The Solwind Comets

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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 semi-regular discoveries from Solwind data marked the beginning of a transition from discoveries limited by the constraints of Earth-based observatories to the new possibilities enabled by observing from space.

Historical Background

The solar corona is one of the 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.

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.

Operation and Comet Discoveries

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, at times taking more than a year to release data from the satellite to its scientists. All discoveries made by the spacecraft were made long after the fact.

The first (and by far the brightest) comet spotted by Solwind's coronagraph was observed on August 29-30, 1979, after roughly 3 months of routine data collection. The first image of the comet occurred after a relatively long imaging gap of about 6 hours, with the bright head of the comet already a substantial way to the Sun. The comet was not immediately recognizable as such, and the initial interpretation of the data was stray light, or worse, a light leak. Stray light had badly plagued the coronagraph on OSO-2, and was an occasional nuisance that disrupted OSO-7 operations (for an example of stray light, see the first image on the bottom row in the Aerobee photo). These problems paled in comparison to a possible light leak, though, which might have allowed direct sunlight to reach the imaging sensor and damage it.

However, the object in the photos was observed to be moving in a systematic way across the sensor, before disappearing in later images. This indicated that it was not an optical artifact, but indeed a comet falling towards the Sun. At peak brightness, late on the evening of August 29 (GMT), it appeared much brighter than Venus, probably peaking at around magnitude -5. The comet did not reappear on the other side of the Sun. However, the corona where the comet was expected to reappear showed a persistent brightening that faded over the next day. The behavior of the corona was initially attributed to effects of the comet actually hitting the Sun. Determining an orbit for the comet, which would provide a definitive answer about how closely the comet could have gotten, was difficult. The large field of view, small sensor size, and a relative lack of bright stars limited the precision of the comet's position. In addition, the short time span over which the comet was observed made it difficult to constrain the comet's trajectory. Through heroic efforts by the Minor Planet Center's Brian Marsden to calculate the comet's orbit from the given data, it was determined to belong to the Kreutz family of sungrazing comets, with a close approach at a very toasty 22,000 km from the surface - with a collision well within the range of error.

Last updated: January 12, 2021