The Solwind Comets Pt. 1 - Background

This write-up covers several aspects of the Solwind mission. For readability, it has been split across several pages.

For a broad swath of space nerds, the Solwind mission is probably just a footnote to an infamous bit of military trivia: it was the target for the first successful anti-satellite missile test. But this satellite holds a much more positive place in the history of planetary science research: it was the vehicle for the first discoveries of Solar System objects from space. In August 1979, one of its instruments captured images of a previously unknown comet plunging towards the Sun. Over the next five years of operation, the spacecraft would spot at least 9 more comets taking the plunge. The discoveries from Solwind data marked the start of a new era, where the census of the Solar System was no longer limited by the constraints of Earth-based observatories.

The Solwind mission was designed to escape one of these constraints, although not for the purpose of discovering Solar System objects. Instead, it was developed to explore the solar corona, one most difficult regions of the Solar System to study. The corona is a large extended cloud of ionized plasma tangled in the Sun's extended magnetic field, which is approximately 40 times larger than the star itself. While in absolute terms, the corona glows very brightly, it does so with only one millionth the amount of light emitted directly from the Sun. Thus, for most of human existence, it could be separated from the Sun's glare only intermittently, during the short interval when the Sun's direct light was blocked by the Moon during a total solar eclipse. Working a precious few minutes at a time over two centuries, astronomers slowly learned more about the corona. Spectrographs pointed at the corona showed a bright green emission line that could not be linked to any known element on Earth. When the lines were finally linked to iron - through mathematical calculation of its energy states, which showed that the iron was ionized to a 13+ state - it demonstrated an environment far beyond what could be recreated in a laboratory setting. Eclipse observers also noted the corona came in a great variety of shapes, changing dramatically from eclipse to eclipse. Some eclipses yielded thin streamers, others brought large butterfly-shaped structures. Over time, astronomers worked out that complexity of the shape generally seemed to correlate to the intensity of sunspot activity, but the reasons underlying this relationship were unclear.

In 1930 Bernard Lyot, an astronomer at Meudon Observatory, began work on an instrument he called the coronagraph. By 1939, he had constructed a way to artificially recreate a solar eclipse within a telescope, using a disk to block out light directly from the Sun and a series of baffles to reduce stray light from the sky. With the coronagraph, astronomers could study the Sun with the support and infrastructure of an observatory, rather than chasing eclipses to the ends of the Earth. While Lyot's invention was revolutionary, it still had severe limits. Scattering of light by Earth's atmosphere only allowed a coronagraph to see a few tenths of a solar radius from the Sun, a fraction of what was visible during a solar eclipse. By the 1950s, additional technical improvements, new high-altitude observatories, and the recognition that the corona emitted polarized light had expanded the coronagraph's reach to about 2 solar radii, but these advancements had run into hard physical limits on what was possible.

At the close of WWII, the US military had taken possession of captured V2 rockets and had begun using them as sounding rockets for scientific research. However, the V2 was in limited supply, and assembling new ones from captured parts proved to be expensive. In response, the Navy Bureau of Ordinance and Naval Office of Research and Inventions developed a new sounding rocket, the Aerobee. The Aerobee proved to be a reliable rocket, quickly becoming the backbone of space environment research for the Navy. One of the researchers involved with the sounding rocket program was Richard Tousey, who had joined the Naval Research Laboratory in 1941 to develop methods to allow wartime pilots to navigate via stars in daylight. During the war he built a research group that focused on different uses of ultraviolet light; this group would be invited to fly an ultraviolet spectrograph onboard a V2 in 1946. Over the next 15 years, Tousey's group expanded their studies to other wavelengths of light, as well as the particle environment of space. The expanding access to space allowed scientists to begin understanding the connection between the solar atmosphere and the environment of Earth's upper atmosphere, and the corona was identified as a possible link between the two.

