TIANWEN: CAMPAIGN FOR AN ASTRONOMICAL OBSERVATORY

The idea of a dedicated astronomical observatory has long been close to the heart of Chinese astronomers and astrophysics, with the United States and Soviet Union first launching such observatories in the 1960s. China has sketched four such missions – Tianwen Weixing, Solar Space Telescope (SST), Hard X-ray Modulation Telescope (HXMT), Kuafu – and made plans for participation in the World Space Observatory.

The first was brought to the design stage in the 1970s and even acquired a name, the Tianwen Weixing, though this project was not identified until many years later by Italian space writer Paolo Ulivi. Work on the 500-kg spacecraft began in 1976 and it was slated to carry astronomical instruments developed by the Purple Mountain Observatory. The purpose of the mission was to observe cosmic rays, x-rays, gamma – ray bursts, and high-energy solar emissions. It would make an all-sky survey through a window of 80 cm2. Instruments were a soft-x-ray imaging telescope, solar H-lyman ionization chamber, solar soft x-ray proportional counter (2-20 KeV), solar ultraviolet detector, solar radiation receiver, 10-cm2 cosmic x-ray source detector, cosmic gamma-ray burst detector, and cosmic gamma-ray burst x-ray detector. The soft x-ray imaging telescope proved exceptionally difficult, delaying the project. Even though the spacecraft was completed and integrated, the mission was canceled in 1985. Its instruments were transferred to other missions: the solar soft x-ray detectors (1-8 A), high-energy proton detector (10-30 MeV), and high-energy electron detector ((0.5-1 MeV) to the first communications satelhtes; the solar and cosmic ion detector to the Feng Yun in 1990; some instruments to Shenzhou 2 (Chapter 8); and others to balloons which made long-distance flights to Japan and over Siberia [11].

China returned to the idea of an astronomical observatory five years later. The Academy of Sciences approved in 1992 a 2-tonne solar observatory called the SST in cooperation with Germany (the proportions being 80:20) but this also made slow progress. By 1997, it had been through a full design study, with five volumes of papers completed. First government funding came in 1999 for the manufacturing of the telescope and attitude control systems, for test facilities in the observatory of Beijing, and for the mirror to be made in Russia. In its final design, the telescope itself would weigh 2 tonnes, carry five instruments, and operate for up to five years at 800-km altitude, carrying a polarizing spectrograph, accompanied by four side – mounted telescopes to examine the Sun in wide-band, X-ray, and hydrogen rays, as follows:

• 1-m main optical telescope, 3,900-6,600 A;

• 12-cm extreme ultraviolet imaging telescope in four spectrum lines;

• 12-cm white-light telescope, 6,562 A;

• wide-band spectrometer with soft and hard x-ray spectrometers and gamma – ray spectrometer to observe solar flares; and

• solar and interplanetary radio spectrometer, 2-50 MHz.

The SST would focus on the solar magnetic field and the detailed structures of the active areas. The intention was that it would be launched from Taiyuan on a Long March 4B for a three-year mission in a circular orbit at 730 km, 99.3 min, 98.3°. The truss for the telescope was 340 kg in weight and the structure holding the telescope 4 m long. The telescope’s observations would be transmitted down to the ground station in Miyun and then relayed 60 km to Beijing by fiber-optic cable. The telescope would collect 1,728 MB a day, which, after compression, will be downloaded in a daily 8-GB x-band transmission. It would have a two-dimensional real-time spectrograph to measure vector magnetic fields on the solar surface with a resolution of 0.1 arc-seconds so as “to achieve a breakthrough in solar physics” [12]. The latest launch date given is 2015.

A second project with long historical roots was the HXMT. This progressed through a number of design stages, too. The idea of an x-ray telescope was first defined in 1994 when two scientists in the High Energy Astrophysics Laboratory and Beijing Observatory, both called Li, proposed the use of 18 hexagonal prisms to

Original solar telescope design, 1990s. Courtesy: COSPAR China.

Revised solar telescope design, 2000s. Courtesy: COSPAR China.

make an x-ray sky survey in the 10-200-KeV range, below the range sampled by the European-Russian INTEGRAL project. Detailed design work was undertaken in the mid-1990s. The principal instrument was to be an 18-sided hexagonal prism for both survey purposes and more detailed imaging of high-interest objects [13].

This was refined in the 11th five-year plan (2008-13), whereby HXMT would operate in a wider energy band of 1-250 keV, with a resolution of 1 arc minute. Its objectives were to go beyond the hard x-ray sky survey, to find out how many supermassive black holes were surrounded by dust, make observations of x-ray emissions from pulsars, and examine the gravitational fields of compact objects and black holes. In 2009, a model was exhibited at the Zhuhai air show and the project was given a 2012 launch date with a budget of ¥lbn (€110m). It was announced that it would carry four telescopes, weigh 1 tonne, and orbit at 500 km for four years.

