US Nancy Grace Roman Space Telescope launches successfully to challenge "mysteries of darkness": Japan also participates

The "Nancy Grace Roman Space Telescope" of the National Aeronautics and Space Administration (NASA), developed as a large space telescope following "Hubble" and "James Webb," successfully launched. The universe expands at an increasing speed, and the telescope investigates mysteries such as unidentified dark energy considered to serve as this expansion's driving force. In the future, technical demonstrations to directly observe exoplanets resembling Earth also plays an important role, and Japan participates through such initiatives as instrument development.

Enduring headwinds of US administration, finally departing

The Roman Telescope was launched on a Falcon Heavy large rocket, made by the US' SpaceX from the Kennedy Space Center in Florida, US, on the night of August 30, Japan time. Approximately 31 minutes later, it entered the designated orbit at an altitude of approximately 700-plus kilometers, and the launch was successful. Taking approximately three months while advancing preparations for observation, it reaches "Lagrange Point 2 (L2)" 1.5 million kilometers away from the Earth, where gravitational forces of the Earth and the Sun are in balance. It serves as an observation location where light objects like a telescope can remain and fuel can be saved. NASA expects the initial release of observation images by early next year.

Capturing the universe in visible light as well, centered on near-infrared light, the Roman Telescope has a length of 12.7 meters and a width of 4.4 meters after deployment, having a size similar to a large bus. The weight excluding fuel reaches 8 tons. Although the aperture of the primary mirror is 2.4 meters, identical to the Hubble Space Telescope, the weight decreased to one-fourth through technological innovation. The observation plan is five years, and development and operation costs will amount to 4.3 billion dollars (approximately 690 billion yen) in total. It was named in 2020 after astronomer Nancy Grace Roman (1925 to 2018), who served as the first chief astronomer of NASA and was called the "Mother of the Hubble Space Telescope."

Development started in the early 2010s. Although the project repeatedly faced the threat of cancellation under the Trump administration's budget proposals, Congress rejected these moves and the plan went ahead. Although it was planned to launch in May next year, it was moved up significantly because development advanced smoothly.

Nicky Fox, Associate Administrator for the Science Mission Directorate at NASA, stated: "The Roman Telescope answers deep human questions regarding the history of the universe to the maximum extent. It possesses a wide field of view and quick observation. It will lead us to a new era of discovery, make invisible things visible, and lay the foundation for life exploration outside the solar system."

Unidentified dark energy, searching for clues

The Roman Telescope carries a total of two main scientific devices. One is a "wide-field imaging instrument," namely a camera. It possesses precision and resolution equivalent to the Hubble Telescope but holds a field of view 200 times as wide. Its main role is attempting verifications regarding dark energy considered to serve as the cause of accelerated expansion of the universe.

It became clear that the universe is expanding in the late 1920s, and in 1998, it was found that its expansion is accelerating rapidly. Due to gravity acting between matter, it was also considered that the universe would eventually contract and collapse, but that view has been radically overturned. An unidentified form of dark energy is hypothesized to provide the energy to explain this. Dark energy accounts for 68.3 percent of matter and energy in the universe, serving as the greatest mystery of modern cosmology.

What served as a clue to discovering accelerated expansion was "Type Ia supernovas," a type of explosion that stars cause at the end of their lives. Since the brightness of explosions becomes physically almost identical in all cases, the distance can be measured considering the apparent brightness from observation points, and it is regarded as a "rule of the universe."

The Roman Telescope cannot pinpoint the identity of dark energy. However, using the wide-field imaging instrument, it observes thousands of Type Ia supernovae and hundreds of millions of galaxies up to 10 billion light-years away, determining the expansion rate of the universe with high precision. Thus, it aims at understanding the degree of influence of dark energy on expansion.

It also investigates the structural origins of the universe, the history of galaxies, and the distribution of dark matter that possesses gravity but remains invisible.

