Richard A Lovett
NASA’s New Horizons spacecraft is closing in on its second encounter with an alien world — a little one nicknamed Ultima Thule, 6.5 billion kilometres from Earth.
New Horizons was launched in 2006. Three years ago, it made news with dramatic photos of Pluto and its giant moon, Charon. But the mission was always designed to be a twofer, with enough fuel to adjust course to fly by another even more distant target, eventually selected to be the object formally classified as 486958 2014 MU69, and nicknamed Ultima Thule.
On 16 August, 2018, New Horizons caught the first glimpse of its destination, so small and dim it’s hard to pick out from the background stars. But the new rendezvous is fast approaching. Shortly after midnight on New Year’s Day, US east coast time, the craft will zip by Ultima Thule at a distance of only 3540 kilometres — more than three times nearer than its closest approach to Pluto.
But with Ultima Thule measuring only 37 kilometres or so in diameter, and the spacecraft moving at 51,000 kilometres per hour, it will very much be a case of “don’t blink.”
As New Year’s Eve becomes New Year’s Day, the spacecraft will approach the tiny object from a distance farther than the Earth is from the moon, whiz by it, and recede away again, all within 15 hours.
This means that every move the spacecraft makes has to be scripted in advance, so it knows where to point its instruments, and what measurements to take when.{%recommended 7747%}
“What’s remarkable is the work that goes into it, prior to such encounters,” says Jim Green, NASA’s chief scientist. “We have to be pointing the camera in the right place, and the object has to be there.”
“We only have one shot at it,” adds Alan Stern, the mission’s principal investigator.
This flyby, Stern says, will be even trickier than the prior flyby of Pluto. To start with, the spacecraft is three years older, including its power source, which runs on heat from the decay of radioactive material.
Also, Ultima Thule is enough farther out from the sun that lighting levels are about half those at Pluto, making it correspondingly more difficult for the instruments to make observations. The greater distance also lengthens the communication delay in talking to the spacecraft.
“Everything about this is more challenging,” Stern says.
Currently, very little is known about Ultima Thule. Even its size is an estimate, based on a couple of times astronomers were able to watch stars wink out as it passed between us and them.
Other information comes from the Hubble Space Telescope, which didn’t even discover the object until 2014, when New Horizons was already well on its way to Pluto. These observations revealed that it’s reddish in colour and very dark — about as dark as potting soil, Stern says.
“All the other things we want to learn are still in our future,” he adds, including whether it has moons or rings that New Horizons might plough into inadvertently.
That said, Ultima Thule has an enormous amount to teach us.
Objects like it are thought to be leftovers from the early days of the solar system — building blocks that never coalesced into something larger.
Other remnants of that era exist in the Asteroid Belt, but those that lie farther out, such as Ultima Thule, have spent all of history locked in a deep freeze.
“The temperature out at Ultima Thule is probably only 40 to 50 degrees above absolute zero,” Stern says. “Ultima Thule has been in this time capsule. Going to it is like making an archaeological dig into the history of our solar system.”
And, as in any Indiana Jones movie, there will be plenty of drama. On Christmas Day, NASA will upload its final command package to the spacecraft — commands that will carry it through the entire next nine days, other than a couple of built-in opportunities for minor tweaks if late observations show the object to be not quite in the expected location.
Then, just as it did at Pluto, the spacecraft will turn away from Earth, dropping out of contact for a few knuckle-biting hours while it devotes its entire attention to collecting data. Eventually, probably between 10 and 11am US east coast time on New Year’s Day, it will turn back to Earth for 15 minutes and send back a brief status report.
“That tells us that it made it through,” says Green.
The data will then trickle back more slowly, requiring an agonising 20 months until it is all received.
All told, Stern says, “It will be an adventure.”
