由于地球电离层的反射、吸收和折射，以及大量的射频干扰，在地面开展低频射电观测受到很大局限，30MHz以下的频段迄今还没有高分辨率的天图，而对于研究宇宙从黑暗时代到黎明演化非常重要的30-120 MHz频段，其地面实验也容易受到各种系统误差的影响。以下我将把0.1-120MHz称为超长波频段。月球轨道可提供极优的超长波观测条件，避免在地面实验中遇到的各种系统效应的影响。我将简介鸿蒙项目(又称DSL)的情况。该项目计划用一次火箭发射将一组卫星部署到环绕月球的圆轨道上，形成沿同一轨道运行的非等间距线形编队。在月球背面开展超长波天文观测，以避免地球产生的射频干扰，在月球正面将数据传回地球。这其中，一颗主星负责星上数据处理和与地面通讯，8颗子星进行0.1-30MHz频段的成像和频谱观测，1颗子星进行30-120MHz频段的高精度频谱观测。超长波频段特别是其中的低频端，迄今为止仍是电磁频谱中观测极少、几乎空白的频段，鸿蒙的观测将首次揭开超长波宇宙的面纱，在宇宙黑暗时代与黎明、射电星系与类星体、银河系星际介质、太阳和太阳系行星等领域获得重要的观测数据，并有可能发现新的未知现象,从而开辟新的超长波天文学领域。

Low-frequency radio observations from Earth are severely limited by ionospheric reflection, absorption, refraction, and significant radio frequency interference (RFI). To date, no high-resolution sky map is available at frequencies below 30 MHz. Moreover, ground-based experiments within the 30–120 MHz band—which is crucial for studying cosmic evolution from the Dark Ages to the dawn of the first stars—are highly susceptible to systematic errors. In this context, the 0.1–120 MHz range is referred to as the ultralong wavelength band. The lunar orbit provides an excellent environment for ultralong wavelength observations, free from the systematic effects that hinder ground-based measurements. This paper briefly introduces the Hongmeng Project, also known as the Discovering the Sky at the Longest Wavelengths (DSL) mission. The project aims to deploy a constellation of satellites into a circular lunar orbit via a single rocket launch, forming a linear array with non-uniform spacing along the orbit. Astronomical observations will be conducted from the far side of the Moon to avoid terrestrial RFI, with data transmission to Earth occurring from the near side. The constellation includes a mother satellite responsible for onboard data processing and Earth communication, eight daughter satellites dedicated to imaging and spectral observations in the 0.1–30 MHz band, and one additional daughter satellite performing high-precision global spectral measurements in the 30–120 MHz band. The ultralong wavelength band, especially at its low-frequency end, remains one of the least explored regions of the electromagnetic spectrum. The Hongmeng observations are expected to reveal the ultralong wavelength universe for the first time,delivering valuable data in areas such as the cosmic Dark Ages and cosmic dawn,radio galaxies and quasars,the Galactic interstellar medium,the Sun,and solar system planets. These data hold the potential to uncover new phenomena and establish the new research field of Ultralong Wavelength Astronomy.

陈学雷，中国科学院国家天文台星系宇宙学部主任，宇宙暗物质暗能量团组首席研究员，中国科学院大学岗位特聘教授，主要从事宇宙学、射电天文学研究。1999年哥伦比亚大学博士毕业，获国家杰出青年科学基金，长江学者讲座教授等，担任暗能量射电探测关键技术研究（天籁实验）负责人，绕月超长波天文阵列项目（鸿蒙计划）首席科学家，已发表研究论文240余篇。

CHEN Xuelei is the Director of the Division of Cosmology and Extragalactic Astronomy at the National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS), and the Principal Investigator of the Cosmic Dark Matter and Dark Energy Research Group of NAOC. He is also a professor at the University of the Chinese Academy of Sciences. His primary research focuses on cosmology and radio astronomy. He earned his Ph.D. from Columbia University in 1999.  He had been selected for the NSFC Distinguished Young Scholars, Changjiang Scholar Lecture Professor. He is the PI of the key technology research in radio detection of dark energy (the Tianlai experiment), and the Chief Scientist of the lunar orbit Discovering Sky at the Longest wavelength (DSL, also known as Hongmeng project). To date, he has published more than 240 research papers.