Beyond Pluto: How will New Horizons spend its remaining fuel?

Ian Evenden February 11, 2016

Zooming past Pluto at 52,000km/h might be considered enough for one car-sized spacecraft’s lifetime, especially when it’s sent back such amazing pictures of a never-before-seen world.

Beyond Pluto: How will New Horizons spend its remaining fuel?

Not for New Horizons, though. Its journey out of the solar system is taking it deeper into the Kuiper Belt, a mysterious region of small bodies beyond the orbit of Neptune.

Its target, 2014 MU

69, is a classical Kuiper Belt Object (KBO), or “cubewano” (the name comes from the first object discovered after Pluto – named 1992 QB1), estimated to be up to 45km in diameter, orbiting around a billion miles from Pluto and 44AU from the sun. An AU, or astronomical unit, is the distance from the Earth to the sun – 93 million miles – and 44AU expressed in miles is the sort of number that will make your calculator run out of zeroes.

“2014 MU69 is a great choice because it is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey [a National Research Council project that flags up potential NASA missions] desired us to fly by,” says New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. “Moreover, this KBO costs less fuel to reach [than other candidate targets], leaving more fuel for the flyby, for ancillary science, and greater fuel reserves to protect against the unforeseen.” new_horizons_after_pluto

The probe won’t reach its target until the very end of 2018 at the earliest, so what else is out there in the Kuiper Belt?

First off, calling it the Kuiper Belt is to forget the work of Irish astronomer Kenneth Edgeworth, who proposed that there were bodies other than Pluto in the space beyond Neptune in the 1940s – almost ten years before Dutchman Gerard Kuiper published his article on the subject. American Frederick Leonard hypothesised such a belt shortly after Pluto’s discovery in 1930, but the name Kuiper Belt has stuck even though some astronomers use Edgeworth-Kuiper Belt.

Pluto was originally thought to be around the size of Earth, and Kuiper believed such a planet would have swept any smaller bodies out of its orbit, meaning that if his proposed belt had once existed, it wouldn’t still be there today. Happily for New Horizons, Pluto is much smaller than Earth, and there are a great many small bodies out at the edge of the solar system to explore. It’s this failure to clear its orbit of other bodies that led to Pluto being downgraded to dwarf planet status in 2006.

These trans-Neptunian objects (defined as anything from the orbit of Neptune – 30AU – to around 1,000AU from the sun) vary greatly in size. The smallest detected so far, spotted by the Hubble Space Telescope, is 975 metres across and 4.2 billion miles away. The largest known is Pluto, although a hypothetical “Planet Nine” inferred by the orbital paths of certain KBOs could be up to ten times the mass of Earth and on a highly elliptical orbit, taking as long as 20,000 years to complete a trip around the sun.

With bodies of all sizes whizzing about, the extreme edge of the solar system is a busy place – and not a simple one to define. “We have to separate what we know is there from our theories of what’s there,” says professor Mark E Bailey, the director of Armagh Observatory in Northern Ireland. “We observed Pluto in 1930, then in 1992 we observed 1992 QB1, and subsequently we’ve found maybe 1,000 objects, but there’s thought to be in the order of 100,000 objects in that trans-Neptunian region bigger than 100km [62 miles] across.”

These objects aren’t randomly distributed though, and Neptune – the third most massive planet in the solar system – has a lot of influence on them. “You’ve got Uranus, then Neptune at 30AU, then you’ve got these objects moving in planet-like orbits,” says Bailey, who studies the origin of comets and has an asteroid, Minor Planet (4050) Mebailey, named after him. “Our understanding of that population has increased to the extent that we recognise that the different objects out there tend to congregate in different classes of orbit. There’s the Pluto types of object which are in a 2:3 resonance with Neptune [making two revolutions around the sun for every three Neptune makes] and are therefore somewhat protected from strong close approaches to Neptune even though some of them, such as Pluto, cross Neptune’s orbit. These are called Plutinos, not a very imaginative name.

“Cubewanos, which have orbits similar to 1992 QB1, are the typical belt objects – they’re moving in orbits somewhat decoupled from Neptune’s with pericentres [the point of closest approach of an astronomical object in an elliptical orbit to its centre of attraction] beyond 32AU and with an outer boundary of 50AU.”

That’s not all, though. “There are other categories of objects that some people call scattered disc objects,” continues Bailey. Some of these objects’ orbits can approach Neptune, from where they can be pulled into the inner solar system by interactions with planets’ gravity and become part of the Jupiter family of comets. “We’ve got a terrible name for them – the near-Neptune high-eccentricity population,” says Bailey. “The last population is the Oort Cloud, which is a swarm of comets that surrounds the solar system, stretching more than halfway to the nearest star.”

Attempting to define the various classes of objects so far from the sun is hamstrung by the fact we can’t easily see them, and by astronomers’ failure to agree on definitions. Take the centaurs – a group of objects that orbit between Jupiter and Neptune. No-one can agree on whether they are locked into this region of space, or have much more eccentric orbits that take them further from the sun and into the Kuiper Belt.

If we can’t define them, can we at least say what they’re made of? Well, no. And that’s one of the questions New Horizons is hoping to answer. “We assume they’re like comets,” says Bailey. “They’ll be made of rock and ice, mostly water ice if they formed in the outer solar system as that’s the most common type of ice.” Selection bias means we have only observed the very largest objects, which makes 2014 MU

69, at only 45km across, something of a mystery. Starting a search in 2011 using some of the largest ground-based telescopes on Earth, the New Horizons team found several dozen KBOs, but none were reachable with the limited fuel supply available aboard the spacecraft.

Using the Hubble Space Telescope, five objects were discovered in 2014, which were eventually narrowed down to three potential targets – 2014 MU69, 2014 OS393 and 2014 PN70. All are in the same size range, ten times larger than the comet selected for the Rosetta landing mission last year, but less than 1% of the size (and about 1/10,000th the mass) of Pluto, and are thought to be similar to the building blocks of Kuiper Belt planets such as Pluto. Essentially, by studying these objects, we’re studying the way the outer solar system formed 4.6 billion years ago.

“There’s so much that we can learn from close-up spacecraft observations that we’ll never learn from Earth, as the Pluto flyby demonstrated so spectacularly,” says New Horizons science team member John Spencer, also of the Southwest Research Institute. “The detailed images and other data that New Horizons could obtain from a KBO flyby will revolutionise our understanding of the Kuiper Belt and KBOs.”

And after that? The course change required to reach 2014 MU69 used up about 35% of New Horizons’ remaining fuel supply, so there’s potential for further flybys if suitable objects can be identified along the probe’s trajectory. As it flies away from the sun, New Horizons has already used its cameras to observe KBO (15810) 1994 JR1 from a distance of 280 million kilometres, close enough to see its shape but few other details.

After the fuel runs out, the 9.75kg of plutonium that powers the probe will provide enough juice to keep its transmitters going into the 2030s. Then, the cold, dead probe will continue to head out of the solar system at around 13km/s, and it’s up to us to catch up with it.

Images: NASA