Miniature X-ray Solar Spectrometer CubeSat

Did You Know?

• MinXSS underwent thermal vacuum chamber cycle testing, thermal balance testing, vibration testing, and end-to-end communications testing before flight.
• The X123 primary science instrument was fully flight-qualified on two sounding rocket flights.
• As of 2018 March 8, 40 graduate, 5 undergraduate, and two high school students had worked on the project.

Mission objective

The MinXSS CubeSat measures the solar soft X-ray spectrum between roughly 0.5 keV and 30 keV, achieving a spectral resolution of about 0.15 keV. This wavelength range experiences the greatest brightening during solar flares and plays a key role in Earth’s ionospheric chemistry. Earlier observations were either low-resolution across a broad band or high-resolution but limited to a very narrow range. MinXSS data help scientists study the solar corona, especially active regions and flares, and are used in models of the upper atmosphere. The mission also marked the first use of Blue Canyon Technologies’ XACT attitude control system, a commercial three-axis system for CubeSats that performed better than its specifications.

Science instrument

The MinXSS CubeSat's main instrument is an Amptek X123 silicon drift detector adapted for space. To cool electronics that rely on air convection on Earth, heat transfer pads were added to create a conductive path in vacuum. A tungsten aperture narrows the field of view to ±4 degrees and prevents photon overload, while an extra beryllium filter cuts down photoelectrons. Two secondary instruments support it: the X-ray Photometer (XP), a single photodiode with a beryllium filter matching the X123's total filter thickness, provides on-orbit cross-calibration; and the Sun Position Sensor (SPS), a visible-light quad-diode with 2.4 arcsecond precision, corrects for signal loss when the Sun is off-axis. All instruments were calibrated at NIST's Synchrotron Ultraviolet Radiation Facility.

On-Orbit success

Communications were established on the first pass over the MinXSS ground station in Boulder, Colorado. First light was presented at a press briefing and poster during the American Astronomical Society's 47th Solar Physics Division Meeting. Over 40 GOES C-class and 7 M-class solar flares occurred in the first weeks and were downlinked for analysis. The Blue Canyon Technologies XACT ADCS performed continuously with 8 arcsecond pointing, exceeding its 11 arcsecond specification.

Follow-on mission (MinXSS-2)

A second MinXSS spacecraft, MinXSS-2, was built in parallel. It is identical except for an upgraded Amptek X123-FastSDD spectrometer, an upgraded BCT XACT, a hard reset circuit, an AstroDev Lithium-2 radio, and minor software updates. MinXSS-2 launched on 3 December 2018 via a SpaceX Falcon 9 as part of the SSO-A SmallSat Express mission. Its polar Sun-synchronous orbit at 575 km altitude provides an estimated 4-year mission life. The mission received two years of NASA H-TIDeS funding and added the Naval Research Laboratory as a science partner.