China's ambitious human space missions get most of the headlines, but its fledgling space science program is quietly gaining strength. The Chinese Academy of Sciences (CAS) last week confirmed plans to launch four new scientific satellites beginning in 2020. Coming on the heels of four successful missions, including one devoted to x-ray astronomy and another that demonstrated quantum entanglement over a record-setting 1200 kilometers, these "phase 2" projects will examine areas including solar physics and the hunt for electromagnetic signals associated with gravitational waves.
Given that China's space science program only started about 10 years ago, the lengthening track record "is impressive, but there are still not many missions given that it's a big country with a big science community," says Xin Wu, a China-born physicist at the University of Geneva in Switzerland who collaborates on China's astrophysics missions. "There is pent-up demand" among Chinese space scientists, he says.
CAS broke with tradition for one of the new missions, the Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM). It fast-tracked selection and development to take advantage of a new scientific opportunity, which Xiong Shaolin, an astrophysicist at CAS's Institute of High Energy Physics in Beijing, and his colleagues identified a month after the U.S. Laser Interferometer Gravitational-Wave Observatory announced its historic detection of gravitational waves in February 2016. They proposed putting two satellites into orbit on opposite sides of Earth that together could watch the entire sky for gamma rays emanating from the events that generate gravitational waves. Funding for technical studies arrived a few months later, and the mission has jumped to the front of the launch queue, with a date of 2020. "When you have this kind of opportunity you can't handle it like a normal mission, with selection and review taking 10 or 20 years," Xiong says.
So far, gamma rays and other electromagnetic signals have only been detected from one kind of gravitational-wave source, a neutron star merger, but they yielded a trove of detail about the enigmatic event. Astrophysicists are still debating whether black hole mergers, the other confirmed source of gravitational waves, also produce electromagnetic emissions. The GECAM team is betting that they do—and that much can be learned from the signals. "I think probably we will find something," Xiong says.
GECAM's observations will complement those of another phase 2 mission, the Einstein Probe (EP), which will survey the sky for the low energy x-rays associated with violent phenomena such as gamma ray bursts and black hole collisions. Combining GECAM, EP, and gravitational wave observations "will allow us to better understand gamma ray burst astrophysics," says Ik Siong Heng, an astrophysicist at the University of Glasgow in the United Kingdom.
China's space scientists have long targeted another area: solar physics. Only the United States produces more papers in the field than China. "But [China's] papers used data from missions developed by Japan and the U.S. and elsewhere," says Gan Weiqun, a solar physicist at CAS's Purple Mountain Observatory in Nanjing. He says China's solar scientists have been pushing for their own mission for 40 years; they've finally gotten the nod for the Advanced Space-based Solar Observatory (ASO-S). "It's very important for us to make original contributions in terms of hardware and data," Gan says. He explains that ASO-S will be the first space observatory to monitor the sun's magnetic field while watching for solar flares and the titanic blasts known as coronal mass ejections. The simultaneous observations could yield clues to how those eruptions are triggered.
The last mission included in the phase 2 list was identified as a priority years ago. The Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), a joint CAS and European Space Agency mission, will pioneer a new technique for imaging Earth's magnetosphere. Previous satellites have made point measurements as they traveled through the magnetosphere. But scientists recently learned that collisions between particles of the solar wind and stray particles of Earth's atmosphere produce low energy x-rays that light up the magnetosphere. By watching these x-rays, SMILE will capture its dynamic behavior.
Last week's announcement bodes well for China's space science program beyond the next four missions. The 4 billion yuan ($605 million) phase 2 budget includes support for development on future missions, particularly the enhanced X-ray Timing and Polarimetry mission, an ambitious international project led by Chinese scientists to study black holes, neutron stars, and magnetars.
China's planetary exploration and astronaut programs will continue to make history; later this year or early next, for example, it plans to land the first probe on the far side of the moon. But the future of its space science efforts also seems assured.