New hori­zons at ul­tima thule

As NASA’s New Hori­zons probe nears its sec­ond tar­get, we in­ves­ti­gate what’s lurk­ing at the edge of the So­lar Sys­tem

All About Space - - Contents - Re­ported by Ian Even­den

The mis­sion’s sci­en­tists re­veal what’ll hap­pen at the Kuiper Belt Ob­ject

The constellation of Sagit­tar­ius, the Archer, is a com­mon sight in the win­ter sky, eas­ily recog­nis­able by the ‘teapot’ as­ter­ism that makes up its front half. Look in the Archer’s di­rec­tion and you’re star­ing into the cen­tre of the galaxy, the Milky Way con­tribut­ing the ‘steam’ from the teapot’s spout.

With a home tele­scope you might see the La­goon Ne­bula within Sagit­tar­ius, or a glob­u­lar clus­ter such as Messier 54. If you have ac­cess to bil­lions of dol­lars’ worth of space tele­scope, how­ever, you may dis­cover other much smaller and fainter things.

This is just what hap­pened in 2014 when the Hub­ble Space Tele­scope was sur­vey­ing the area, hop­ing to find a new tar­get for the New Hori­zons mis­sion after ground-based tele­scopes had failed to

“By ob­serv­ing KBOs up close we hope to learn a lot about how the early for­ma­tion stages of the plan­ets took place”

Dr Alan Stern

find any­thing there. Back then, NASA’s mis­sion to Pluto was still in­com­plete, but it was clear the probe would be head­ing in the di­rec­tion of Sagit­tar­ius fol­low­ing its en­counter with the dis­tant dwarf planet and, if an even more dis­tant ob­ject were found in its path, it could carry on and en­counter that too. With plenty of plu­to­nium diox­ide on board to gen­er­ate power the probe is ex­pected to keep func­tion­ing for many more years, time enough to ex­plore the outer reaches of the So­lar Sys­tem at over 58,000 kilo­me­tres (36,040 miles) per hour.

The ob­ject Hub­ble dis­cov­ered was named (486958) 2014 MU69. The num­ber in brack­ets is its mi­nor planet num­ber – we know of over half a mil­lion – while 2014 is the year of dis­cov­ery. M is for the sec­ond half of June, and U64 in­di­cates it’s the 1,745th ob­ject dis­cov­ered dur­ing those two weeks. Mod­ern tech­niques and space tele­scopes are dis­cov­er­ing a lot of ob­jects. 2014 MU69 quickly picked up a nick­name, Ul­tima Thule, as a re­sult of a pub­lic vote. Thule, in Greek and Ro­man lit­er­a­ture, was the fur­thest north you could go, of­ten as­so­ci­ated with Green­land or Ice­land. The later ad­di­tion of ‘ul­tima’, mean­ing ‘fur­thest’, was used to mean a place beyond the bor­ders of the world.

There are many thou­sands of un­known worlds out there at the bor­ders of our So­lar Sys­tem beyond the or­bit of Nep­tune, but very few were in the right place to be vis­ited by New Hori­zons. Col­lec­tively known as Trans-Nep­tu­nian Ob­jects (TNOs), they’re dimly lit and enor­mously spread out, with dis­tances of 1 As­tro­nom­i­cal Unit – the dis­tance from the Earth to the Sun – be­tween them. The best known, Pluto, is the most mas­sive known ob­ject in an area of space be­tween 30 and 55 AU from the Sun, known as the Kuiper Belt.

We know of a few other big things in the belt – with two of them clas­si­fied as dwarf plan­ets like Pluto – and there’s some­thing heav­ier than Pluto too, the dwarf planet Eris, but it’s three-times fur­ther from the Sun than the de­moted ninth planet, and isn’t classed as a Kuiper Belt ob­ject (KBO) thanks to its ex­treme dis­tance – it falls into an area known as the Scat­tered Disc. Many sci­en­tists also be­lieve there’s ev­i­dence of a larger body in the or­bits of smaller ones, but it hasn’t been seen yet.

An ob­ject needs a sin­gle-body di­am­e­ter of at least 300 kilo­me­tres (186 miles) to be con­sid­ered a dwarf planet. Most known KBOs are much smaller than Pluto’s 2,377-kilo­me­tre (1,477-mile) width, how­ever.

