­date its trajectory once a week, Adam says). Video an­i­ma­tions of the space­craft’s planned or­bits look like an elab­o­rate cos­mic bal­let.

In 2020, af­ter 18 months of ob­ser­va­tions, Osiris-Rex will swoop close to Bennu and ex­tend a long robotic arm equipped with its sam­ple-col­lect­ing in­stru­ment, called Tagsam. With a puff of ni­tro­gen gas, it will blow some ma­te­rial off the as­teroid’s sur­face, gath­er­ing as much as 4.4 pounds of rock in the head of the sam­ple. Then it must turn around and re­trace its path back home.

Fi­nally, on Septem­ber 24, 2023, a cap­sule con­tain­ing the sam­ple will streak through Earth’s at­mos­phere and land in the Utah desert.

As Nasa sci­en­tists learn more about Bennu, they’ll be com­par­ing their find­ings with coun­ter­parts from the Ja­panese Space Agency, whose Hayabusa 2 space­craft ar­rived at the as­teroid Ryugu ear­lier this year. The Ja­panese mis­sion was the first to land mov­ing rovers on the sur­face of an as­teroid, and it will also re­turn sam­ples to Earth in 2020. As­teroid sci­en­tists are ea­ger for the “bo­nanza” of dis­cov­er­ies that await, as one re­searcher at mis­sion con­trol puts it.

Osiris-Rex sci­en­tists ex­pect to re­veal the re­sults of their early sur­veys of Bennu this month at the au­tumn meeting of the Amer­i­can Geo­phys­i­cal Union in Washington.

Asked how he was feel­ing at the mo­ment of ar­rival, prin­ci­pal in­ves­ti­ga­tor Dante Lau­retta tweeted, “re­lieved, proud and anx­ious to start ex­plor­ing!”

Af­ter a two-year chase, a Nasa space­craft has ar­rived at the an­cient as­teroid known as Bennu, its first vis­i­tor in bil­lions of years.

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