NASA’s New Horizons spacecraft is about to fly past and send back data about a planetary object whose existence was not even known when it was launched in 2006. Nicknamed Ultima Thule, it is a small ball of ice, rock, or both in a region of our solar system called the Kuiper Belt. Planetary bodies in the Kuiper Belt are thought to be unchanged from when the solar system was formed 4.5 billion years ago. One billion miles past Pluto, Ultima Thule will be the farthest object visited by a probe from Earth when New Horizons makes its closest approach on January 1 at 12:33 am ET.

New Horizons already made breakthrough discoveries when it flew past Pluto in 2015, its primary objective. The spacecraft still had fuel and power and the science instruments were still working, so the mission was extended to allow it to observe an object further out in the Kuiper Belt.

At one point in the project’s long history, visiting Pluto and a Kuiper Belt Object (KBO) were both primary objectives and it was called the Pluto-Kuiper Express. NASA killed that project in 2000 because of cost and schedule challenges, but in November 2001 approved a lower cost mission focused on Pluto. Principal investigator (PI) Alan Stern of the Southwest Research Institute (SwRI) and the Johns Hopkins University Applied Physics Laboratory (JHUAPL) had only about four years to design and build the spacecraft and instruments to meet a do-or-die launch date of January 2006 dictated by celestial mechanics.

The success at Pluto and the spacecraft’s continuing good health, plus the identification in 2014 of a KBO along its route, won approval for the extended mission that will reach its climax on Tuesday.

Today, New Horizons is 4.1 billion miles (6.6 billion kilometers) from Earth. It takes a radio signal about 6 hours 7 minutes to make that trip.

The spacecraft will fly as close as 2,200 miles (3,500 kilometers) to Ultima Thule, which is only about 20 miles (30 kilometers) long. Although the spacecraft is just four days away, Ultima Thule is only now getting to be close enough to fill one pixel of an image. By New Year’s Eve, it will fill 1,000 pixels. During a press briefing today, Stern said Ultima Thule would appear in the spacecraft’s cameras as big as a full moon on Earth.

No one knows for sure what Ultima Thule is composed of or even very much about its appearance. It was detected by scientists in 2014 using the Hubble Space Telescope. Its official designation is 2014 MU69. Ultima Thule (pronounced TOO-lee) is a nickname chosen by the mission team following a public naming contest. It essentially means “a place beyond the known world.”

At today’s briefing, Stern, Missions Operations Manager Alice Bowman and Science Operations Manager Kelsi Singer were asked what they expect it to look like. Singer said it is difficult to predict. “I’m ready to be excited and surprised by what we find.” Bowman concurred. “It will amaze us just like Pluto did.” Stern agreed it is wise not to make predictions and, in any case, everyone will know in just four days. “I don’t have any idea” what to anticipate. “I can’t wait to find out.”

Generally speaking, KBOs are thought to be primarily icy bodies that formed at the same time as the solar system. Those is circular orbits like Ultima Thule have remained there throughout the solar system’s 4.5 billion year history in temperatures close to absolute zero. Scientists are anxious to learn whatever they can about an object that has been so well preserved over eons.

Ultima Thule will not be the first KBO visited by a spacecraft because Pluto itself is categorized as a KBO. It is the farthest, however — 1 billion miles (1.6 billion kilometers) beyond Pluto.

New Horizons will speed by Ultima Thule at 32,000 miles per hour (52,000 kilometers per hour) with closest approach at 12:33 am ET on January 1 collecting data and images that will be transmitted back to Earth. First, however, it will send a simple signal to tell mission controllers that all is well. That “phone home” signal is expected to arrive at Earth about 10:29 am ET January 1.

The data and images will begin arriving thereafter, but slow data rates mean it will take 20 months for everything to be transmitted and received.

What’s next for New Horizons? The spacecraft will continue on its course in perpetuity. Stern said today the spacecraft and its instruments are in excellent health and it still has plenty of fuel and power. He expects it to continue operating until the late 2030s, pointing out that it is only halfway through the Kuiper Belt. It will remain in the Belt until the end of the 2020s and he hopes to get permission to fly past another KBO in that decade.