The Moon provides an attractive location both from a exploration- and a scientific perspective and over the recent years the interest in the Moon has been revived, both from agency driven programmes (e.g. NASA’s Artemis & CLPS, CNSA Chang’E) as well as commercial entities (e.g. Astrobotics, Intuitive Machines, Chandrayaan) which seek to land on the moon within the decade. Here we present the status of the concept for a far side low-frequency radio telescope that is done in the European Space Agency (ESA) Topical Team for an Astronomical Lunar Observatory (ALO), as part of the ESA’s Argonaut Lander programme (Taerrea Novae 2030+).

Following the detection of gravitational waves, and taking into account the wealth of astronomical instrumentation across the electromagnetic spectrum, the radio frequency range below ~30 MHz remains the last virtually unexplored frequency domain. The Earth’s atmosphere reflects back all radiation from space below its ionospheric plasma frequency (around 20 MHz), and the turbulent ionosphere gives rise to “radio seeing”, making ground-based radio observations of the sky more difficult at frequencies below ~100 MHz but certainly prohibiting observations at the lowest frequencies. An additional complication at lower radio frequencies is that strong man-made Radio Frequency Interference (RFI) levels close to Earth require either locations which provide partial or complete obscuration of the Earth, or locations that are sufficiently remote so that the RFI levels are significantly attenuated. In addition, stable temperature and gain conditions are essential to allow for careful calibration of the radio antennas. These requirements are met only for a number of locations of which the lunar far side, is by far the most promising one. 

There is a wealth of science to be addressed in the low-frequency radio regime, ranging from the study of Solar and Jupiter emission to the detection of exo-planets, but the real treasure-trove is the detection and imaging of the redshifted 21-cm line emission from the neutral Hydrogen in the pristine periods of the universe known as the Dark Ages (DA) and the Cosmic Dawn (CD; see e.g. Jester & Falcke, 2009). The only conceivable signal from the CD and DA comes from the hyperfine 21-cm line from neutron Hydrogen caused by the spin flip of the electron. The high redshift involved causes this emission to redshift in the frequency range between 1.4 – 140 MHz, with the global DA and CD signals peaking around 30 MHz and 70 MHz, respectively, and being rather broad, hence requiring space-based or lunar-based low-frequency radio instrumentation.

The Astrophysical Lunar Observatory, or ALO, is an ESA topical team to prepare for a Lunar far side low-frequency interferometer in the context of ESA’s Terrae Novae 2030+ program. As coordinator of the ALO topical team I will provide the latest status of the activities, in particular I will focus on a dedicated Concurrent Design Facility activity that was performed with a team of ~50 ESA engineers and present a preliminary baseline design for a far side radio array. I will present the identified enabling technologies and the required R&D developments that we are planning for the coming years. To coordinate the international activities we announce an International Lunar Science Working Group, a collaboration between the ESA ALO team and the NASA SSERVI institute and we extend an invitation to other international partners.

Marc KELIN WOLT 博士是荷兰奈梅亨拉德堡德大学拉德堡德射电实验室的主任及联合创始人，并担任天体物理学系的助理教授。拉德堡德射电实验室目前拥有约 20 名成员，致力于为空间和地面天文学开发仪器设备。Marc Klein Wolt 博士是纳米比亚非洲毫米波望远镜的项目主任。该望远镜建成后将加入全球事件视界望远镜网络。拉德堡德射电实验室团队正牵头为 EHT 升级和安装甚长基线干涉测量 (VLBI) 设备及 VLBI 监测系统。此外，Marc Klein Wolt 博士是中国嫦娥四号月球任务中射电天文载荷 (NCLE) 的首席科学家 (Principle Investigator)。该载荷是荷兰对首次月球背面着陆任务的贡献，是首个也是目前唯一一个用于低频射电天文研究的在轨射电天文台。他同时担任欧洲航天局 (ESA) 月球背面天文台专题小组 (Topical Team for an Astronomical Lunar Observatory on the far side of the moon) 的协调员。最后，Marc Klein Wolt 博士与埃因霍温理工大学 (TU/e) 的本图姆教授 (Bentum) 共同担任虚拟天文仪器中心 (Center for Astronomical Instrumentation, CAI) 的主任。该中心是拉德堡德大学与埃因霍温理工大学的合作机构。

Dr. Marc KELIN WOLT is the Director and co-founder of the Radboud Radio Lab (RRL) of the Radboud University Nijmegen, and assistant professor at the department of Astrophysics. The Radboud Radio Lab, which now consists of about 20 staff members, aims at developing instrumentation for space- and ground-based astronomy, and projects are started in close collaboration with research institutes and industry partners with a strong focus on technological Innovation. Dr. Klein Wolt is the Project Director of the Africa Millimetre Telescope (AMT) in Namibia which will be realised to become part of the global Event Horizon Telescope (EHT) network and the team of the RRL is leading the efforts for the upgrade and installation of the VLBI equipment and the VLBI Monitor for the EHT. In addition, Dr. Klein Wolt is the Principle Investigator of the radio astronomy payload (NCLE) for the Chinese Lunar Chang’e 4 mission, which is the Dutch contribution to the first ever landing on the Lunar farside and the only space-based radio observatory for low-frequency radio astronomical research, and coordinator of the ESA Topical Team for an Astronomical Lunar Observatory on the far side of the moon. Finally, together with Prof. Bentum (TU/e), Dr. Klein Wolt is the director for the virtual Center for Astronomical Instrumentation (CAI), a collaboration between the Radboud University and the Technical University of Eindhoven.