7 instruments zoom in on primitive 'world'

Ultima Thule might be a fraction of the original planetary seeds that collected into the planets of the Solar System. By examining the make-up of the ice world, New Horizons’ instruments can peek back into the early Solar System.

Radiometer indicates atmospheric pressure and temperature

If Ultima Thule has an atmosphere, the probe’s radiometer can tell its pressure and temperature.

Particle spectrometer 2 measures radiation

Heading into the Kuiper Belt, the probe’s other particle spectrometer measures the quantity of charged, high-energy particles, which typically come from outer space. Together with the measurements of the first spectrometer, this isto show how the solar wind reacts with plasma from interstellar space on the way to the outskirts of the Solar System.

Particle spectrometer 1 measures the solar wind

The Sun is constantly emitting charged particles into the solar wind that extends to the outskirts of the Solar System. One of the probe’s two particle spectrometers observes charged particles with low energy, which is typical for solar wind particles, hence measuring the solar wind force around the ice world.

Dust collector studies the rings

A large dust collector is to reveal if Ultima Thule is surrounded by dust rings and if so, what the dust is made up of. Moreover, the instruments are to measure the quantity of dust in the Kuiper Belt and so show the conditions under which small planet fragments form.

Telescopes have observed several stars (coloured lines), but Ultima Thule (dark spot) blocked out the light.

Telescope camera maps out world

New Horizons’ telescope camera takes photos of the ice world from different angles, as the probe moves on. The images are to show if Ultima Thule stretches 30 km or whether it is made up of two smaller globes that orbit each other. The camera also counts the number of craters on the surface.

Spectrometer reveals atmospheric chemistry

If Ultima Thule has a thin atmosphere, the gases in it will absorb UV radiation differently. The probe’s ultraviolet spectrometer can hence determine the contents of the atmosphere. More-over, the instrument will measure the quantity of hydrogen gas deeper into the Kuiper Belt. Based on this, astronomers can calculate the gas density on the outskirts of the Solar System.

Camera shows the make-up of the surface

A spectrometer detects the infrared heat radiation from Ultima Thule and so measures the surface temperature – including deviations in the shape of cold craters, etc. The spectrometer cooperates with an optical camera that detects the colours of the surface. Combined, the different types of data show what the surface is made up of. Horizons also provided us with lots of new data about Pluto’s five moons. The largest one, Charon, has no atmosphere, and old impact craters show that unlike Pluto, the moon is now a dead rock without any geological activity. But in Charon’s youth, intense geological activity resulted in major, ice-covered plains, huge gorges with depths of up to 13 km, and mountains that rise several km. However, the most surprising observation is that the north pole is covered in a reddish ice sheet made of methane and nitrogen, which must have escaped Pluto over time only to freeze on to the moon’s cold pole. Astronomers have never observed anything like it. Scientists estimate that the entire system of Pluto and the five moons formed in the early Solar System more than four billion years ago in a collision between two major planetary seeds – i.e. primitive worlds that formed at the same time as the planets, which have diameters ranging from a few m to hundreds of km.

Probe examines 25 worlds

Already before the launch in 2006, NASA had planned that after its visit to Pluto, the USD 700 million probe was to continue its observations deeper inside the Kuiper Belt, if its instruments worked, and there was any fuel left. Both conditions were fulfilled, and the prolonged mission was finally agreed in 2016.

The preparations began in 2011, when scientists used the Magellan telescope in Chile and the Subaru telescope in Hawaii to search for a suitable ice world. It had to be located within one degree of the satellite’s path to minimize the need for course changes and so fuel consumption. The search identified 143 potential targets, but all of them had to be given up, as they required too extensive course changes. Not until the scientists were granted Hubble Telescope observation time, they found three promising candidates – also known as Potential Target 1, 2, and 3. In late 2015, candidate No. 1 was chosen – Ultima Thule – which was located closest to New Horizons’ path.

By the time Ultima Thule was chosen, scientists believed that the ice world had a 3040 km diameter and was shaped like a peanut, but new observations from Earth indicate that there might be two small ice worlds orbiting one another. The mystery will not be solved until the satellite sends the first images back to Earth. If everything goes according to plan, the satellite will pass by Ultima Thule on New Year's Day at a distance of only 3,500 km or one third of its distance from Pluto. The observation period will be shorter, as the probe will pass quickly by the small planetary seed. The resolution by Ultima Thule will be three times that by Pluto, and so, New Horizons will be able to see details the size of a basketball court.

Ice world shows planet births

Ultima Thule is the most original and unchanged object from the early Solar System that has ever been closely examined.

The observations will show us the ice world’s shape, temperature, surface make-up, and atmosphere/dust rings, if any. If Ultima Thule is a fragment of a large planetary seed knocked off in a collision, the entire interior make-up of the original seeds will be directly visible in the surface. If so, scientists will be able to map out the seeds’ development in details and find out how they formed in the early Solar System in collisions between lumps of ice, dust, and rock. That would provide scientists with a clearer image of how the planetary seeds grew big, collided, and united into planets.

According to plan, up until 2021, New Horizons will observe at least 25 other ice worlds from longer distances spanning from 15 million km for some of the small ice worlds to 1.5 billion km for the potential dwarf planet of Quaoar, which is located eight billion km from Earth at the same distance from Ultima Thule as Earth is from the Sun.

The observations will reveal the ice worlds’ shape and determine the make-up of the surfaces. So, astronomers can enter the detailed studies of Ultima Thule into a larger context and develop a new, improved model for how the Kuiper Belt and the exterior planets formed. If more of the observed planetary seeds are included in dual systems with two worlds orbiting each other, it could explain how planetary seeds formed the four exterior planets 4.5 billion years ago.

In 1951, Dutch-American astronomer