|Comet Halley's last visit before the space age: a photographic plate captured at Yerkes Observatory on 6 June 1910. Image credit: Yerkes Observatory.|
Herman Michielsen was a Senior Staff Scientist at Lockheed Missiles & Space Company's Palo Alto Research Laboratory in California in August 1967, when he presented a paper on possible missions to Comet Halley to an American Institute of Aeronautics and Astronautics (AIAA) conference in Huntsville, Alabama. His paper was the earliest oft-cited work describing options for exploring Comet Halley using spacecraft during its 1985-1986 apparition, the first that would take place since the advent of spaceflight in 1957.
Lockheed funded Michielsen's Comet Halley research under its Independent Research Program, which gave its scientific staff opportunities to perform studies on company time outside their normal range of work. At the time he presented his Comet Halley paper, much of Michielsen's work had focused on calculating lunar and planetary ephemerides using advanced computers and on Earth satellite tracking. He was an important figure in the Independent Tracking Coordination Program, which aimed to supplement the limited number of professional Earth satellite visual observations with those of skilled amateurs around the world.
Comet Halley requires little introduction; it is the one recurrent comet the name of which is widely known to non-astronomers. Observations of Comet Halley were recorded in China as early as 240 BC. Not until the 18th century, however, was it understood that Comet Halley follows an elliptical Sun-centered path that brings it to a perihelion (closest point in its orbit about the Sun) between the orbits of Venus and Mercury about every 76 years.
The comet is named for Edmond Halley, the English astronomer who wrote in 1705 that comets observed in 1531, 1607, and 1682 were in fact a single comet. Halley successfully predicted that the comet would return in 1758, though he did not live to see its return.
Michielsen noted that short-period comets — that is, any comet with a period of 200 years or less — are typically visible only using telescopes and barely show a tail. Comet Halley is a short-period comet but bucks this tendency, making it an object of interest for future exploration using robot probes. The Lockheed scientist predicted that its return in 1985-1986 would become "a culmination point in the field of cometary probes."
Comet Halley is, however, not an ideal target for a spacecraft because it follows a retrograde path around the Sun. The great majority of Solar System bodies orbit their primary — the Sun, a planet, or any of the various categories of small body — in a prograde direction, which is to say counterclockwise. For its part, Comet Halley orbits the Sun clockwise. Michielsen called this "an unfortunate condition."
Michielsen calculated that spacecraft on a prograde intercept path would encounter Comet Halley at Earth's distance from the Sun (one Astronomical Unit, or AU) moving at about 60 kilometers per second (km/sec) relative to the comet; at Comet Halley's perihelion distance, 0.59 AU from the Sun, the relative intercept speed would exceed 90 km/sec. High encounter speeds near and at perihelion would mean that a probe could view the comet's nucleus, which was expected to measure at most a few tens of kilometers across, for only a very short time, making impossible any in-depth observations when the comet was most active.
At the time Michielsen presented his work, most comet scientists favored astronomer Fred Whipple's "dirty snowball" model of the structure of the comet nucleus. It should be noted, however, that in 1967-1968 rival models had supporters. Confirming the nature of the nucleus was among the most important justifications for comet exploration until the 1980s.
Michielsen proposed that an effort be made in time for the 1985-1986 apparition to place a robot probe into a retrograde Sun-centered orbit that would enable it to rendezvous with and travel beside Comet Halley for weeks or months. He wrote that a rendezvous mission would permit "a return of useful data many orders of magnitude greater than that from even a number of high-speed intercepts." A rendezvous would, however, be extremely challenging in terms of propulsive energy required.
A Comet Halley rendezvous might approach feasibility, he wrote, if the rendezvous probe were first launched into an elliptical Sun-centered orbit with an aphelion (farthest point in its orbit about the Sun) at about seven AU (that is, between the orbits of Jupiter and Saturn, which orbit the Sun at 5.2 AU and 9.5 AU, respectively). He proposed a launch in 1978, with the probe approaching aphelion in 1982.
