25143 Itokawa

25143 Itokawa, also called 1998 SF₃₆, is a small space rock that orbits close to Earth. It was found in 1998 by the LINEAR program and named after the Japanese rocket engineer Hideo Itokawa. This asteroid is shaped like a peanut and is about 330 meters, or 1,100 feet, wide. It spins around once every 12.1 hours and is made of many pieces of rock held together, like a pile of loose boulders.

Itokawa was very important because it was the first asteroid visited by a spacecraft that brought pieces of it back to Earth. The Japanese probe Hayabusa collected over 1,500 tiny pieces of dust from Itokawa’s surface in 2005. When the probe returned to Earth in 2010, scientists studied these pieces closely. This helped them learn more about how our Solar System changed and grew over time. Before the DART mission to Dimorphos in 2022, Itokawa was the smallest asteroid ever seen up close by a spacecraft.

Discovery and naming

Itokawa was discovered on 26 September 1998 by astronomers with the Lincoln Near-Earth Asteroid Research (LINEAR) program at Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico, in the United States. It was given the provisional designation 1998 SF₃₆. The minor planet was named to honor Japanese rocket scientist Hideo Itokawa, who helped start rocket science in Japan. The official naming was shared by the Minor Planet Center on 6 August 2003.

Orbit and classification

Itokawa is part of a group called the Apollo asteroids. These are special rocks that cross Earth’s path and are one of the biggest groups of objects close to our planet. Itokawa travels around the Sun at a distance between 0.95 and 1.70 times the distance from Earth to the Sun, or 0.95–1.70 AU. It completes one trip around the Sun every 18 months. Its path around the Sun is slightly stretched out and tilts a little compared to Earth’s path. It comes quite close to Earth, sometimes coming within about 1,960,000 kilometers of our planet.

Exploration

In 2000, Japan chose this asteroid as the target for its Hayabusa mission. The spacecraft reached Itokawa on September 12, 2005. Because Itokawa’s gravity was too weak to hold the spacecraft in orbit, Hayabusa moved to stay in line with the Sun and the asteroid.

Hayabusa landed on Itokawa on November 20 for a short visit. Although it tried to collect soil samples, it was not successful during this first attempt. A second landing was tried on November 25. The spacecraft returned its sample capsule to Earth, landing in Woomera, South Australia on June 13, 2010. Scientists later confirmed that the dust collected came from Itokawa.

Surface features

Scientists from the Hayabusa mission suggested names for the main features on Itokawa, which were approved by a group of experts from the International Astronomical Union. Smaller features still have temporary names for study purposes. Below are tables showing the names of some geological features on the asteroid.

Craters

Ten impact craters on Itokawa received their names on February 18, 2009.

Regiones

Regiones are large areas on a planet or asteroid that look different from nearby areas because of brightness or color. Several regiones have been named on Itokawa.

Physical characteristics

Itokawa is a stony S-type asteroid. In 2001, images taken by Goldstone showed it to be shaped like an ellipsoid, measuring about 630 meters long and 250 meters wide.

The Hayabusa mission confirmed these findings and suggested Itokawa might be made of two or more smaller asteroids that stuck together. The mission showed Itokawa has a rough surface covered in large rocks and very few craters. Its density is too low to be solid rock, indicating it is made of loosely connected fragments.

Since 2001, many observations of Itokawa’s rotation have been made. These showed it spins once every 12.132 hours and has a non-spherical shape. Scientists also determined two possible directions for its spin axis.

In 2011, scientists studied dust collected by Hayabusa and found it matched a common type of meteorite. They also discovered the dark color on Itokawa’s surface came from tiny space particles scraping its surface.

In 2018, researchers found water in different particles from Itokawa. The water’s composition matched that found in our solar system and on Earth.

In 2020, more studies showed water and organic materials in another particle from Itokawa.

In 2021, further studies supported the idea that some of Earth’s water may have come from hydrogen in space dust that landed on asteroids like Itokawa.

The asteroid also has areas where dust collects in depressions, known as dust ponds. These were found in regions called Sagamihara and Muses-Sea. The dust particles range in size from millimeters to less than a centimeter.