There’s Lempo, a bi­nary sys­tem with at least one ad­di­tional satel­lite and a to­tal di­am­e­ter of around 400 kilo­me­tres (249 miles), the same as Saturn’s moon Mi­mas. It’s named after the god of love from Fin­nish mythol­ogy, and like a lot of KBOs ap­pears ex­tremely red. Then there’s Drac, only 90-kilo­me­tres (56-miles) across and named after Bram Stoker’s fa­mous count. It’s no­table be­cause of its high in­cli­na­tion and the fact its or­bit is ret­ro­grade – the op­po­site di­rec­tion to most other ob­jects.

Ul­tima Thule could be a bi­nary sys­tem too, but with a di­am­e­ter of just 30 kilo­me­tres (18.6 miles) it’s a bit hard to make out from Earth. It was cho­sen as the new tar­get be­cause of its po­si­tion – less fuel was re­quired to reach it. A brighter, and there­fore prob­a­bly larger ob­ject was also con­sid­ered, but the fuel needed to reach it would have left less in the tank for fu­ture ma­noeu­vring.

But why worry about some­thing so small and far away? Study­ing KBOs like this tells us about the way the So­lar Sys­tem was a long time ago. “The belt is anal­o­gous to the So­lar Sys­tem’s at­tic,” says New Hori­zons’ prin­ci­pal in­ves­ti­ga­tor Dr Alan Stern. “It’s an an­cient re­gion, very far from the Sun, which has been pre­served in a deep freeze. It’s the equiv­a­lent of an ar­chae­o­log­i­cal dig into the his­tory and for­ma­tion of the plan­ets. So sci­en­tif­i­cally it’s a gold mine, and by go­ing there with a space­craft and ob­serv­ing KBOs up close, like we’ll be do­ing with Ul­tima, we hope to learn a lot about how the early for­ma­tion stages of the plan­ets took place.”

And that’s not all, as the in­ter­ac­tions of the small ob­jects out there can tell us a lot about the move­ments of big ob­jects. Re­ally big ones. “What we know of the Trans-Nep­tu­nian re­gion is that it’s the left­over rem­nants of the ob­jects that didn’t make it into be­ing plan­ets,” says Dr Michele Ban­nis­ter, a post-doc­toral re­searcher at Queen’s Univer­sity Belfast, who helps dis­cover mi­nor plan­ets as part of the Outer So­lar Sys­tem Ori­gins Sur­vey. “These lit­tle rocky and icy worlds were formed in the ini­tial disc of ma­te­rial around the

Sun, the ones that never grew up into be­ing plan­ets in their own right. Since then they’ve been sculpted by changes in the or­bital po­si­tions of the gi­ant plan­ets, par­tic­u­larly Nep­tune.”

The idea that plan­ets move around rather than just placidly or­bit the Sun can be hard to process given the enor­mous size and mass of the outer plan­ets, but ac­cord­ing to the Nice model of So­lar Sys­tem for­ma­tion – named after the place in France, not merely be­cause it’s pleas­ing – as the So­lar Sys­tem gath­ered to­gether from its pro­to­plan­e­tary disc, ev­ery­thing formed much closer to the Sun.

The outer edge of the Kuiper Belt was once 30AU from the Sun in­stead of 55, and Uranus was the outer planet in­stead of Nep­tune. There’s even a hy­poth­e­sis that there could have been a fifth gi­ant

planet, ejected from the So­lar Sys­tem fol­low­ing an en­counter with Jupiter.

Grav­i­ta­tional in­ter­ac­tions be­tween the four gi­ants we know of led to Nep­tune mov­ing out­wards past the or­bit of Uranus, pro­duc­ing the Kuiper Belt we see to­day. “What we see there to­day are ma­te­ri­als from that ini­tial disc,” says Ban­nis­ter. “Some of them are fa­mil­iar, like wa­ter ice and rock, but some of them are un­fa­mil­iar, like kitchen clean­ing chem­i­cals you have un­der your sink, in solid form.”

And even though it’s called a belt, don’t imag­ine it’s com­pletely flat. “A lot of the ob­jects have never had any­thing hap­pen to them; they’re on round, flat or­bits, but a lot of them have had en­ergy put into them,” con­tin­ues Ban­nis­ter. “They can be a lot more ec­cen­tric; their or­bits are long, thin el­lipses and they’re tilted com­pared to the plane of the So­lar Sys­tem. Some of them are in an or­bital bal­let with Nep­tune, called a mean mo­tion res­o­nance, where Nep­tune goes around the Sun three times for ev­ery two times one of these ob­jects goes round – that’s the res­o­nance Pluto is in. A lot of the ob­jects are in places where they can do this, and that res­o­nant ob­jects ex­ist at all when the spa­ces in be­tween are free of ob­jects is a sig­na­ture that Nep­tune mi­grated out­wards in the early So­lar Sys­tem.”

