Earth’s de­fences against as­ter­oids


IF you watched the 1998 movies Ar­maged­don or Deep Im­pact, you’d have the im­pres­sion that the way to deal with a big as­ter­oid hurl­ing to­wards earth would be to send some as­tro­nauts to land on it and place some bombs on it. Us­ing bombs to de­flect an as­ter­oid’s tra­jec­tory is a tech­nique that’s fea­si­ble but the mis­sion wouldn’t in­volve any as­tro­nauts. “That’s some­thing rel­e­gated to the movies — it makes a good movie, but we do not see in our stud­ies any tech­nique that would re­quire the in­volve­ment of as­tro­nauts,” said NASA’s plan­e­tary de­fence of­fi­cer, Lind­ley John­son, in June af­ter re­leas­ing a re­port en­ti­tled “Na­tional NearEarth Ob­ject Pre­pared­ness Strat­egy and Ac­tion Plan”. NASA is the USA’s Na­tional Aero­nau­tics and Space Ad­min­is­tra­tion, ar­guably the lead­ing space agency in the world.

The re­port out­lines the steps that NASA will take over the next decade to pre­vent dan­ger­ous as­ter­oids from hit­ting Earth. “This plan is an out­line not only to en­hance the hunt for haz­ardous as­ter­oids, but also to bet­ter pre­dict their chances of be­ing an im­pact threat well into the fu­ture and the po­ten­tial ef­fects that it could have on Earth,” said John­son, adding that the plan will help NASA “...step up our ef­forts to demon­strate pos­si­ble as­ter­oid de­flec­tion and other mit­i­ga­tion tech­niques”.


An as­ter­oid is a rocky or metal­lic ob­ject or­bit­ing the sun. They’re now de­fined as be­ing larger than one me­tre in di­am­e­ter with ob­jects smaller than that be­ing called me­te­oroids. The largest as­ter­oid im­pact in recorded his­tory is known as the Tun­guska event, where an as­ter­oid ex­ploded over Siberia on June 30, 1908.

That ex­plo­sion knocked down some 80 mil­lion trees over 2000 square kilo­me­tres of for­est. Re­mark­ably there was no known hu­man ca­su­alty be­cause it was such a sparsely pop­u­lated part of Siberia. Sci­en­tists clas­sify it as an im­pact event even though the as­ter­oid — es­ti­mated to be be­tween 60 and 190 me­tres — is be­lieved to have dis­in­te­grated at an al­ti­tude of 5 to 10km above the earth and didn’t ac­tu­ally hit the earth. The 15-mega­ton im­pact of the ex­plo­sion though is equal to about 1,000 times that of the atomic bomb dropped on Hiroshima dur­ing World War II.

The Tun­guska event, as dev­as­tat­ing as it was, is noth­ing com­pared to the one that sci­en­tists be­lieve hit the Earth 65 mil­lion years ago. Es­ti­mated to be be­tween 10 and 15km in di­am­e­ter, that as­ter­oid’s im­pact threw so much dust into the air that it would have cut off sun­light all over the world and thus pre­vented pho­to­syn­the­sis as well as low­ered tem­per­a­tures con­sid­er­ably.

The im­pact would have also caused mega-tsunamis while ejected melted rock would have caused wide­spread for­est fires. Sci­en­tists be­lieved that that event re­sulted in the ex­tinc­tion of some 70 per cent of the species on Earth.

To pre­vent an­other as­ter­oid like that — or worse still, one that’s even big­ger — from ever im­pact­ing Earth, sci­en­tists have devel­oped var­i­ous mit­i­ga­tion tech­niques, most of which in­volve de­flect­ing the as­ter­oid so that it steers off course.


The way they did it in Deep Im­pact and Ar­maged­don was to use some kind of ex­plo­sion to de­flect the as­ter­oid. NASA has de­ter­mined that a se­ries of stand­off nu­clear ex­plo­sions could push an as­ter­oid off course. The idea isn’t to ex­plode a nu­clear war­head on the as­ter­oid it­self — be­cause that would risk it break­ing into sev­eral smaller pieces which could still be just as deadly — but to det­o­nate it near the as­ter­oid. The heat from the ex­plo­sion would sear one side of the as­ter­oid and as ma­te­rial va­por­ises from its sur­face, the as­ter­oid would ac­cel­er­ate in a di­rec­tion away from Earth.


