NASA’s STEREO-B spacecraft called home on Aug. 21, 2016, after having lost contact 22 months ago on Oct. 1, 2014.
STEREO-B is one of NASA's Solar Terrestrial Relations Observatories. Two nearly identical spacecraft were launched in 2006 into orbits around the Sun in Earth-like orbits. The mission was originally expected to last 3 years. Nine years and nine months later, STEREO-A is still fully operational while STEREO-B is being revived after a 2 year outage.
Orbits
The STEREO-Ahead spacecraft has an orbit that is a little bit closer to the Sun than Earth, and therefore orbits a little bit faster than Earth. STEREO-Behind, on the other hand, has an orbit just slightly outside Earth's, and is thus a little bit slower. The net result is that each spacecraft seems to slowly drift in opposite directions away from Earth by about 22 degrees per year. On February 6, 2011, the two spacecraft were exactly 180 degrees apart from each other, allowing the entire Sun to be seen at once for the first time.
The two spacecraft enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.
The following video from NASA describes the mission and the lead up to the 2014 events, when each spacecraft would separately be behind the Sun for a few months, from the vantage point of Earth.
Why STEREO?
STEREO is a key addition to the fleet of space weather detection satellites by providing more accurate alerts for the arrival time of Earth-directed solar ejections with its unique side-viewing perspective.
Coronal mass ejections (CMEs) are powerful eruptions that can blow up to 10 billion tons of the Sun's atmosphere into interplanetary space. Traveling away from the Sun at speeds of approximately one million mph (1.6 million kmph), CMEs can create major disturbances in the interplanetary medium and trigger severe magnetic storms when they collide with Earth's magnetosphere. CMEs and solar flares accelerate cosmic-ray-like electrons, protons, and heavier ions to near the speed of light. They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays,
Large geomagnetic storms directed towards Earth can damage and even destroy satellites, are extremely hazardous to Astronauts when outside of the protection of the International Space Station performing Extra Vehicular Activities (EVAs), and they have been known to disrupt communications and power grids on Earth.
The principal benefit of the mission is stereoscopic images of the Sun and observation of parts of the Sun that are not visible from Earth. This permits NASA scientists to directly monitor the far side of the Sun, instead of inferring the activity on the far side from data that can be gleaned from Earth's view of the Sun.
STEREO has also been used to study other celestial objects and has discovered 122 eclipsing binaries (stars) and has studied hundreds of variable stars.
STEREO-B Loss of Contact
Communication with STEREO-B was lost on October 1, 2014, during a test of the spacecraft’s command loss timer. The spacecraft is designed to perform a hard reset if it loses communications with Earth for 72 hours and then periodically re-try to establish contact. The STEREO team was testing this function in preparation for something known as solar conjunction, when STEREO-B’s line of sight to Earth – and therefore all communications – would be blocked by the sun, between Jan 22 and March 23, 2015.
A series of problems in the guidance and control system led to the failure.
The Star Tracker failed to provide proper data, which caused a failure of the high gain antenna to be properly pointed at Earth, resulting in the initial faint signal, which then drifted even further so that the signal was lost completely.
The limited telemetry received after the spacecraft reset also indicated that one of the laser gyros in the Inertial Measurement Unit (IMU) had failed, and was providing bad data to the attitude control system. Thus, two simultaneous failures had occurred in the attitude control system—the Star Tracker and the IMU—and the ability of the spacecraft's guidance and control electronics to cope with multiple failures is limited.
With only that limited amount of information, it's unclear what happened next. If the spacecraft managed to recognize that the IMU had an anomaly, it would have disabled it, and fallen back on the only system left for determining orientation, the solar aspect sensors. There are five solar aspect sensors on the spacecraft, among them covering the full sky. With just these sensors operating, the Behind spacecraft should at least have kept itself well enough pointed to keep the solar panels pointed at the Sun, and maintain power on the spacecraft.
However, if the spacecraft did not detect that the IMU was providing bad data, it may have fired its thrusters to stop the roll that it thought it was in. With bad data coming from the IMU, instead of stabilizing the spacecraft, this would have sent it into a spin about its principal axis of inertia. The solar panels may have then stopped getting enough sunlight to keep the spacecraft powered, draining the batteries and shutting the spacecraft down.
See stereo-ssc.nascom.nasa.gov/… for details on the diagnosis of the failure.
