15 March 2015

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

Comet Ikeya-Seki lights the pre-dawn sky at Kitt Peak on 29 October 1965. Image credit: Roger Lynds/NOAO/AURA/NSF
The comet now designated C/1965 S1 - better known as Comet Ikeya-Seki, for its discoverers, Kaoru Ikeya and Tsutomu Seki - was perhaps the intrinsically brightest comet of the past 1000 years. Ikeya and Seki independently discovered the inbound "dirty snowball" on 18 September 1965. As it neared the Sun in mid-October 1965, it blazed in daylight skies as magnitude -10 (that is, it was 100 times brighter than the planet Venus, which is typically the brightest object in earthly skies after the Sun and moon).

Shortly before it became lost in the Sun's glare, Comet Ikeya-Seki was observed to have broken into three major pieces. On 21 October 1965, the three parts of the comet passed a mere 450,000 kilometers (km) from the Sun. The fractured comet then emerged from the glare into dawn skies sporting a very bright tail, possibly because its fragmentation had exposed fresh ice. The fragments remained close together as Comet Ikeya-Seki retreated from the Sun. By early 1966, it was lost to sight. Its fragments might reenter the Inner Solar System as separate comets in about 900 years.

Soon after Comet Ikeya-Seki left the Inner Solar System, Tim Kreiter, with NASA Lewis Research Center in Cleveland, Ohio, prepared a brief report in which he outlined what might have become NASA's first missions to a comet and an asteroid. Kreiter sought to demonstrate that the space agency could aim for targets other than the moon, Venus, and Mars - all of which had been reached by mid-1965 - using existing or near-term spacecraft technology and "intermediate-sized" Atlas-derived launch vehicles.

Kreiter's target comet was 6P/d'Arrest, named for its 19th-century discoverer, German astronomer Heinrich Louis d'Arrest. Kreiter cited a NASA-funded March 1965 Illinois Institute of Technology Research Institute (IITRI) study that singled out the 1976 d'Arrest opportunity as the most favorable of any comet mission opportunity between 1965 and 1986.

Comet d'Arrest, a relatively faint comet, follows an elliptical Sun-centered orbit with an aphelion (point where it is farthest from the Sun) just beyond Jupiter's orbit and a perihelion (point where it is nearest the Sun) just beyond Earth's orbit. As the comet neared perihelion, solar heating would cause its surface to become active, making it an interesting target for spacecraft exploration.

A flyby spacecraft that intercepted Comet d'Arrest at the time of its 1976 perihelion would do so less that 0.16 Astronomical Units (that is, 0.16 times the Earth-Sun distance - about 23.9 million km) from Earth. Relatively close proximity to Earth would permit a typical low-powered spacecraft transmitter to return data to Earth at a relatively high rate and enable astronomers using Earth-based telescopes to closely monitor Comet d'Arrest during the flyby.

Jupiter's gravity often reshapes comet orbits. Because of this, Kreiter wrote, astronomers would need to track Comet d'Arrest for at least two months before spacecraft launch to precisely determine its orbit. Again citing the 1965 IITRI study, he explained that Comet d'Arrest would need to attain a visual magnitude of at least 20 before it could be spotted using Earth's largest telescopes. This would, he wrote, occur seven or eight months before planned launch of the comet probe, providing space navigators with ample time to plot the comet's orbit.

Kreiter considered Atlas rocket/upper stage combinations designated SLV-3A/Agena, SLV-3C/Kick, and SLV-3C/Centaur. These could in 1976 launch to Comet d'Arrest robotic spacecraft of considerable mass: about 430 kilograms (kg), 700 kg, and 1050 kg, respectively. For comparison, Mariner 4, which left Earth on a Atlas SLV-3/Agena on 28 November 1964 and flew past Mars on 15 July 1965, was limited to a mass of about 260 kilograms.

