Arokos: The most distant "sky"

On January 1, 2019, NASA's New Horizon probe flew past the "snowman"-shaped Kuiper Belt asteroid Arrokoth, which is 6.5 billion kilometers away and is the most distant asteroid ever visited by a human probe. It was once nicknamed "the end of the world." The data sent back by New Horizons not only shows us a strange world we have never seen before, but is also expected to reveal more mysteries related to the origin and evolution of the solar system.

On November 13, 2019, NASA announced that the official name of 2014 MU69 is "Arrokoth". The New Horizons flyby detection of Arrokoth was selected as one of the top ten scientific breakthroughs of 2019 by SCIENCE magazine.

Now, follow Xiao Zi (hereinafter referred to as "Zi") to visit Ji Jianghui (hereinafter referred to as "Ji"), Director of the Key Laboratory of Planetary Science of the Chinese Academy of Sciences and Researcher of the Planetary Science and Deep Space Exploration Laboratory of Purple Mountain Observatory, to learn some details.

Zi: What is the mission of New Horizons?

Ji: The New Horizons probe was launched by NASA on January 19, 2006. It is mankind's first messenger sent to Pluto and the Kuiper Belt. It will bring humans information about the surface features, geological conditions, internal composition and atmosphere of these distant solar system bodies, and unveil the mystery of the outer edge of the solar system for us.

The first composite "portrait" of Arokos | Source: AAAS/Science

Zi: Why flyby Pluto?

Ji: After human spacecraft visited the eight planets in the solar system from Mercury to Neptune, New Horizons was given the glorious mission of the "Terminator" of solar system exploration, because Pluto was still the ninth planet in the solar system when it was launched, and it was downgraded to a dwarf planet by the International Astronomical Union 7 months after the launch. Unlike the rocky terrestrial planets (Mercury, Venus, Earth and Mars) and the gaseous Jovian planets (Jupiter, Saturn, Uranus and Neptune), Pluto and its largest satellite Charon belong to the third category: icy dwarf planets, which have solid surfaces and are mostly composed of icy substances.

The images and scientific data sent back by New Horizons show that Pluto's surface has a heart-shaped area composed of vast frozen plains, with a large amount of methane ice distributed on the surface, and a young iceberg as high as 3,500 meters near the equator. New Horizons also obtained for the first time the complex and diverse landforms and surface features of several of Pluto's satellites.

Zi: Why did you choose the Kuiper Belt next?

Ji: The Kuiper Belt is a disk-shaped region about 40 to 50 astronomical units (AU, 1AU is about 150 million kilometers) from the sun outside the orbit of Neptune. Thousands of small icy bodies have been discovered so far, and they are believed to come from the fragments of the protoplanetary disk surrounding the sun. As early as the 1950s, astronomers Kuiper and Edgeworth predicted the existence of such a "marginal" zone in the solar system, but it was not until 1992 that the "first" Kuiper Belt Object (KBO) was found. So far, thousands of Kuiper Belt Objects have been discovered.

Pluto (discovered in 1930) is the largest known object in the Kuiper Belt, and its demotion also virtually brought forward the discovery of the first Kuiper Belt object by 62 years. New Horizons is the fifth human spacecraft to fly through the Kuiper Belt after Voyager 1, 2 and Pioneer 10, 11, but it is a pioneer in scientific exploration of the Kuiper Belt, and it will bring us information about the evolution of these icy bodies.

A probe flying over the Kuiper Belt | Image source: NASA/JHUAPL/SwRI/Magda Saina

Zi: What new discoveries have been made during the flyby exploration of Arrokoth?

Ji: First of all, this is the first time that a human probe has carried out an exploration mission deep into the Kuiper Belt, revealing this unexplored remote territory in the solar system. The celestial bodies in the Kuiper Belt are believed to be the material left over from the early formation of the solar system. Because they are located at the outer edge of the solar system, these small celestial bodies contain the most primitive information on the formation and evolution of the solar system planets. Arrokoth has well preserved the appearance of the solar system when it was first formed 4.5 billion years ago. Secondly, this is also the first time that humans have explored a well-preserved planetesimal and captured its clear image at an unprecedented close distance and perspective.

Data from the New Horizons spacecraft show that this snowball-shaped asteroid is about 35 kilometers long, 20 kilometers wide, and 10 kilometers thick. Scientists have also made new achievements in the study of Arrokoth's origin, evolution mechanism, surface structure, and material composition. The strange shape of Arrokoth's head and body, which are coaxial, indicates that it may not be formed by a violent collision, but by the slow merger of two lobes after long-term orbital dissipation. This provides direct evidence for the streaming instability model of planetesimal formation.

Arrokoth has bright spots and patches, hills and valleys, pits and protrusions on its surface. Spectral data also indicate the presence of methanol, water ice and organic molecules on its surface. The red color of its surface may be caused by the modification of organic matter: simple molecules are reassembled into complex organic polymers.

Arokos geomorphology map (Stern et al., Science 2019)

Zi: What is the significance of the flyby detection of Arokos?

Ji: Arokos means sky in the Powhatan/Argonne language, implying a yearning for the sky and curiosity about other planets and worlds outside the Earth. The name also pays tribute to the most distant flyby of a human spacecraft from the Earth in history. The exploration of small bodies in the Kuiper Belt has enabled humans to understand the structure of the solar system and the evolution of planets from a new perspective. The detailed study of more "living fossils" in the solar system - ancient small bodies and the results of organic molecule detection will help humans explore the important scientific issue of the origin of life on Earth.

Zi: What is the status of research development in this field in China? What gaps are there?

Ji: my country's Chang'e-2 probe achieved a close flyby of the potentially threatening near-Earth asteroid Toutatis during its extended mission, achieving important scientific results and having a great impact internationally. Since 2010, relevant domestic units have organized demonstrations of China's deep space exploration plan for 2030, which includes the implementation of small body sampling and return and main-belt comet exploration in the next few years. my country's deep space exploration of small bodies started late, the research foundation is relatively weak, key technologies need to be broken through, and the research and development capabilities of scientific payloads need to be further improved. The deep space exploration mission is a long and arduous journey.

It is worth mentioning that in recent years, domestic scientists have proposed a plan for solar system edge exploration, namely, to achieve edge exploration of the solar system beyond 100 astronomical units in 2049, launch probes to carry out large-scale three-dimensional spatial characteristics of the heliosphere, the edge of the solar wind and the characteristics of matter in neighboring interstellar space, visit the asteroid belt, Jupiter and other gaseous large planetary systems, and detect Kuiper belt objects at the same time.

Zi: Finally, could you please share our outlook on the future development trends and possible breakthroughs of deep space exploration?

Ji: In the future, deep space exploration and planetary science will focus on key scientific issues, combine advanced aerospace technology, scientific research and economic development levels, use scientific goals to guide and drive technological innovation and the development of emerging industries, and expand international cooperation. With the launch of more deep space exploration missions, scientists will make new breakthroughs in major scientific issues such as the origin of the solar system, the origin of life on Earth, and the threat of small celestial bodies to the survival of the Earth and human beings.

This article was originally published in the 2nd issue of China Science Foundation in 2020, with appropriate modifications.

Editor-in-Chief: Mao Ruiqing

Rotating Editor-in-Chief: Zhao Haibin

Editors: Wang Kechao, Gao Na