Ul­tima Thule is one of the less ec­cen­tric ob­jects, and is not in a res­o­nance with Nep­tune. Known as a ‘clas­si­cal’ KBO, it’s also part of the ‘cold’ pop­u­la­tion, which means it has never re­ceived any en­ergy from col­li­sions or grav­i­ta­tional in­ter­ac­tions. It’s just been sit­ting there, do­ing rel­a­tively lit­tle, since the So­lar Sys­tem formed. Much of what we know about it comes from Hub­ble ob­ser­va­tions, or from oc­cul­ta­tions where the ob­ject passes in front of a back­ground star. The dip in the star’s bright­ness tells us about what’s block­ing it, and three oc­cul­ta­tions by Ul­tima Thule in 2017 were stud­ied by a spe­cial group of as­tronomers formed by the New Hori­zons team. This study of oc­cul­ta­tions is the same process that is used to ob­serve ex­o­plan­ets around dis­tant stars, but even with this data Ul­tima Thule re­mains a mys­tery.

“We don’t know if it’s two ob­jects, or if it’s bi­nary, but we know its shape is not round,” says Ban­nis­ter. “Bi­nary sys­tems are very com­mon in the pop­u­la­tion to which this lit­tle world be­longs, and this ties di­rectly into how they formed. A So­lar Sys­tem starts off be­ing made of dust and gas, and this starts form­ing lit­tle ob­jects, and they have to get over about a me­tre in di­am­e­ter and sud­denly they’re full-on as­ter­oids that can start ac­cret­ing ma­te­rial much, much faster. This whole process is some­thing peo­ple are very ac­tively work­ing to un­der­stand, but bi­nary ob­jects might be im­ply­ing that, when you ini­tially make lit­tle worlds, you make them bi­nary, so it tells us a lot about what physics to put into sim­u­la­tions of how plan­ets are formed.”

What­ever Ul­tima Thule is like, New Hori­zons is well equipped to tell us all about it, as the star­tling im­ages of red-and-white plains and moun­tains on Pluto showed. “We have a very pow­er­ful set of seven sci­en­tific in­stru­ments,” says Stern. “They will map its sur­face com­po­si­tion, search for an at­mos­phere, search for satel­lites, search for rings and make other kinds of stud­ies. And I hope that we put to­gether a very com­plete pic­ture of what this typ­i­cal Kuiper Belt ob­ject is like, be­cause not only is this the first time that an ob­ject like this has been ex­plored, but no­body’s plan­ning an­other mis­sion out to the Kuiper Belt, so I think that this dataset is some­thing that’s go­ing to be valu­able sci­en­tif­i­cally for decades to come.”

How long New Hori­zons can carry on send­ing back this kind of re­mark­able data is lim­ited by the power and fuel sup­plies on the space­craft, as well as the avail­abil­ity of suit­able tar­gets in its path. When you’re trav­el­ling over 58,000 kilo­me­tres (36,040 miles) per hour, chang­ing di­rec­tion isn’t easy.

Stern isn’t wor­ried about the fu­ture, how­ever.

“We have a very healthy space­craft,” he says. “We have the fuel and the power in our nu­clear bat­tery to run it for at least 15 years, maybe 20 years. If NASA con­tin­ues to fund it, if NASA judges that it’s sci­en­tif­i­cally worth­while, this space­craft will be oper­ated into the mid-2030s or later. It’s very much like the Voy­agers which fin­ished their ex­plo­ration of the plan­ets in the 1980s, but are still re­turn­ing use­ful sci­en­tific data 40 years after launch.”

“I think that this dataset is some­thing that’s go­ing to be valu­able sci­en­tif­i­cally for decades to come”

Dr Alan Stern

New Hori­zons launches from Cape Canaveral Air Force Sta­tion, Flor­ida, with a nine-year flight ahead of it be­fore its en­counter with Pluto

Five over­laid Hub­ble im­ages show­ing Ul­tima Thule. Taken in 2014, these show the KBO’s move­ment at ten-minute in­ter­vals

An artist’s im­pres­sion of New Hori­zons’ en­counter with Ul­tima Thule, which is de­picted as a bi­nary ob­ject with a third, more dis­tant satel­lite

An im­age from April 2017 shows Ul­tima Thule as an un­recog­nis­able dot among back­ground stars – it’siden­ti­fied by its mo­tion

An artist’s im­pres­sion of the New Hori­zons probe, seen ap­proach­ing Charon in the Pluto sys­tem

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