New data about the outer solar system changes ideas about its size
The New Horizons spacecraft, which made a historic encounter with Pluto in 2015, continues its mission and moves through the most distant corners of the Solar system — through the Kuiper Belt. But having encountered a cosmic storm there, he «tells» that more mysterious processes are taking place in this part than astronomers assume.
In space there is dust consisting of tiny particles, several microns in size. Much of this dust is left over from the formation of planets and numerous collisions between objects in the solar system. Subsequently, fresh — joined the ancient dust. from the surface of asteroids and comets as a result of micrometeorite impacts. This dust, both ancient and recent, creates a phenomenon known as «Zodiac light». It is noticeable even in the most remote corners of the solar system. Although astronomers are still not entirely sure what its boundary looks like.
The Kuiper Belt is so distant, and its icy inhabitants so small and dim, that the first Kuiper Belt Object (KBO) beyond Pluto was not discovered until 1992. But since then, thousands of other Kuiper belt objects have been discovered, and astronomers have been able to begin mapping the outer solar system.
Beyond the Kuiper Belt is the Scattered Disk, «inhabited» objects that were scattered by the gravitational influence of the outermost planet of the solar system — Neptune. Scattered Disc objects have highly elliptical orbits, highly inclined from the plane of the Solar System and can extend hundreds of astronomical units from the Sun.
Behind the Scattered Disk there is an even larger space — The Oort Cloud, which is a vast spherical region of icy objects extending over a distance of more than a light year. Although the Oort Cloud itself has not been observed directly, the orbits of long-period comets suggest the existence of such a region.
However, unexpected data obtained from the New Horizons mission is poised to revolutionize our understanding of the outer solar system. «New Horizons is making the first direct measurements beyond Neptune and Pluto, so each observation could lead to a discovery», — said astronomer Alex Doner from the University of Colorado.
The distance between the outer edge of the Kuiper Belt and the Sun is believed to be about 50 astronomical units. On January 1, 2019, New Horizons collided with a Kuiper Belt object named Arrokoth at a distance of 44.5 AU. from the sun. And currently New Horizons are at a distance of 58.25 AU. from the Sun, crossing the line of 50 AU. in April 2021. Over the past five years «New Horizons» was supposed to cross the edge of the Kuiper belt. However, since the KBOs are separated by millions of kilometers, the probe will not notice that they are left behind. Instead, the sign will be a decrease in interplanetary dust levels.
Using the Small Dust Counter (SDC) instrument on board the spacecraft, scientists measure the level of dust along the spacecraft's path. Surprisingly, SDC has yet to detect a significant reduction in dust levels, leaving astronomers perplexed.
The SDC instrument, located on the front surface of New Horizons, consists of 14 plastic film detectors, each measuring 14.2 by 6.5 cm and only 28 microns thick. Twelve of these detectors are exposed to space matter, while the other two are shielded and serve as reference detectors, recording any non-dust events to prevent false alarms. When a dust particle hits one of the detectors, it leaves a small dent in the film, which slightly changes the electrical conductivity of the surface.
One hypothesis is that excess dust formed closer to the Sun and was forced out by solar pressure. However, researchers consider this version unlikely. Instead, the continued presence of dust is thought to indicate that the probe is still within the Kuiper Belt, which appears to be much wider and farther from the Sun than hitherto believed.
This is confirmed not only by the amount of dust. Astronomers use machine learning algorithms to process observations made by the Subaru Telescope. with a diameter of 8.2 meters on Mount Mauna Kea in Hawaii and the «Blanco» telescope 4 meters in diameter at the Chilean Observatory. These observations led to the discovery of 154 objects in New Horizons' path, including about 20 that the spacecraft will approach within about 8 million kilometers, which is sufficient for some observations. However, it appears that some of these objects are located outside the Kuiper Belt, in the Scattered Disk region.
The predecessors of «New Horizons» there have only been four spacecraft in the history of solar system exploration — «Pioneer-10» and «Voyager 1», as well as «Voyager 1» and «Voyager 2» and none of them were equipped with a dust detector.
While the previous missions had already completed their tasks, «New Horizons» have sufficient fuel and energy until the 2040s, when it will be well beyond 100 AU. from the sun. By the time its energy is exhausted, it will likely completely redraw the map of the solar system, providing valuable information about this mysterious region.