Kreiter had his Comet d'Arrest flyby spacecraft leave Earth between 22 March and 21 April 1976. Daily launch windows lasting about 2.5 hours would occur throughout the planned launch period. The Atlas would boost the Agena, Kick, or Centaur upper stage and attached spacecraft into a 185-km-high parking orbit about the Earth. The upper stage and spacecraft would then loiter for from 10 to 20 minutes before the former ignited to place the latter on course for Comet d'Arrest.

Travel time to Comet d'Arrest would range from 115 to 145 days, with the precise duration and flyby date determined by the date of launch from Earth. The spacecraft would zoom past the comet moving at about 12.8 km per second, Kreiter calculated, so would spend only about 4.4 hours within 100,000 km of its target. This might mean, he wrote, that some desirable scientific investigations - for example, imaging the comet's mysterious heart, a feat which would demand precise camera aiming - would probably not be feasible.

30 May 1966: An Atlas-Centaur rocket launches Surveyor 1, the first lunar soft-lander. Kreiter's 1974 Eros and 1976 Comet d'Arrest missions might have launched on a similar rocket. Image credit: NASA

Kreiter's flyby mission to Near-Earth Asteroid 433 Eros would launch between 9 August and 8 September 1974, about 18 months ahead of his Comet d'Arrest mission. He estimated that a Atlas SLV-3A/Agena could launch to Eros a spacecraft with a mass of about 470 kg; the corresponding masses for the SLV-3C/Kick and SLV-3C/Centaur launches were, respectively, 740 kg and 1050 kg. The Eros flyby spacecraft and its upper stage would orbit Earth for from 20 to 40 minutes before the latter ignited to send the former on its way.

The Eros spacecraft would fly past its target between 140 and 170 days after launch, with the precise encounter date being dependent on its launch date. As with the Comet d'Arrest mission, the flyby would occur near the target body's perihelion, when Eros and the Eros spacecraft would pass no more than 0.13 Astronomical Units from Earth. Moving at about 7.2 km per second, the spacecraft would spend about 7.9 hours within 100,000 km of Eros.

American space scientists would have to wait many years for NASA comet and asteroid missions. In the meantime, they made do. The International Comet Explorer (ICE) - originally designated International Sun-Earth Explorer-3 - was launched in 1978, repurposed and renamed in 1982, and flown through 21P/Giacobinni-Zinner's tail on 11 September 1985. The Galileo Jupiter orbiter flew past Main Belt asteroid 951 Gaspra on 29 October 1991 on its way to its second gravity-assist Earth flyby.

The first successful NASA spacecraft designed especially to explore an asteroid and a comet did not leave Earth until 1996 and 1999, respectively. The former was the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft, which entered orbit around Eros on 14 February 2000, and the latter was Stardust, which flew by 81P/Wild 2 on 24 September 2003.

Both missions were far more ambitious that Kreiter's simple flybys. After orbiting Eros for almost a year, NEAR Shoemaker landed on the asteroid on 12 February 2001 and operated for about two weeks before draining its batteries. Stardust completed its primary mission by dropping off at Earth a reentry capsule containing Comet Wild 2 dust samples on 15 January 2006; in its extended mission, it flew past 9P/Tempel 1 on 15 February 2011, where it imaged a crater blasted by the Deep Impact spacecraft on 4 July 2005.

Sources

"Splendor in the Night," M. Jewell, Time Magazine, 22 October 1965, p. 90

"The Great Comet of 1965," B. Marsden, Sky & Telescope, December 1965, pp. 332-337

"Reports on Comet Ikeya-Seki (1965f)," L. Robinson, Sky & Telescope, January 1966, pp. 52-55

Intercept Missions to Asteroid Eros in 1974 and Comet d'Arrest in 1976, NASA Technical Memorandum X-1288, T. Kreiter, October 1966

More Information

Cometary Explorer (1973)

A 1974 Plan for a Slow Flyby of Comet Encke

Catching Some Comet Dust: Giotto II (1985)

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