Beyond farthest frontiers

IN less than four weeks, the intrepid spacecraft called New Horizons will pass by the most distant solar system object yet encountered: a space rock orbiting well beyond the dwarf planet Pluto. It is called — for now, at east — Ultima Thule.

You may never have heard of the name Ultima Thule, but neither did anybody until relatively recently, when it was given that name as a result of a competition. The name means beyond the farthest frontiers, and it is a fitting name for the object! It is certainly easier to remember than the designation 2014 MU69, indicating that it was the 1745th object discovered in the second half of June 2014. Ultima Thule is only a temporary name, and a permanent one will eventually be assigned.

You may, however, have heard of New Horizons. If that name rings a bell, this is the spacecraft that flew past the dwarf planet Pluto in 2015, revealing, for the first time, the appearance of Pluto when seen close up. It was one of the most challenging space projects ever attempted, and it was a spectacular success. When the spacecraft flew past Pluto, it was 4773 million kilometres from Earth — so far from home that its radio signal, travelling at 300,000km a second, took four hours and 25 minutes to reach our antennas on Earth.

There was a time when it looked like the New Horizons mission may not have taken place. However, common sense prevailed, partly because of the discovery of the atmosphere of Pluto in the late 1980s.

At that stage, Pluto was at about its closest point to the sun on its elongated 248-year orbit, and it was realised that during the early part of the 21st century, the atmosphere may freeze back onto the surface of Pluto as its increasing distance from the sun resulted in lower temperatures. The opportunity to study the dwarf planet while its atmosphere was still present would not come again for about another 200 years.

And so it was that the mission survived a possible axe, but there was a proviso: to get the most out of the mission, New Horizons would need to be designed so that it would be capable of continuing to operate long after leaving Pluto, to enable it to study another object.

It was already known that there was a multitude of objects orbiting farther out than Pluto — the first was discovered in 1992 — and the ability to study at least one more body in this so-called Kuiper Belt of objects could yield valuable information about the formation of the solar system four and a half billion years ago.

That was accepted, and in 2006 New Horizons lifted off on a nine-year journey to what was then called the planet Pluto. Pluto was assigned “dwarf planet” status by astronomers later that same year.

The time for the next encounter is almost upon us. The fly-by of Ultima Thule will take place on January 1, 6526 million kilometres from Earth.

We already have some knowledge of this object. Earth-based measurements have told us to expect to see a body about 30km across, about the distance between Kingston and Bridgewater.

It is also suspected of being a “double” object, as we have obtained some clues as to its shape.

Such situations are not unknown. A famous example is the comet called ChuryumovGerasimenko, which has two distinct components joined together by a narrow “neck”, giving it the appearance of a rubber duck. In that case, analysis of the observations by the Rosetta spacecraft revealed that the two components started off being separate, joining only later.

We also know that the orbit of the object around the sun is nearly circular, indicating it formed in roughly the orbit that it currently has. This makes Ultima Thule very important in our research on early conditions in the outer solar system.

As with the Pluto fly-by, there is only one opportunity to get everything right; there is no second chance. Just as with the passage past Pluto, it will be a relatively short encounter. I’m sure that mission controllers will be checking and rechecking the plan many times in the next few weeks.