486958 Arrokoth

Arrokoth is a small, icy object located in the Kuiper belt.

This region is a vast ring of icy bodies orbiting the Sun beyond Neptune and Pluto. Arrokoth is a contact binary, which means it consists of two distinct lobes joined together. These lobes are connected by a narrow neck of material called a waist. This unique shape makes it look somewhat like a snowman. Because it has changed so little over billions of years, it serves as a primitive window into the early solar system.

The formation of Arrokoth likely involved a very gentle process. Scientists believe the two lobes were once separate planetesimals. A planetesimal is a small, solid object that helped build planets long ago. These two bodies likely formed from the same cloud of icy particles. They probably moved toward each other and merged in a slow, low-speed collision.

This slow merger left the lobes in a near-parallel alignment. Their long axes are almost aligned with each other and the object's rotational axis. This alignment suggests they were mutually locked by tidal forces before they finally touched.

Arrokoth is composed of two specific parts named Wenu and Weeyo. Wenu is the larger lobe, also known as Wenu Lobus. It has a slightly flattened shape and measures about 35 kilometers across its longest axis. Weeyo is the smaller lobe, or Weeyo Lobus, and is roughly spherical. Weeyo measures about 26 kilometers across its longest axis. The volume of Wenu is nearly exactly twice that of Weeyo.

The entire object is about 35 kilometers long and roughly 21 kilometers thick. A bright band of material called Akasa Linea encircles the narrow neck.

Astronomers discovered Arrokoth on June 26, 2014. Marc Buie and the New Horizons Search Team found it using the Hubble Space Telescope. Before it had a permanent name, it was nicknamed "Ultima Thule." This name referred to a distant place beyond the known world in ancient literature. However, the name faced criticism due to its historical associations with 19th-century racist ideologies. Consequently, the International Astronomical Union officially named the object Arrokoth in November 2019.

The name comes from the Powhatan people of the Tidewater region. In the Powhatan language, the word likely meant "sky" or "cloud."

On January 1, 2019, the NASA spacecraft New Horizons flew past Arrokoth. This mission made Arrokoth the farthest object in the solar system ever visited by a spacecraft. The surface of the object is notably red. This color comes from tholins, which are complex organic compounds. These tholins are produced through photolysis. This happens when cosmic rays and ultraviolet solar radiation hit simple organic compounds.

The spectrum of Arrokoth shows the presence of methanol and sulfur-rich organic compounds. Interestingly, there was no evidence of water ice found on the surface.

The surface of Arrokoth is very smooth and has very few craters. This suggests that impact events in the Kuiper belt are uncommon. Because objects in this region move at slow orbital speeds, impact speeds are also low. Typical impact speeds are expected to be around 1 kilometer per second.

Because of this low impact rate, the surface has remained preserved for 4.5 billion years. Scientists have identified small pits on the surface, measuring about 100 to 200 meters. The exact cause of these pits is still unknown. They might be caused by impacts, the collapse of material, or the escape of gases from the interior.

Studying Arrokoth helps scientists understand the broader history of our solar system. It acts as a time capsule of the materials that existed during the solar system's birth. The presence of certain chemicals, like methanol and acetylene, helps researchers track how volatiles behave in deep space. By looking at the way these small pieces merged, we learn how larger planets might have grown. Arrokoth provides a direct link to the primitive building blocks of the planetary system we live in today.