Cometary Explorer (1973)

Explorer 50/Interplanetary Monitoring Platform-J (IMP-J) satellite. Image credit: NASA.
The world's longest-running spacecraft series is the Explorer series, which began with the launch of Explorer 1, the first U.S. Earth satellite, on 31 January 1958. The U.S. Army carried out Explorer missions until NASA opened its doors on 1 October 1958. NASA Headquarters tapped NASA Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, to manage the Explorer Program. Explorer 6, launched 7 August 1959, was the first in the series to reach Earth orbit under NASA auspices.

The NASA Explorers were envisioned as low-cost science satellites. Explorer 6, a 142-pound spheroid with four paddle-like solar panels, carried lightweight, relatively simple radiation and micrometeoroid detectors. Simple did not, however, mean insignificant: Explorer 6 conducted the first detailed survey of the Van Allen radiation belts, which contain solar radiation particles trapped by Earth's magnetic field. Subsequent Explorers took many forms, but Sun-Earth interactions and the interplanetary environment remained major Explorer Program areas of interest.

NASA launched Explorer 50/Interplanetary Monitoring Platform-J (IMP-J) on 26 October 1973. The following month, a 35-member NASA GSFC team, the Cometary Explorer Study Group, completed a report for the Greenbelt center's Cometary Study Office which laid out a design for a low-cost dual-comet Explorer mission.

One goal of the proposed mission, which aimed to carry out ballistic (unpowered) intercepts of Comet Grigg-Skjellerup and Comet Giacobini-Zinner in 1977 and 1979, respectively, was to gain experience ahead of a Comet Halley mission. Halley, well known to the public and of significant scientific interest, was due to return to the inner Solar System in 1985-1986.

That NASA GSFC should seek a leading role in comet exploration is not surprising. Comets interact profoundly with the Sun and the interplanetary environment. The Greenbelt center staked its claim to comets as early as March/April 1970, when the NASA GSFC-managed Orbiting Astronomical Observatory-2 (OAO-2) spacecraft turned its ultraviolet telescopes toward Comet Bennett, a long-period comet discovered in December 1969.

OAO-2 revealed a large "halo" of hydrogen gas surrounding the comet, which implied that it had a nucleus made up at least partly of water ice. This helped to lend support to astronomer Fred Whipple's "dirty snowball" comet model.

Cometary Explorer spacecraft. A = solid-propellant kick motor; B = hydrazine propellant tank (one of eight); C = science instrument ring; D = solar cell ring (one of three); E = hydrazine thruster (one of six); F = main antenna reflector spin motor; G = main antenna reflector. The right side of the image shows a cutaway of the spacecraft; the left side shows its exterior appearance. Image credit: David S. F. Portree/NASA.
The Cometary Explorer Study Group based its 450-kilogram spacecraft on the drum-shaped Explorer 50/IMP-J design, which was very similar to Explorer 43/IMP-H (launched 13 March 1971) and Explorer 47/IMP-I (launched 23 September 1972). The spacecraft's 1.4-meter-wide, 1.8-meter-tall structure would be made up of four stacked 16-sided "rings." Of these, three rings would carry on their outer surfaces solar cells which together would generate 162 watts of electricity.

Within the fourth ring would be mounted most of Cometary Explorer's dozen science instruments. The instrument ring would have attached to it six appendages: four evenly spaced, 61-meter-long cable antennae for measuring interplanetary electric fields and a pair of instrument booms, each about three meters long.

Top view of Cometary Explorer spacecraft. A = cable antenna (one of four); B = instrument boom (one of two); C = counter-rotating high-gain radio antenna. Image credit: David S. F. Portree/NASA.
For stability, Cometary Explorer would spin about its long axis at least 15 times per minute. Most of its equipment would not be affected by its spin or would be aided by it; for example, acceleration ("artificial gravity") the spin would create within the spacecraft would help to move hydrazine propellant from eight small pressurized tanks in the lower solar-cell ring to six thrusters spaced around the spacecraft.

Cometary Explorer's top-mounted high-gain antenna, on the other hand, would become useless if it spun with the spacecraft. An electric motor in the antenna base would thus turn the antenna against the spin so that it would remain stationary relative to the rest of the spacecraft. This would help to keep it fixed on Earth throughout Cometary Explorer's two-and-a-half-year mission.

