Transition Region and Coronal Explorer (TRACE, or Explorer 73, SMEX-4) was a NASA heliophysics and solar observatory designed to investigate the connections between fine-scale magnetic fields and the associated plasma structures on the Sun by providing high-resolution images and observation of the solar photosphere, the transition region, and the solar corona. A main focus of the TRACE instrument was the fine structure of coronal loops low in the solar atmosphere. TRACE was the third spacecraft in the Small Explorer program, launched on 2 April 1998, and obtained its last science image on 21 June 2010, at 23:56 UTC. It reentered the atmosphere 600 km south of Perth, Australia on 18 July 2025, at 11:37 UTC.
Mission
The Transition Region and Coronal Explorer (TRACE) was a NASA small explorer mission designed to examine the three-dimensional magnetic structures which emerge through the Sun's photosphere (the visible surface of the Sun) and define both the geometry and dynamics of the upper solar atmosphere (the transition region and corona). Its primary science objectives were to: (1) follow the evolution of magnetic field structures from the solar interior to the corona; (2) investigate the mechanisms of the heating of the outer solar atmosphere; and, (3) determine the triggers and onset of solar flares and mass ejections.
Spacecraft
thumb|TRACE in cleanroom during assembly
The satellite was built by NASA's Goddard Space Flight Center. Its telescope was constructed by a consortium led by Lockheed Martin's Advanced Technology Center. The optics were designed and built to a state of the art surface finish by the Smithsonian Astrophysical Observatory (SAO). The telescope had a aperture and 1024 × 1024 charge-coupled device (CCD) detector giving an 8.5 arcminute field of view (FoV). The telescope was designed to take correlated images in a range of wavelengths from visible light through the Lyman alpha line to far ultraviolet. The different wavelength passbands corresponded to plasma emission temperatures from 4,000 to 4,000,000 K. The optics used a special multilayer technique to focus the difficult-to-reflect extreme ultraviolet (EUV) light; the technique was first used for solar imaging in the late 1980s and 1990s, notably by the MSSTA and NIXT sounding rocket payloads.
TRACE was a single-instrument, three-axis stabilized spacecraft. The spacecraft attitude control system (ACS) utilized three magnetic-torquer coils, a digital Sun sensor, six coarse Sun sensors, a three-axis magnetometer, four reaction wheels, and three two-axis inertial gyros to maintain pointing. In science mode, the spacecraft used an instrument-provided guide telescope as a fine guidance sensor to provide a pointing accuracy of less than 5 arcseconds. Power was provided to the spacecraft through the use of four panels of gallium arsenide (GaAs) solar cells with a total area of . The solar array actually produced power of around 220 watts, 85 W of which was used each orbit by the spacecraft and 35 W of which was used by the instrument each orbit. The remaining power was used for operational and decontamination heating of the spacecraft and telescope. A 9 A-hour nickel–cadmium battery (NiCd) provided energy during time when the spacecraft was in the Earth's shadow. Communications were provided via a 5 W S-band transponder, providing up to 2.25 Mbit/s downlink data transmission and 2 kbit/s uplink. Data were transmitted up to six times daily. Data were stored onboard using a solid-state recorder capable of holding up to 300 MB. The command and data handling system used a 32-bit 80386/80387 processor.