In June 1963, Tousey's group launched their first coronagraph on an Aerobee rocket, which returned photographs of details in the solar corona out to 10 solar radii. Continued sounding rocket photography, with some launches separated by only a day or two, showed major changes in the shape of the corona. This indicated that understanding coronal events would require a more frequent imaging cadence, which sounding rockets would struggle to provide given their expense and short operational lifetime. Development then turned to creating an orbiting coronagraph, which could collect images over a longer time period, and perhaps with a higher imaging cadence. Tousey developed a coronagraph instrument to fly on NASA's Orbiting Solar Observatory 2 (OSO-2) mission in 1965, but this instrument was badly affected by both stray light and mechanical problems with its detector.

Photographs taken on June 3, 1963 from a coronagraph placed on an Aerobee sounding rocket. (From Tousey et al. 1967)

Technical improvements with vidicon imaging sensors spurred some of Tousey's proteges, led by Michael Koomen, to develop a new coronagraph instrument to fly onboard the OSO-7 mission, which launched in 1971. This instrument was capable of delivering 256x256 pixel images of the corona from 3 to 10 solar radii, and produced daily images of the corona until OSO-7 malfunctioned in 1974. The success of this instrument, alongside the coronagraph on Skylab's Apollo Telescope Mount, captured the first images of "coronal transients" (now known as coronal mass ejections, or CMEs). These two instruments demonstrated a connection between these events and the radiation environment in near-Earth space, spurring the development of follow-on missions. The first of these was developed by the Naval Research Laboratory through the Department of Defense's Space Test Program (STP).

In ways, the STP was a continuation of the military's space program in progress prior to 1958, when many of its functions were placed under civilian control and reassigned to the newly-formed NASA. The Air Force was allowed to continue its space program, with the benefits of spy satellites (and the eventual planned launch of the Manned Orbital Laboratory) cited as a national security benefit. Supporting this program was the Titan IIIC, which was developed as the launch rocket for the X-22 Dyna-Soar space plane. The Dyna-Soar program was cancelled just as the Titan IIIC was being brought to operational status. The Titan IIIC was then repurposed to fly experimental military satellites, which carried payloads designed to test new instrumentation ranging from scientific investigations of the space environment to improved surveillance hardware. The mission developed by the Naval Research Lab, named Solwind (or P78-1 according to the STP nomenclature) would use a spare Orbiting Solar Observatory bus provided by NASA, and would fly gamma, x-ray and particle spectrometers, an extreme-ultraviolet imager, an aerosol monitor, and a white-light coronagraph (a spare from the OSO-7 mission).

Solwind was launched on February 24, 1979 onboard an Atlas-F rocket into a 500 km high sun-synchronous orbit with a 97-degree inclination and 97 minute orbital period. This orbit was designed such that overhead passes occurred on a noon-midnight schedule. The coronagraph was capable of producing one image every 10 minutes, but with several limitations. The orbital placement of the satellite meant that it spent one half of its orbital period on the dark side of the Earth, creating approximately 40 minute data gaps on every orbit. In addition, the imaging cadence was further reduced by ground station availability and the amount of data that could be crammed into the spacecraft's three data recorders between downlink periods. Finally, data was not analyzed in real-time to capture ongoing events. In fact, the Air Force was slow to process the telemetry and data returned by the spacecraft. For much of the mission, scientists at the Naval Research Laboratory were looking at data more than a year old by the time it arrived at their facility. Although this did not strongly affect the scientific operation of the mission which was strongly tied to ground station availability, it did limit possible follow-up observations from ground-based observatories.

The mission was highly productive through early spring 1985, but a combination of factors placed the mission on the chopping block. First, Solwind's batteries had degraded, reducing the amount of time over each orbital period when the spacecraft had enough power to operate. This also meant that operators had to be careful not to overtax the electrical system, or the spacecraft would default to a low-power safe mode. Additionally, the scientific return was drastically reduced by the failure of its tape drives. Solwind carried three tape drives for data storage. Over the years, two of these drives had failed, but expanding Air Force downlink capacity allowed more frequent downlinks to offset the loss of storage capacity. However, the failure of the last drive on March 14, 1985 required a new operation mode entirely. From then on, science operations could only occur when the spacecraft could transmit directly to ground stations. This limited data collection to at most 15 minutes when the satellite flew overhead of a ground station, although special arrangements were occasionally made to pass off communications between multiple stations for longer observation periods. However, the limited amount of data returned and the headache of dealing with safe mode events led the Air Force to propose terminating the mission.