Cometary Explorer would lift off from Cape Canaveral, Florida, on 4 November 1976, at the start of a 10-day launch opportunity. A Delta rocket would place the spacecraft and a solid-propellant upper stage into Earth parking orbit. At the appropriate time, the solid-propellant motor would ignite to place Cometary Explorer on course for Comet Grigg-Skjellerup. Its job complete, the spent upper stage would separate; the spacecraft's small hydrazine thrusters would then tweak its Sun-centered orbital path to ensure a successful comet intercept.

Cometary Explorer inside its 2.44-meter-diameter streamlined launch shroud. A = outline of launch shroud; B = Cometary Explorer spacecraft; C = solid-propellant upper-stage motor. Image credit: David S. F. Portree/NASA.
At the time the Cometary Explorer Study Group prepared its report, Comet Grigg-Skjellerup orbited the Sun once every 5.1 years. Its elliptical orbit had a perihelion (point closest to the Sun) of 0.99 Astronomical Units (AU), and an aphelion (point farthest from the Sun) of 4.93 AU. An AU, incidentally, is equal to the mean Earth-Sun distance (149,597,871 kilometers).

Grigg-Skjellerup's orbital elements were the result of a close (0.33 AU) pass by Jupiter in early 1964; prior to that encounter, its orbital period had been 4.9 years and its perihelion distance 0.86 AU. Though Grigg-Skjellerup's orbit had been precisely determined following the Jupiter encounter, the Group advised that observatories on Earth should locate and track the comet before Cometary Explorer's launch to help to ensure a successful intercept.

Cometary Explorer would pass about 1000 kilometers from the Sun-facing side of Grigg-Skjellerup on 11 April 1977, traveling at 15.2 kilometers per second relative to its target. At time of intercept, comet and spacecraft would orbit the Sun only 0.2 AU from Earth. In addition to collecting data on the comet's interactions with the Sun and interplanetary space and the composition of its gas and dust, scientists would attempt to image Grigg-Skjellerup's nucleus.

Departure from Grigg-Skjellerup would mark the start of Cometary Explorer's "extended mission," which would last nearly two years. The spacecraft would for a time follow the initial orbit that had taken it past Grigg-Skjellerup; then, on 26 October 1977, nearly a year after its launch, it would return to Earth to perform a gravity-assist flyby at a distance of about 42,000 kilometers.

Before and after its Earth flyby, Cometary Explorer would pass through and attempt to define the limits of Earth's magnetotail, the part of its magnetosphere pushed outward by the solar wind. During the flyby, the spacecraft would ignite the solid-propellant kick motor embedded in the "thrust tube" at the center of its lower solar-cell ring. This, combined with Earth's gravity, would bend its course toward its second target, Comet Giacobini-Zinner. As Earth grew small behind it, flight controllers would use Cometary Explorer's hydrazine thrusters to refine its trajectory.

The Giacobini-Zinner intercept would take place 1.83 AU from Earth on 19 February 1979. Relative to the comet, the spacecraft would zip along at a speed of 20.8 kilometers per second.

The Cometary Explorer Study Group explained that the Grigg-Skjellerup and Giacobini-Zinner encounters would occur in "the proper order," meaning that the least perilous comet intercept would occur first. Grigg-Skjellerup, Cometary Explorer's primary target, was not a dusty comet, so the group felt that the spacecraft would not suffer crippling damage as it flew past. Giacobini-Zinner, on the other hand, was a dusty comet, so was more likely to damage or destroy Cometary Explorer.

In the foreword to its report, the Cometary Explorer Study Group warned readers that NASA had already rejected its proposed mission. The space agency had cited its rapidly shrinking budget when it turned down the NASA GSFC plan.

The Group argued, however, that its report was still worthy of publication because it had "established the framework for investigating future ballistic intercept missions to comets." In the decade that followed the Cometary Explorer study, several of the Group's members — but most notably Robert Farquhar, Mission Definition Manager for the study — would continue to plan inexpensive, pioneering missions to comets. More often than not, these would aim to prepare NASA to explore Comet Halley in 1985-1986. Several of these proposed missions will be described in future posts.


System Definition for "Cometary Explorer": A Mission to Intercept the Comets Grigg-Skjellerup (1977) and Giacobini-Zinner (1979), NASA TM X-70561, NASA Goddard Space Flight Center, November 1973.

Encyclopedia: Satellites and Sounding Rockets of Goddard Space Flight Center - 1959-1969, NASA, no date (1970).

"NASA Facts: Explorer Satellites," E-10-62, NASA, 1962.

More Information

A 1974 Plan for a Slow Flyby of Comet Encke

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)

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