Aiming at broad understanding of exoplanets

Another important observation target is exoplanets. They literally mean planets outside the solar system, discovered for the first time around a neutron star in 1992 and around a star in 1995. Over 6,300 planets have already been discovered. Aspects greatly differing from the solar system became clear, such as giant planets like Jupiter or scorching rocky planets being discovered one after another in close proximity to stars. They compel revisions in conventional understandings of planet formation. Whether "a second Earth" possessing signs where life could exist, such as oxygen, methane, and water, can be found serves as a major interest. Research on exoplanets contributes greatly to deepening understanding of the solar system, Earth, and existence of life by knowing "others."

The Roman Telescope is expected to discover approximately 1,400 exoplanets near the center of the Milky Way Galaxy using the wide-field imaging instrument. Its method captures phenomena where the gravity of a planet acts like a lens, amplifying the light of a star passing behind, called "gravitational microlensing." In ground observations, the "transit method" observing dimming when a planet passes in front of a star and the "Doppler method" capturing changes in light wavelength as a star wobbles due to gravity of a planet have been mainly adopted.

Many discovered exoplanets exist as hot planets or heavy planets located near parent stars. Limits exist in observation methods and precision, and they are not yet captured broadly like "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune" in the solar system. Therefore, the Roman Telescope undertakes the observation of stars distant from parent stars and stars of diverse weights using gravitational microlensing. It will capture a wide range of exoplanets, and understanding planet formation is expected to deepen.

Japan is also undertaking the development of equipment for exoplanet exploration

Another device being used is a "coronagraph instrument." A coronagraph was originally a device developed to observe the atmosphere (corona) of the Sun. It blocks the light of the main body of the Sun with a disk, making the light of the surrounding corona visible. The Roman Telescope applies this to exoplanet observation, hiding bright stars to observe faint planets located in close proximity. NASA aims to observe planets resembling Earth in the next-generation large space telescope concept "Habitable Worlds Observatory," and the device was developed as a technology demonstrator.

Professor Toru Yamada, who serves as team leader of the Roman Telescope at the Institute of Space and Astronautical Science (ISAS) of the Japan Aerospace Exploration Agency (JAXA), explained at a press conference before launch: "Contrary to the wide-field imaging instrument, the coronagraph instrument observes places close to the primary star (star) while reducing the light of the primary star as much as possible. With the Roman Telescope, observing planets similar to Earth is not possible yet. However, we expect that observing larger planets will become possible."

Japanese research groups provided mask substrates and optical elements for the coronagraph instrument. Other contributions include receiving ultra-large-capacity observation data via the JAXA Misasa Deep Space Station (Nagano Prefecture), and coordinated observations via the National Astronomical Observatory of Japan Subaru Telescope (Hawaii, US) and the PRIME Telescope of the University of Osaka and partners (South Africa). Centered on JAXA ISAS, researchers at Osaka University, Astrobiology Center, National Astronomical Observatory of Japan, Fukui University of Technology, the University of Tokyo, and other institutions participate. Thus far, Japanese researchers have achieved research results through observations of the Hubble and James Webb Telescopes, but it is stated that this is the first organizational participation in development and operation of a large space telescope.

Other non-American institutions participating in this endeavor include the European Space Agency (ESA), French National Centre for Space Studies (CNES), and Max Planck Institute for Astronomy in Germany.

Regarding the Roman Telescope, Specially Appointed Professor Motohide Tamura at the Astrobiology Center states: "At what distance from their stars are low-mass planet like Earth located? In other words, the question is whether the solar system is unique. Being finally able to provide a statistical answer to this matter is highly significant. It functions also as a technical demonstration for Habitable Worlds Observatory attempting to see signs of life, serving as a truly impressive telescope."

It is said that when asked "what was the most interesting discovery by the Hubble Telescope?" Roman answered dark energy. The Hubble Telescope backed up initial measurement results of accelerated expansion of the universe, contributing to a major discovery. Roman also wrote a paper in 1959 touching upon observation possibilities of exoplanets. The mysteries of the universe are endless. Roman, now "reborn" as a telescope, has taken up the baton of intellectual curiosity.