Deep space exploration technology demonstrator（DESTINY⁺）Project

DESTINY+ (Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science) is a space exploration program to flyby the asteroid (3200) Phaethon, the parent body of the Geminid meteor shower. The high relative velocity of Phaethon to the ecliptic plane, which results from its large orbital eccentricity and high inclination, prevents sample return from the asteroid and rendezvous with it. Accordingly, a flyby is currently the most feasible means of exploration. The highly elliptic orbit of Phaethon around the Sun with perihelion and aphelion at 0.14 and 2.4 au (au: the mean Earth-Sun distance), respectively, intersects the ecliptic plane at a speed of 36 km/s (Fig. 1).

 Phaethon delivers vast amounts of meteoroids (dust) to the Earth every year in mid-December observed as the Geminid meteor shower. It is the largest asteroid (about 6 km in diameter) to come closest to the Sun and its surface temperature exceeds 700℃ at perihelion.

 It is also called an “active asteroid” owing to its comet-like activity that ejects meteoroids and micrometeoroids (dust) from the surface. Since ground-based telescopes have never succeeded in detecting the ejection of dust from the active asteroid Phaethon, the dust-ejection mechanism has long been the subject of debate in the fields of astronomy and planetary science. It is also worth noting that the asteroid orbits the Sun in a 1.4-year period without disintegrating like sunskirting comets, despite its sunskirting orbit.

 In the DESTINY+ project, we will, therefore, observe the detailed topography of Phaethon and analyze the chemical composition of dust particles in the vicinity of the asteroid and their physical properties such as mass during the closest approach of the spacecraft to the asteroid. Being an active asteroid intensely exposed to solar heating and annually delivering dust to the Earth, we seek to elucidate the actual conditions of Phaethon through DESTINY+ in-situ observations!

 During its closest approach to Phaethon, the DESTINY+ spacecraft will image the topography of the asteroidal surface with a telescopic camera onboard the spacecraft while tracking the asteroid at a relative speed of about 36 km/s from approximately 500 km away. It will also resolve the distribution of various surface materials using an onboard multi-band camera.

 In addition, the element composition, impact velocity, and mass of dust ejected from Phaethon will be measured by a dust analyzer onboard the spacecraft. During the interplanetary cruise from the Earth to Phaethon, the dust analyzer will also conduct in-situ measurements of interplanetary and interstellar dust. DESTINY+ will carry out scientific in-situ observations using three onboard instruments: a tracking telescopic camera, a multi-band camera, and a dust analyzer.

 In December 2017, Phaethon at a geocentric distance of roughly 10 million km was observed by a number of ground-based telescopes, among which Arecibo radar observations in Puerto Rico revealed a crude three-dimensional shape of the asteroid.

[1] Solar System Dynamics. (Small-Body Database Lookup). https://ssd.jpl.nasa.gov
[2] Kim, M. -J. et al., 2018, A&A 619, A123.
[3] Hanuš, J. et al., 2018, A&A 620, L8.
[4] Beniyama, J. et al., 2023, Publ. Astron. Soc. Japan 75(2), 297.
[5] Geem, J. et al., 2022, MNRAS 516, L53.
[6] Marshall, S. et al., 2023, In DESTINY+ Science Working Team meeting 2023.

 This logo is by designer Hyu Kurokawa, who created the PERC logo and the Meteor Project logo.
The design is filled with the scientific background and keywords(the Geminid meteor shower, dust trails, Phaethon, parent bodies of meteor showers, and near-Sun asteroids) of the DESTINY+ scientific mission.

