SPY­ING ON THE NEIGH­BOURS

Hub­ble’s suc­ces­sor, the James Webb Space Tele­scope, will look far­ther back in time and space than ever be­fore. But this gi­ant tele­scope could also be turned to tar­gets right in our own cos­mic back­yard, as Ben­jamin Skuse re­veals

Sky at Night Magazine - - CONTENTS - Dr Ben­jamin Skuse is a math­e­ma­ti­cian turned sci­ence writer based in Bris­tol, UK.

The James Webb Space Tele­scope won’t only peer into the dis­tant Uni­verse; here’s what we hope to learn about our So­lar Sys­tem.

Big­ger and more pow­er­ful than any space ob­ser­va­tory ever launched, the James Webb Space Tele­scope’s (JSWT’s) in­frared gaze will stretch to the very first stars and gal­ax­ies be­ing born, of­fer­ing new in­sight into the Uni­verse’s ori­gins. Its eyes will also scan ex­o­plan­ets in the search for the build­ing blocks of life beyond our cos­mic doorstep, look­ing for an­swers to the peren­nial ques­tion: ‘Are we alone?’

What many do not re­alise though is that JWST will not solely be peer­ing at the farthest reaches of the Uni­verse. In fact, with some clever re­con­fig­ur­ing, Webb will be able to cast its spy­ing eye on our clos­est cos­mic neigh­bours, hop­ing to un­cover some of the se­crets hid­den within our So­lar Sys­tem.

Adapt­ing JWST for the lo­cal na­ture of So­lar Sys­tem sci­ence, how­ever, is fraught with dif­fi­cul­ties. The big­gest is that the tele­scope is de­signed for de­tect­ing the faintest, most dis­tant ob­jects. Its ex­tremely sen­si­tive sen­sors there­fore need to be pro­tected at all times from the over­pow­er­ing light and heat from the Sun, which is why it is equipped with a ten­nis court-sized sun­shield. This would not be a prob­lem but for the fact that Webb will be lo­cated at the sec­ond La­grangian point (L2), some 1.5 mil­lion km beyond Earth’s or­bit. As it is, the sun­shield per­ma­nently shrouds Mer­cury, Venus, Earth and the Moon from Webb’s gaze.

The clos­est of our neigh­bours Webb will be able to track are near-Earth ob­jects (NEOs) like Eros and Hal­ley’s Comet.

“The Earth’s at­mos­phere makes it very dif­fi­cult to ob­serve NEOs in cer­tain wave­length re­gions, some of which are very in­for­ma­tive and di­ag­nos­tic of things like wa­ter and or­gan­ics,” says NASA re­search sci­en­tist Cristina Thomas. “If we want to fo­cus on ori­gins of life ques­tions, then go­ing out­side the at­mos­phere helps us.”

The bright­ness dilemma

The sec­ond near­est tar­get, Mars and its moons, will only be within JWST’s spy­glass ev­ery two years. Webb will add an in­frared view to the Mars tool­box of rovers and satel­lites tasked with study­ing the planet and its po­ten­tial for host­ing life.

NASA plan­e­tary sci­en­tist Geron­imo Vil­lanueva be­lieves this ca­pa­bil­ity will be in­valu­able: “JWST will open a new win­dow into the planet’s cur­rent and past hab­it­abil­ity,” he says. Vil­lanueva should know. Among other achieve­ments, he was the co-dis­cov­erer of meth­ane on the planet (a pos­si­ble biosig­na­ture) and mapped deu­terium to hy­dro­gen ra­tios in Mars’s at­mo­spheric wa­ter – lead­ing to the re­al­i­sa­tion that the Red Planet had an an­cient ocean. “New ob­ser­va­tions

“What many do not re­alise is that JWST will not solely be peer­ing at the farthest reaches of the Uni­verse”

are ur­gently needed to con­firm these find­ings,” he says.

The Red Planet brings us to the sec­ond main chal­lenge in us­ing Webb to look over the gar­den fence: over­ex­po­sure. Es­sen­tially, Mars is far too bright for the Webb’s sen­si­tive de­tec­tors to cope with. “Even Pluto is bright enough that if we took full-frame data with our widest fil­ters it would sat­u­rate,” says John Stans­berry, a Space Tele­scope Sci­ence In­sti­tute (STScI) sci­en­tist. “So bright has a dif­fer­ent def­i­ni­tion for JWST!”

To get round this, NASA will com­mand the in­stru­ment to just process a tiny square right in the mid­dle of the full de­tec­tor

ar­ray. “In­stead of hav­ing a 4-megapixel im­age, we’ll take a much smaller postage stamp in the mid­dle,” says NASA space sci­en­tist Conor Nixon. “That way we can read that out re­ally quickly be­fore it be­comes over­ex­posed.”

Beyond Mars is where JWST will re­ally have to start get­ting busy. With

A full-size mock up of JWST’s 21x14m sun­shield, the de­vice that will keep the space scope’s in­frared in­stru­ments cool enough to work prop­erly

Eros is a near-Earth as­ter­oid that could pose an im­pact risk – so it’s worth keep­ing an eye on

Mars and its moons will be too bright to process us­ing the ca­pac­ity of JWST’s de­tec­tors (NIRCam im­ager shown left) so only a tiny sec­tion of the sen­sor will be used to im­age them

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