Meanwhile, STEREO-A continued to perform its mission and send back valuable science data.
Current Status
NASA's Deep Space Network, or DSN, a set of Earth based tracking stations and networks, which tracks and communicates with missions throughout space, established a lock on the STEREO-B communications signal at 6:27 p.m. EDT on Aug 21. NASA had not given up on STEREO-B and had been scanning for signals from it ever since the time contact was lost.
The signal was monitored by the Mission Operations team over several hours to characterize the attitude of the spacecraft and then the transmitter high voltage was powered down to save battery power. The STEREO Missions Operations team plans further recovery processes to assess observatory health, re-establish attitude control, and evaluate all subsystems and instruments.
On Aug 23, after reviewing the downlink signal levels, it was concluded that STEREO-B is most likely rotating about its principal axis of inertia. While this uncontrolled orientation appears to be power positive now (enough power is being generated), it will drift off, and the estimate is that it may not return until next summer. The ~2 minute rotation/wobble is ~3 deg/sec which is beyond what the momentum wheels can handle. This means that any attempt to de-spin the spacecraft at the present time would result in saturation (the electrically operated momentum wheels reach maximum speed) and subsequent autonomous momentum dump (automatic firing of thrusters to reduce spacecraft and wheel spin), for which the spacecraft has not yet been prepared.
STEREO Data and Communication Systems (Geek-Speak)
Each spacecraft carries two CPUs, one for command-and-data handling and one for guidance and control. Both are radiation hardened 25 MHz IBM RAD6000 processors, based on POWER1 CPUs (predecessor of the PowerPC chip found in older Macintoshes). For comparison, a modern iPhone has two 64-bit cores running at 1.85 GHz each, but radiation hardening means lower clock speeds.
STEREO also carries Actel FPGAs that use triple modular redundancy for radiation hardening (3 identical chips execute the same hardware logic, a failure in one of the chip is detected by comparing against the results of the other two, the faulty chip memory locations are fixed). The FPGAs hold the P24 MISC and CPU24 soft microprocessors.
For data storage, each spacecraft carries a solid state recorder able to store up to one gigabyte each. Its main processor collects and stores on the recorder images and other data from STEREO's instruments, which can then be sent back to Earth.
The spacecraft consumes an average of 475 watts of power, generated by the solar panels. The average hair dryer consumes 1500 watts!
Communications
The spacecraft transmit 63 watts to a 1.1 meter dish at 8.4 GHz X-band, with an EIRP of approximately 85 dBm. The spacecraft has a transmission capacity of between 427 and 750 kbps. (My home WiFi operates at 108 Mbps, but then the spacecraft communicates from a distance of up to 186 million miles.)
The forward error correction was changed, in April 2013, from convolutional encoding to Turbocode.
NASA ground stations receive and forward the data via the Internet to NASA GSFC for processing where it is posted on the web in near real time for all to use, including the various world wide organizations monitoring and predicting Space Weather. The ground network for the low speed data is organized and coordinated by the SWPC in Boulder, Colorado.
The Solar Dynamics Observatory (SDO)
The SDO is another Sun-observation spacecraft and was launched on February 11, 2010, on a 5 year mission (with extension to 10 years; most NASA spacecraft seem to be over-achievers; Voyager 1, launched in 1977, is still going, 20 billion kilometers away). SDO is in an inclined geo-synchronous orbit around Earth, 22,3000 miles above Earth, 24-hour orbit period, and is providing another vantage point for observing the Sun.
The Sun — in Ultra-HD
The following is a cool time-lapse video of the Sun, based on data from the SDO. It presents “the "dance of the ultra-hot material on our life-giving star in extraordinary detail, offering an intimate view of the grand forces of the solar system.” In addition to keeping life alive on Earth, the sun sends out a constant flow of particles called the solar wind, and it occasionally erupts with giant clouds of solar material, called coronal mass ejections, or explosions of X-rays called solar flares.
References:
- www.nasa.gov/...
- stereo.gsfc.nasa.gov
- STEREO (Solar Terrestrial Relations Observatory) — en.wikipedia.org/...
- Solar Dynamics Observatory — en.wikipedia.org/…
- Spacecraft Attitude Control — en.wikipedia.org/…
- Stereo A/B Spacecraft Telemetry Reception at Bochum — www.amsat.org/...