 The acronym DESTINY+ stands for Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science. The first part of the official mission name refers to the engineering objective of “demonstrating advanced technologies for future deep-space exploration”, and the second part, starting with ‘with’, represents the scientific objective of ‘understanding the nature of dust from the asteroid (3200) Phaethon through a flyby’. As the name implies, this is a joint mission of science and engineering, achieved by sharing an engineering demonstrator with scientific instruments. The science mission is based on “a mission proposal for asteroid Phaethon”[2], which was submitted in response to a public call[1] “Planetary exploration in a coming decade activity” from the Japanese Society for Planetary Sciences (JSPS) in 2010. It turned out to be the first case that a bottom-up proposal from JSPS was accepted as a project. In the first session of JSPS's panel discussions on so-called top sciences, our proposal received a favorable evaluation by the panel of small-body exploration. It was, however, eliminated in the second round of selection, because it was deemed unfit for flagship missions (medium missions) [3]. Subsequently, we had groped for independent opportunities to realize our proposed mission with the advice and support of science and engineering experts worldwide through frequent discussions with them. In our original proposal, we set out on a sample return mission, but the proposal was revised to a flyby mission for technical feasibility. Moreover, we consulted with German experts on the development of a dust analyzer that is capable of analyzing the chemical composition of dust in situ. Because a dust analyzer enables us to obtain information on the chemical composition of surface materials, compensating for the drawback of a flyby, we decided to include it in the proposal. We also joined the DESTINY Working Group established in FY2011 as observers to explore the possibility of sharing an engineering mission with us. We submitted our proposal to the first JAXA's competitively chosen medium size satellite program in FY2013 as an example of scientific applications to the technology validation mission DESTINY, the successor of which is DESTINY+, but the proposal was not selected, losing out in the final screening to Smart Lander for Investigating Moon (SLIM). Our updated proposal re-submitted in FY2015 to the second JAXA's competitively chosen medium size satellite program was selected in August 2017 as DESTINY+, a joint mission of engineering and science with a Phaethon flyby. However, the catastrophic failure of the X-ray astronomy satellite “Hitomi” has caused delays in our review process and a drastic revision in our proposal, which increased cost and weight. Once we overcame these issues, the proposal was upgraded to a pre-project (a preliminary step to a project) on June 1, 2020 and finally to a project on May 1, 2021. The DESTINY+ project successfully completed the Preliminary Design Review (PDR) in December 2022 and is currently in the process of moving into the Critical Design Review (CDR). The failed launch of Epsilon-6 on October 12, 2022 and the explosion of Epsilon S small rocket engine during a ground combustion test of the second stage engine on July 14, 2023 made a change of our development timelines inevitable. At the meeting of the Committee on Space Science and Exploration, which is a section of the Committee on National Space Policy (CNSP), it was reported on October 27, 2023 that the launch of DESTINY+ would be postponed from FY2024 to FY2025. At the subsequent meeting of the Committee on Space Science and Exploration/CNSP on October 9, 2024, there were announcements about a further delay in the launch to FY2028 and a replacement of the launch vehicle from Epsilon S to an alternative such as H3 being in the process of coordination.

[1] Namiki et al. 2010, Planetary People 19, 221.
[2] Arai et al. 2012, Planetary People 21, 239.
[3] Namiki et al. 2012, Planetary People 1 21, 215.

DESTINY+ is a joint mission of science and engineering by means of observation instruments onboard a technology demonstrator. JAXA is responsible for the development of a launch vehicle and a spacecraft, while Chiba Institute of Technology is taking a central role in the promotion of scientific programs and the development of observation equipments.

MEISEI ELECTRIC CO., LTD.

is responsible for the development of the panchromatic telescopic camera (TCAP) and the multiband camera (MCAP).

GENESIA Corporation

is responsible for the development of the optics of the panchromatic telescopic camera (TCAP).

COSINA CO., LTD.

is responsible for the development of the optics of the multiband camera (MCAP).

 More than 40,000 tons of extraterrestrial micrometeoroids (i.e., dust) is supplied to the Earth every year. Analyses of cosmic dust collected in the stratosphere and on the Earth's surface have revealed that the abundances of carbon and organic matter in interplanetary dust are several times higher than in meteorites. It is, therefore, natural to assume that cosmic dust is a major source of extraterrestrial carbon and organic matter to the Earth as well as a key substance for testing the hypothesis that life on the Earth originated from extraterrestrial materials. In recent years, cosmic dust has been extensively studied in planetary science and astronomy to test the above hypothesis.

