TRACE

Lore & Background

TRACE was a single-instrument, three-axis stabilized spacecraft built by NASA's Goddard Space Flight Center. Its telescope was constructed by a consortium led by Lockheed Martin's Advanced Technology Center, with optics designed and built by the Smithsonian Astrophysical Observatory. The telescope had a 30 cm aperture and a 1024 × 1024 CCD detector, giving an 8.5 arcminute field of view. It was designed to take correlated images in wavelengths from visible light through the Lyman alpha line to far ultraviolet, corresponding to plasma emission temperatures from 4,000 to 4,000,000 K. The optics used a special multilayer technique to focus extreme ultraviolet light, a technique first used for solar imaging in the late 1980s and 1990s by the MSSTA and NIXT sounding rocket payloads.

The spacecraft's attitude control system used 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. In science mode, an instrument-provided guide telescope served as a fine guidance sensor, providing pointing accuracy of less than 5 arcseconds. Power came from four panels of gallium arsenide solar cells totaling 2 m², producing about 220 watts. A 9 A-hour nickel–cadmium battery provided energy during Earth's shadow. Communications used a 5 W S-band transponder, with data stored on a solid-state recorder holding up to 300 MB. The command and data handling system used a 32-bit 80386/80387 processor.

The telescope was of Cassegrain design, 1.6 m long, with a focal length of 8.66 m. Its primary and secondary mirrors had normal-incidence coatings divided into quadrants, with three coatings for specific iron emission bands (Fe IX at 17.3 nm, Fe XII at 19.5 nm, Fe XV at 28.4 nm) and one for broadband ultraviolet centered on 500 nm. A filter wheel allowed further UV selection, including continuum observations at 170 nm and emission bands for C I and Fe II, C IV, and H I Lyman-alpha. The primary mirror assembly was based on designs from the SWATH and NIXT projects, and some hardware from the MDI instrument on SoHO was also used.

Reader's Guide

TRACE's significance lies in its focused investigation of the three-dimensional magnetic structures that emerge through the Sun's photosphere and define the geometry and dynamics of the upper solar atmosphere. Its primary science objectives were to follow the evolution of magnetic field structures from the solar interior to the corona, investigate mechanisms of heating of the outer solar atmosphere, and determine the triggers and onset of solar flares and mass ejections. By providing high-resolution images with a spatial resolution of one arcsecond and temporal resolution under one second, TRACE allowed scientists to observe fine-scale magnetic features and coronal loops in unprecedented detail. Its observations spanned a wide range of plasma temperatures, from 4,000 K to 4,000,000 K, enabling studies of the transition region and corona. The mission operated for over 12 years, from its launch in April 1998 until its last science image in June 2010, and its data contributed to understanding solar activity and space weather. The spacecraft reentered Earth's atmosphere in July 2025, ending its operational legacy.

Did You Know?

• TRACE was the third spacecraft in NASA's Small Explorer program.
• Its telescope used a multilayer coating technique first used for solar imaging by the MSSTA and NIXT sounding rocket payloads.
• The spacecraft's attitude control system achieved pointing accuracy of less than 5 arcseconds using a guide telescope.
• TRACE reentered the atmosphere 600 km south of Perth, Australia on 18 July 2025.