Funding for HXMT was not finally forthcoming until March 2011, with a new launch date set for 2015. Now it would carry both a low-energy instrument (1­15 keV) and a medium-energy one (5-30 keV). The 2,700-kg satellite will be put into an orbit of 550 km, 43°, for a four-year mission with the following final configuration of instruments:

• high-energy x-ray instrument, 20-250 keV;

• medium-energy x-ray experiment, 5-30 keV; and

• low-energy x-ray experiment, 1-15 keV.

As before, HXMT will perform sky surveys of the galactic plane, but its objectives were made more precise: to monitor variables and detect fresh sources; make large observations of the cosmic x-ray background; obtain the x-ray spectra of active galactic nuclei in order to determine their components and geometry; and observe the timing, physics, and extreme physical conditions near x-ray binaries.

The fourth was Kuafu, named after a figure in Chinese mythology who traveled to the Sun and followed a similarly named earlier mission with Shi Jian 4. This also dated to the 1980s and made no progress until Professor Tu Chuanyi formally proposed on 24th January 2003 that it be merged with a conceptual study for a Solar Wind and Auroral Storm Explorer (SWASE). International partners were invited to a presentation in Frankfurt, Germany, in December 2004 and it quickly attracted support in Germany, France, Britain, Belgium, Canada, Austria, and Ireland, with others joining later (Finland, Italy, Norway, and Switzerland). One hundred and five participants attended a Kuafu symposium in palm-tree-Uned Sanya, China, in 2007. Later, it became part of the International Living with a Star (ILWS) program and also found its way into the national 11th five-year plan (2008-13).

Kuafu is a development of Tan Ce, but this time with three spacecraft on a 10- year mission. The aim of the mission is to follow the complete chain of disturbance from the solar atmosphere to geospace, with a particular interest in solar flares, Coronal Mass Ejections (CMEs), interplanetary clouds, shock waves, magnetic storms, sub-storms, and auroras. According to Rainer Schwenn of the Max Plank Institute, “despite the enormous progress in recent years, there is still a lack in understanding of several key links in the long chain of actions and reactions that connect our Earth to its parent star, especially the origin of disturbances at the Sun, like flares and CMEs and our inability to predict them; their effects on the Earth;

Kuafu, showing locations of the three probes. Courtesy: NASA.

Table 7.6. Kuafu instruments.

Kuafu A

Extreme ultraviolent disk imager Coronal dynamics imager Radio burst instrument Solar wind instrument package Solar energetic particle sensor X-ray detector

Kuafu В

Extreme ultraviolet aurora monitoring camera

Aurora spectrograph

Wide-field aurora imager

Fluxgate magnetometer

High-energy particle experiment

Neutral atom imager (Ireland)

their ability to enter the Earth’s system and the magnitude of their terrestrial effects”. It is anticipated that each spacecraft will be 700 kg and fly 670,000 km from the Earth at three distinct points, the launcher being a CZ-3B (single launch) or CZ-2 (double launch). A will be located at Lagrange point LI, 1.5m km inward from the Sun, while B1 and B2 are put into terrestrial polar orbit, north and south, respectively, thereby giving simultaneous observations of solar storms and auroras at the peak of the next solar maximum.

Kuafu A will carry instruments for measuring solar extreme ultraviolet, white – light CMEs, the local plasma and magnetic field, solar wind as well as radio waves, while B1 and B2 would study high-energy particles and the magnetic field, and make 24-hr continuous imaging of the auroral regions. Instruments cover the 20- 300-MeV range, while the magnetic field measurer has a resolution of 0.01 nT. The payload for Kuafu A is 89 kg with a data transmission rate of 160 kbps, while for В the payload is 60 kg and the data rate is 500 kbps [14]. Table 7.6 describes the payloads.

An abortive project was SMESE (Small Explorer for Solar Eruptions), aimed to study solar flares and CMEs and the connections between the two at solar maximum. Even though a mission design had been completed, France pulled out of the project in April 2009. Instrumentation had even been agreed (Lyman disk imager, EUV disk imager, infrared telescope, Lyman alpha coronagraph, x-ray spectrometer, and gamma-ray spectrometer).

Finally, China indicated its willingness to participate in the World Space Observatory Ultraviolet (WSO-UV), a joint Russian-European-Chinese project with a 1.7-m primary mirror to study the structure of the universe and the atmospheres of distant planets, with China offering to contribute a long slit spectrograph. Participation will require advances in instrumentation, especially in focusing telescopes, focus plane detectors, sub-millimeter wave detectors, infrared detectors, and cooling systems. This project is still at an early stage of definition.