 There are two main routes for dust transport to the Earth (Figure 2). One is as “interplanetary dust” that is a large collection of micrometer-sized particles spread out across the ecliptic plane (the orbital planes of planets such as the Earth around the Sun) after ejection from a number of comets and active asteroids. These particles may transit the Earth's orbit as they spiral gradually toward the Sun due to the relativistic effect of solar radiation (path ① in Figure 2). The other is as dust trails of meteor showers (path ② in Figure 2). A meteor shower is a celestial event in which a large number of meteors appear to originate from one point in the sky when the Earth crosses a dust trail consisting of submillimeter-sized particles confined in a belt along the orbit of their parent-body comet or asteroid. The trajectory of a dust particle larger than several hundred micrometers is observed as a meteor, since the entry of the particle into the Earth's atmosphere forms an incandescent trail of a hot and dense plasma in the atmosphere. Dust in a meteor shower is of great importance, provided that the parent body of the dust has been identified. The parent bodies of meteor showers are generally comets, although there are exceptions such as the Geminids and the Quadrantids of asteroid origin (Table 1).

DESTINY+ address the following three scientific issues.

The Overall Picture of Interplanetary Dust and the Origin of Individual Particles

 Interplanetary dust is a collection of dust particles from a number of comets and asteroids. Until now, many researchers have attempted to understand the overall picture of interplanetary dust through analyses of individual dust particles collected on the Earth and telescopic observations of the zodiacal dust. Nevertheless, as the ratio of cometary to asteroidal dust is still a matter of debate, we have not obtained a clear overall picture of interplanetary dust yet. In this mission, we aim at settling this dispute by in-situ measurements of the size, velocity, arrival direction, and element composition for individual particles in interplanetary space. Interplanetary dust collected on the ground or in the stratosphere is not pristine due to heating and destruction upon atmospheric entry. Accordingly, we expect that in-situ analysis of dust in interplanetary space will reveal the pristine nature of interplanetary dust prior to its entry into the Earth's atmosphere.

Chemical Composition of Interstellar Dust Penetrating into  1 au

 It is known that approximately 5% of dust in interplanetary space is interstellar dust. In-situ measurements of interstellar dust by the HELIOS, GALILEO, ULYSSES, and CASSINI missions reveled that the radius of interstellar dust is 10 micrometers at most, smaller than interplanetary dust. Infrared observations with telescopes indicate that both dust and gas components of interstellar clouds are rich in C, N and O and that organic matter comprises roughly 40% of interstellar dust mass. In contrast, the Cosmic Dust Analyzer (CDA) onboard Cassini-Huygens detected 36 particles of interstellar origin, but no particles contained organic matter nor carbon. We will elucidate the presence or absence of organic matter and carbon in interstellar dust, in other words, the abundance of organic matter through ceaseless in-situ measurements of interstellar dust penetrating into the solar system during the two-year interplanetary cruise phase of the mission.

Identification of Dust Ejection Mechanism from Active Asteroids 

 Recent observations using ground-based telescopes and the Hubble Space Telescope have discovered comets with asteroid-like orbits or asteroids with comet-like activities in the main asteroid belt, and even ice and organic matter on the surfaces of asteroids. Since a series of minor bodies with characteristics of both comets and asteroids (main belt comets, extinct or dormant comets, and active asteroids) have been discovered, the traditional classification of comets and asteroids is gradually being revised. Phaethon is a near-Earth asteroid with an Apollo-type orbit and the parent body of the Geminid meteor shower and the ejection of dust would take place near perihelion. Although Phaethon is an active asteroid, the absence of a coma or a dust tail like a comet provides no clue about the dust ejection mechanism from Phaethon. In our mission, we will clarify how active asteroids like Phaethon eject dust by means of topographic and geologic observations at the closest approach to Phaethon.

 DESTINY⁺ will be equipped with three science instruments: the DESTINY⁺ Dust Analyzer (DDA), the Telescopic CAmera for Phaethon (TCAP), and the Multiband CAmera for Phaethon (MCAP). The DDA is a small, lightweight, high-performance model that is an improved version of the Cosmic Dust Analyzer (CDA) on board the Cassini spacecraft. It is being developed by a team from the University of Stuttgart in international collaboration with Germany. The development of TCAP and MCAP is being led by Chiba Institute of Technology (Fig.3).