— How to spot an alien space­ship


Cosmos - - Contents -

Know­ing we are not alone in the uni­verse might de­pend on iden­ti­fy­ing an alien emis­sary mil­lions of kilo­me­tres away. LAU­REN FUGE ex­plains how we could do so.

THE OB­JECT HAD BEEN hurtling through the So­lar Sys­tem for years by the time as­tronomers spot­ted it, just 33 mil­lion km away – 20 mil­lion km closer than Mars ever comes to the Earth. Highly elon­gated and about the size of a WWII bat­tle­ship, its tra­jec­tory proved it was an in­ter­stel­lar in­ter­loper – our So­lar Sys­tem’s first iden­ti­fied vis­i­tor from deep space.

As­tronomers named it `Ou­mua­mua – Hawai­ian for “a mes­sen­ger from afar ar­riv­ing first” – be­cause Hawaii’s PAN-STARRS tele­scope was the first to spot it, in Oc­to­ber 2017. Was it, as some arm­chair sci­en­tists spec­u­lated, an alien emis­sary? Such wild con­jec­ture turned out to be a stretch. Anal­y­sis pointed to it sim­ply be­ing an oddly shaped as­ter­oid. Ul­ti­mately there was noth­ing to hint it was more than “a big chunk of rock”, says as­tronomer Olivier Hain­aut, of the Euro­pean South­ern Ob­ser­va­tory.

Too bad for alien en­thu­si­asts. So how did sci­en­tists work out what it was? If it had been an alien space­ship, how would we have known?


The first thing that stood out about ‘Ou­mua­mua was its or­bit. Though pass­ing through the So­lar Sys­tem, it was not cap­tured by the Sun. “It is the only ob­ject seen so far with a strongly hy­per­bolic or­bit,” says David Je­witt, an as­tronomer at the Univer­sity of Cal­i­for­nia, Los An­ge­les, “mean­ing it is trav­el­ling so fast that the Sun’s grav­ity can­not hold it back.”

This in­di­cated it could be some­thing novel, says Jonti Horner, an as­tro­bi­ol­o­gist at the Univer­sity of South­ern Queens­land. But “ex­tra­or­di­nary claims re­quire ex­tra­or­di­nary ev­i­dence, so peo­ple across the planet went into a frenzy to get more ob­ser­va­tions and lock things down”.


A key ob­ser­va­tion is to de­ter­mine if an ob­ject is sur­rounded by a fuzzy cloud, or ‘coma’, of dust and gas: this is the sig­na­ture of a comet heat­ing up and re­leas­ing gas as it ap­proaches the Sun. ‘Ou­mua­mua didn’t show any such ac­tiv­ity, rul­ing out it be­ing a comet, though that didn’t prove it was an alien space­craft.


The next thing to look at is how an ob­ject’s bright­ness changes over time. As­teroids have ir­reg­u­lar shapes and tend to spin, so they ap­pear brighter or dim­mer as they tum­ble in the sun­light. The bright­ness of a space­ship, on the


other hand, would be more stable. ‘Ou­mua­mua showed sig­nif­i­cant fluc­tu­a­tions in bright­ness, sug­gest­ing it was an as­ter­oid. . An ob­ject that is ro­tat­ing might be a hint it is cre­at­ing ar­ti­fi­cial grav­ity – think the ro­tat­ing ring of the space­craft in Andy Weir’s Her­mes The Mar­tian, or Dis­cov­ery One 2001: A Space Odyssey.

in Spin pro­duces a cen­trifu­gal force that can mimic the ef­fect of grav­ity. The faster the spin, the greater the force. As­tronomers could see ‘Ou­mua­mua was ro­tat­ing but each ro­ta­tion took seven to eight hours – way too slow to repli­cate any mean­ing­ful grav­i­ta­tional ef­fect for an ob­ject its size. To pro­duce ar­ti­fi­cial grav­ity sim­i­lar to what we ex­pe­ri­ence on Earth, it would need to ro­tate more like once a minute. An ob­vi­ous give­away could be found by lis­ten­ing for ra­dio trans­mis­sions across a range of wave­lengths. Says Hain­aut: “Nar­row ra­dio emis­sions, es­pe­cially if they are mod­u­lated in some way, don’t re­ally hap­pen in na­ture.” Lis­ten­ing for signs of alien civil­i­sa­tions is not a new idea – pro­grams like SETI have long been mon­i­tor­ing dis­tant so­lar sys­tems for life – but we rarely have cause to tune into our own. In De­cem­ber 2017 the Break­through Lis­ten pro­gram fo­cused the 100-me­tre Green Bank Tele­scope on ‘Ou­mua­mua but found no in­di­ca­tion of ar­ti­fi­cial sig­nals.


As­tronomers can also learn about the ob­ject’s sur­face by analysing the spec­trum of re­flected light. Un­ex­pected sig­na­tures could point to ma­te­ri­als such as space­craft paint. See­ing bright, short flashes might in­di­cate an ar­ti­fi­cial pol­ished sur­face. ‘Ou­mua­mua was found to have a dark red­dish hue, per­haps in­di­cat­ing a sur­face cov­ered with dense, metal-rich rock, red­dened from cos­mic ray bom­bard­ment.


A space­craft might give off a heat sig­na­ture from an en­gine or an in­ter­nal en­ergy source, vis­i­ble to us in the ther­mal in­frared. Its en­gine could also give off de­tectable emis­sions. An­other in­di­ca­tion of an en­gine might be an ob­ject stray­ing off the path of a nat­u­ral grav­i­ta­tion­ally driven or­bit. How­ever, out­gassing can also dis­turb the or­bits of comets, so it would take a large vari­a­tion to sig­nal an ar­ti­fi­cial space­craft.

02 | An artist’s im­pres­sion of ‘Ou­mua­mua, the first in­ter­stel­lar as­ter­oid to be iden­ti­fied, by the PAN-STARRS 1 tele­scope on 19 Oc­to­ber 2017.

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