The Wind spacecraft, launched on 1 November 1994, is a critical element in NASA’s Heliophysics System Observatory (HSO) – a fleet of spacecrafts created to understand the dynamics of the Sun‐Earth system. The combination of its longevity, its diverse complement of instrumentation, and high resolution and accurate measurements has led to it becoming the “standard candle” of solar wind measurements. A recent article published in Reviews of Geophysics describes the contributions of Wind to heliophysics and astrophysics. We asked the lead author what Wind has been doing for the past 25 years and what the future might hold.
What was the initial motivation behind launching the Wind spacecraft in November 1994?
Wind was designed and launched as part of the stand-alone Global Geospace Science (GGS) program, a subset of the International Solar Terrestrial Physics (ISTP) program.
The ISTP program was an international collaboration that included the Japanese Geotail spacecraft, the European four-spacecraft Cluster mission and SoHO spacecraft, the four Russian Interball spacecraft, and the two GGS spacecraft, Wind and Polar.
The goals for Wind were to investigate basic plasma processes in the near-Earth solar wind and magnetosphere and to provide baseline, near-Earth solar wind observations for inner and outer heliospheric missions.
This mission was one of the initial, coordinated efforts to better understand the Sun-Earth connection and the near-Earth space environment. This all has been lumped into the broader term “space weather.”
Wind is also one of the first missions (if not the first) that flew a Russian instrument, which came at the end of the Cold War. The scientific studies enabled by Wind have evolved over time owing to its longevity and diversity of instruments and diversity of space environments explored.
How is Wind distinct from other heliophysics missions?
There are several things that make Wind unique to nearly all NASA missions, not just heliophysics missions. For instance, the diversity of environments explored in the near-Earth space has yet to be repeated by any single mission. Wind made at least 11 lunar flybys including through the lunar wake. Wind made large, east-west prograde orbital loops to several hundred Earth radii from Earth. Wind explored the second Earth-Sun Lagrange point, approximately 235 Earth radii behind Earth along the Earth-Sun line. Wind made about 67 petal-shaped (i.e., highly elliptical) orbits through Earth’s magnetosphere. All of these were performed prior to June 2004, after which Wind was placed in its final orbital location at the first Earth-Sun Lagrange point. Wind also has an unusually broad and redundant set of instrument suites. Wind is also the second oldest NASA Heliophysics mission in operation, but unlike many other missions well past their design lifetime, Wind is still fully operational.
What instruments are on board and what types of data have they been gathering?
Wind has an unusually broad range of instrumentation compared to most other NASA missions still operating. Multiple instruments measure the low energy, thermal particles allowing for redundancy to cross-calibrate each instrument. Wind also measures electric and magnetic fields from quasi-static all the way up to radio frequencies. Wind can measure energetic particles up to approximately 50 megaelectron volts per nucleon and distinguish ion species from hydrogen to uranium including carbon, nitrogen, oxygen, neon, silicon, sulfur, iron, argon, etc. Wind also had two gamma ray instruments, one still operating that was provided by Russia. So, Wind can measure particles from a few electron volts to tens of megaelectron volts, electromagnetic fields from about 0 Hz to 13 MHz, and gamma rays with energies up to about 15 megaelectron volts. (For context, a typical blue photon would have an energy of about 3 electron volts, which is about 5 million times less energy per photon.) Note that 1 electron volt is the energy gained by an electron being accelerated by a 1 volt electric potential.
How have the contributions of Wind advanced the fields of heliophysics and astrophysics?
Wind data have led to paradigm shifting results in studies of statistical solar wind trends, magnetic reconnection, large-scale solar wind structures, kinetic physics, electromagnetic turbulence, the Van Allen radiation belts, coronal mass ejection topology, interplanetary and interstellar dust, the lunar wake, solar radio bursts, solar energetic particles, and extreme astrophysical phenomena such as gamma-ray bursts and magnetars.
The impact of Wind’s data on science is reflected in the more than 5,810 refereed publications from launch to the end of 2020, which have received more than 156,880 citations. So, while Wind is now 26 years old, it is still making new discoveries, still used as a calibration baseline for other missions, and is still advancing the field of heliophysics.
What is the Wind mission best known for?
This would depend upon with whom you speak. The magnetospheric community regard Wind as the upstream monitor, i.e., the spacecraft that informs them of what the solar wind is doing. A solar wind researcher may look at Wind as a great mission to investigate large-scale, transient magnetic structures like interplanetary coronal mass ejections or as one of the best missions for investigating small-scale, kinetic phenomena like plasma instabilities. A solar physicist will look to Wind for its extremely accurate and reliable remote-sensing radio instrumentation to investigate solar radio bursts or to its gamma ray instruments to investigate solar flares. Planetary and solar system scientists may look to Wind for its large statistical dataset of micron-sized dust impact measurements. This dataset includes both interplanetary and interstellar dust. Astrophysicists rely upon Wind for detecting gamma ray bursts and soft gamma ray repeaters (magnetars). Wind is truly special for a wide array of reasons to multiple fields of study.
After a quarter of century in flight, how much longer is the Wind mission expected to continue? What is the future potential of Wind?
After 25 years, Wind is by no means done. The spacecraft still has enough fuel for over 50 more years of flight and all instruments remain nominal. Wind‘s solar arrays can provide enough power for at least 20 more years with the current consumption rates. There are several mitigation strategies to extend this even further, if necessary. Thus, it is fully expected that Wind will live to at least 40 years of age barring any unexpected mechanical failure of things like thrusters or magnetic tape drives.
Wind is now a critical collaborator with the newer missions such as the Magnetospheric Multiscale (MMS), Parker Solar Probe, and the European Solar Orbiter missions. It continues to make novel discoveries, the team is working to release new data products for public consumption and publish scientific findings. In fact, Wind had 11 publications in Nature journals in 2020 alone, and there are at least four more Nature and two Science papers already in 2021 as of March.
III Wilson, L.B. (2021), Wind: Discoveries and impacts of a venerable spacecraft, Eos, 102, https://doi.org/10.1029/2021EO156801. Published on 18 May 2021.
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