NASA’s pre­ferred op­tion, how­ever, is a ki­netic impactor called Dou­ble As­ter­oid Redi­rec­tion Test (DART) which is ex­pected to launch in 2021. DART would be “...our first tech­nol­ogy demon­stra­tion of the ki­netic im­pact tech­nique to de­flect an as­ter­oid,” said John­son. The DART space­craft, just about the size of a re­frig­er­a­tor, will en­counter the Didy­mos as­ter­oid which is ex­pected to fly by Earth in 2022. Didy­mos is a bi­nary sys­tem con­sist­ing of a larger ob­ject, 780 me­tres, and a smaller one, 160 me­tres, which is or­bit­ing the larger as­ter­oid. DART is pro­grammed to strike the smaller ob­ject while trav­el­ling at the speed of six kilo­me­tres per sec­ond. The im­pact of this hit should dis­rupt the or­bit of smaller as­ter­oid and pro­vide cru­cial data for such at­tempts on a larger scale.


An­other ap­proach also favoured by NASA is some­thing called a grav­ity trac­tor. This ap­proach doesn’t re­quire any space­craft to im­pact the as­ter­oid. Rather, it utilises the force of grav­ity to do this. A space­craft fly­ing along­side an as­ter­oid for years would have enough grav­i­ta­tional pull to change its path. Over the long haul, it would then be able to guide the as­ter­oid away from earth. How­ever, the tech­nique has never been tried in prac­tice and would re­quire lit­er­ally decades of test­ing, ac­cord­ing to NASA.


As they say, there’s more than one way to skin a cat. Over the years, sci­en­tists have pro­posed many dif­fer­ent op­tions for de­flect­ing as­ter­oids. These in­clude paint­ing the as­ter­oids white to change the amount of so­lar ra­di­a­tion. Ap­par­ently this would af­fect its tra­jec­tory. Other ap­proaches in­clude at­tach­ing so­lar sails to as­ter­oids and us­ing ro­botic lan­ders to mount thrusters to grad­u­ally change the di­rec­tion of the as­ter­oid’s flight path.


What­ever the ap­proach adopted, NASA can’t do it alone. The US, as the world’s lead­ing su­per­power, would nat­u­rally lead the way but it would need the help of other na­tions with strong space pro­grammes.

To help the rest of the world pre­pare for an as­ter­oid strike, NASA’s Plan­e­tary De­fense Co­or­di­na­tion Of­fice is work­ing with the United Na­tions Com­mit­tee on the Peace­ful Uses of Outer Space to look at what should be the in­ter­na­tional re­sponse should a dan­ger­ous as­ter­oid be found to be head­ing to­wards earth.


In Hol­ly­wood movies about as­ter­oids, they ap­pear rather sud­denly, re­quir­ing a rapid re­sponse by space agen­cies. In real life, sci­en­tists are able to see and project the tra­jec­tory of dan­ger­ous as­ter­oids not just decades and but cen­turies in ad­vance. NASA pre­dicts that there’s less than a 0.01 per cent chance of a po­ten­tially haz­ardous as­ter­oid mak­ing an im­pact in the next 100 hun­dred years.

Sci­en­tists have pro­jected that in 2135, an as­ter­oid called Bennu will fly pretty close to the Earth.

Al­though there’s no dan­ger of a col­li­sion, its flyby will ac­tu­ally be within the moon’s or­bit, which could change its path such that there is a 0.037 per cent (or 1 in 2,700) chance that it could hit the earth some­time be­tween 2175 and 2199. Be­tween now and then, there will be gen­er­a­tions of sci­en­tists from NASA and other na­tions’ space agen­cies work­ing on ef­fec­tive de­flec­tion sys­tems. In any event, the 500-me­tre-wide Bennu isn’t con­sid­ered an ex­tinc­tion-level as­ter­oid.

Of course, if you look long term enough, per­haps a few cen­turies or thou­sands of years from now, an ex­tinc­tion-level as­ter­oid might in­deed head to­wards the Earth. For­tu­nately for hu­mankind, to­day’s sci­en­tists have al­ready started work­ing on so­lu­tions to this prob­lem and would have surely per­fected it by the time the Earth ac­tu­ally needs it.

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