Ryugu, that was explored by Hayabusa2, has an equatorial diameter of about 1km. The destination for the Extended Mission, 1998 KY26, is expected to be only about 30m in diameter. These small celestial bodies are the asteroids, but they are not the smallest bodies in the Solar System.

For example, meteors have been observed falling to Earth when stone grains with sizes from a few centimetres to a few millimetres burn up in the atmosphere. There is also a large amount of dust in the Solar System with sizes less than 1mm that we call “interplanetary dust”. More than 100 tons of interplanetary dust pours onto the Earth every day. Interplanetary dust is thought to have been released from comets, or during a collision between asteroids. Hayabusa2 also generated and scattered interplanetary dust during the spacecraft’s two touchdowns on Ryugu, and during the creation of the artificial crater. Exploring how this interplanetary dust is distributed is important for understanding how mass is transferred within the Solar System. We will therefore wanted to try to measure the interplanetary dust distribution during the Hayabusa2 Extended Mission.

But how do you measure the distribution of dust that is smaller than 1mm through the vastness of the Solar System?

Using Hayabusa2’s ONC-T telescopic optical navigation camera, which was used extensively during the exploration of Ryugu, we are trying to investigate interplanetary dust by observing “zodiacal light”.

Interplanetary dust scatters sunlight, making the entire Solar System appear to faintly shine. This shine is the zodiacal light. Zodiacal light can even be seen with the naked eye on Earth in a dark enough location.

The figure below shows observation data of the zodiacal light acquired by the ONC-T on August 9, 2021. The data we obtain will all be an ordinary starry sky image such as this one. From this image, the brightness of the zodiacal light can be derived from measuring the brightness of the dark night sky where celestial bodies such as stars are not visible.

Figure 1: An example of the zodiacal light observation data.
Image captured with the ONC-T on August 9, 2021 in the direction of Cetus
(credit: JAXA, Tokyo City University, Kwansei Gakuin University, Kyushu Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST).

By observing the zodiacal light, we can investigate the interplanetary dust. However, observing from the Earth or close by with a space telescope has a problem. Since the light scattered by the foreground interplanetary dust overlaps with light scattered from the background dust, it is difficult to investigate the spatial distribution. This is where the Hayabusa2 Extended Mission can provide invaluable insight. Hayabusa2 will fly over a range of 0.7 - 1.0 au (*1) from the Sun until the Earth swing-by in 2027, and then over the range 1.0 - 1.5 au, allowing the spacecraft to observe the zodiacal light in three dimensions at different locations from the Earth. This allows the zodiacal light to be observed from a variety of positions within the Solar System, clarifying the distribution of interplanetary dust. Such observations cannot be made from the Earth, which constantly orbits at 1 au from the Sun, so a plan such as the Hayabusa2 Extended mission is needed to navigate a wide area of the Solar System over a long period of time.

Observing zodiacal light not only investigates the small interplanetary dust in the Solar System, but also the history of the formation of stars and black holes after the birth of the Universe in the Big Bang. Light in the Universe is mainly emitted from stars and black holes (*2), so if we can measure how much light fills the Universe—that is, measure the “brightness of the Universe”—you can estimate how many celestial bodies much be present. The biggest obstacle to this plan is the zodiacal light, which makes the brightness of the Solar System many times brighter than that of the Universe. In other words, the Solar System is so dazzling that it is difficult to measure the brightness of the Universe. Therefore, by improving the measurement of zodiacal light with Hayabusa2, we gain a better understanding of the zodiacal light, helping remove this obstacle to measuring the brightness of the Universe.

We would like take over the baton of Hayabusa2 that included the successful exploration of Ryugu and the sample return, to make the best use of the unique opportunity of the Extended Mission and achieve similar successes in the zodiacal light observations.

*1: 1 au (astronomical unit) is the average distance between the Sun and the Earth; approximately 150 million km.
*2: Light is emitted from material falling into the black hole.

Reference:
Below we label some of the celestial bodies in the photographed area to help identify the region.

(Credit: JAXA, Tokyo City University, Kwansei Gakuin University, Kyushu Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST).

The bright star near the centre is the star Iota (ι) in the constellation of Cetus. This is the star near the tip of the tail of the whale, and is a yellow-orange giant with an apparent magnitude of 3.6 mg. There are no first magnitude stars in Cetus, but there is a famous variable star call Mira. Mira means “wonderful” in Latin and its brightness changes from 2nd magnitude to 10th magnitude over a 332 day average cycle. The “whale” of Cetus is no ordinary whale, but a sea monster that is attempting to eat the Princess Andromeda in Greek mythology.

Hayabusa2 Project ONC Team / Zodiacal Light Team (written by Kohji Tsumura)
2021.8.25