The timing of this failure is how Solwind came to be the target of an anti-satellite test. The test was the rushed culmination of an anti-satellite arms race that had developed during the 1970s, when both the United States and USSR had produced prototype anti-satellite weapons. In 1978, Jimmy Carter ordered the Department of Defense to begin developing a new missile-based anti-satellite system, in response to the Soviet Union restarting their successful Istrebitel Sputnikov (IS) anti-satellite system under Leonid Brezhnev. The American anti-satellite system matured under Reagan, and in early 1985, the administration began making plans for a demonstration test. Congress, too, was not thrilled with the prospects of anti-satellite testing. Aware that the Soviet Union had ended testing of IS after Brezhnev's death, Congress wanted the Reagan administration to negotiate for a full anti-satellite weapons ban, rather than push ahead with a test that might lead to the Soviets restarting their program in response. In 1985, Congress passed a ban on anti-satellite tests to take effect in October 1985.

The closing window for an anti-satellite test prevented the Department of Defense from developing a target satellite, and high-altitude balloons designed for the first phases of anti-satellite testing were delayed by technical issues. Solwind fit the bill for a stand-in test satellite, as it was owned by the Department of Defense, essentially was at the end of its lifespan, and still had the ability to communicate with the ground to signal a successful test. As soon as the missile became available in August 1985, Reagan authorized the test to go forward, ahead of a November meeting with Mikhail Gorbachev that November. However, the specifics of the test and target were kept secret as long as possible. Naval Research Laboratory scientists were only told that their satellite would be shut down sometime after August 1, 1985, and it wasn't until September 6 that they learned that Solwind was the target satellite through a Washington Post article. There was a flurry of last minute objections by heliophysicists (although not those at the Naval Research Laboratory, who were prevented from speaking on the issue), who strongly argued that the satellite was still an extremely powerful research tool. NASA also objected, arguing that debris from the test would likely remain in orbit for more than a decade and force redesigns of the shielding on their planned space station.

Nevertheless, the test continued as planned, and the satellite was shot with an ASM-135 missile on September 13, 1985. Data collection continued until the end of the mission, with the last coronagraph image returned at 2036 UT on September 13, only minutes before the missile impact occurred (sometime around at 2040 UT). Interestingly, the fact that Solwind had collected data right up until its destruction was at odds with the political messaging that accompanied the test. Defense Secretary Caspar Weinberger was quoted describing Solwind as a "burnt out satellite". Additionally, Air Force officials also provided press statements describing the satellite as being beyond its operational lifetime. (Which, in true spindoctor fashion, was technically true - its operational lifespan had been technically reached in 1982, at the end of its three-year primary mission.) However, a week after the test, Robert MacQueen was quoted in the Washington Post as saying it was "deplorable" that a scientifically useful satellite had been used for a military test.

Ultimately, the test provided the first look at the effects of a hypervelocity impact in Earth orbit, generating more than 200 pieces of debris. Much of this debris did not reenter Earth's atmosphere until the early 1990s (slightly faster than NASA's predictions due to increased atmospheric drag related to the strong solar maximum in 1989). The amount of debris generated, the greater than expected difficulty in tracking it, and the now-demonstrated threat of orbital debris cascades to other American military satellites bolstered support for a continued Congressional ban. The anti-satellite program continued for another three years, but with little enthusiasm from many of its proponents. Many of the Solwind's capabilities, particularly its wide-field coronagraph, were not replicated until the launch of the Solar and Heliospheric Observatory mission in 1995.

Next page - Operational Discoveries of Solwind Comets (Coming Soon)

Last updated: January 17, 2021