Near aphelion, the spacecraft would move relatively slowly, so could place itself into a retrograde orbit using a propulsive maneuver (an "aphelion pulse") that changed its speed by only about 9.3 km/sec. Combined with Earth-departure and fine-targeting maneuvers, the total propulsive velocity change required to carry out a Comet Halley rendezvous in 1985 would amount to about 31 km/sec.
|Diagram of Comet Halley and rendezvous spacecraft paths during Michielsen's aphelion-pulse mission. Please click on image to enlarge. Image credit: DSFPortree.|
Other options would enable a Halley rendezvous with even less propulsive velocity change, Michielsen added. Departing Earth in 1973 would, for example, trim the aphelion pulse velocity change by 2.5 km/sec. The 12-year flight time from Earth launch to Halley rendezvous might, however, be seen as excessive.
In the early-to-mid-1960s, many planners considered the possibilities of propellant-saving gravity-assist maneuvers. Michielsen explained that a spacecraft launched on 13 September 1977 that passed in front of Jupiter on 16 September 1978 would be slowed and its course bent onto a retrograde path that would permit a rendezvous with Comet Halley on 27 May 1985, 254 days before its predicted perihelion on 5 February 1986. He also described a mission launched from Earth on 16 October 1978 that would encounter Jupiter on 14 October 1979 and rendezvous with Comet Halley on 10 September 1985, 148 days ahead of predicted perihelion.
Jupiter would be better positioned for the gravity-assist flyby in the 1977 opportunity, Michielsen added, thus reducing the required Earth-departure velocity and the velocity at which the spacecraft would approach Comet Halley. The propulsive velocity change from Earth departure through Halley rendezvous would total 24.6 km/sec for the mission launched in 1977 and 25.6 km/sec for the 1978 mission.
Michielsen then briefly explored the possibility of a Saturn gravity-assist flyby, which he said was suggested at the August 1967 AIAA meeting by Maxwell Hunter, who was a National Space Council member from 1962 until he joined Lockheed in 1965. A Saturn flyby Comet Halley rendezvous mission launched from Earth on 30 August 1973 would require a total propulsive velocity change of 22.2 km/sec; one launched on 14 September 1974 would need 22.9 km/sec. Saturn flyby would occur on 19 January 1976 for the 1973 launch and on 14 January 1977 for the 1974 launch; Comet Halley rendezvous would take place on 18 April 1985 or 21 June 1985, respectively.
In the second half of his paper, Michielsen gave close attention to the problem of precise prediction of Comet Halley's return, and it is in this context that his work is most often cited today. He noted that digital computers had enabled researchers to confirm that the gravity of the planets — in particular, Jupiter, Earth, and Venus — had caused Comet Halley's orbital period to vary by up to 1000 days over the centuries. In addition, a non-gravitational effect — the explanation of which he declared was beyond the scope of his paper — caused a shift in the perihelion date of about four days during each of the six apparitions spanning the period from 1456 to 1835.
The non-gravitational effect Michielsen was loath to explain had been attributed to jets of gas and dust that form when a comet nucleus is heated by the Sun. These jets would, it was believed, behave like natural rocket motors. This hypothesis would eventually be confirmed, but the Lockheed scientist was probably wise to treat the potentially controversial problem as an unnecessary distraction when he presented his study of Comet Halley rendezvous methods.
The shift in perihelion date meant that a Comet Halley probe launched in the late 1970s would need to perform additional propulsive maneuvers to ensure a close rendezvous. The magnitude of the maneuvers required would begin to become apparent, he predicted, in November 1983, when Earth's largest telescopes would begin to photograph Comet Halley between the orbits of Saturn and Jupiter at a distance of 8.5 AU from the Sun. Michielsen expected that, if reacquisition took place at that time, then a sufficient number of observations could occur to ensure that maneuvers requiring a total propulsive velocity change of just 1.2 kilometers per second would yield a "worthwhile rendezvous mission." Later reacquisition might demand a greater propulsive velocity change.
As it turned out, the advent of CCD technology enabled reacquisition of Comet Halley more than a year ahead of Michielsen's predicted date. On 16 October 1982, observers using the 200-inch Hale Telescope at Mount Palomar in California became the first humans to glimpse Comet Halley since 1911. The comet, which had yet to show a tail, lay beyond the orbit of Saturn when it was reacquired.
Advances in astronomy technology mean that Comet Halley has remained visible since its 16 October 1982 reacquisition. When it reaches perihelion in July 2061, it will have been visually tracked for 79 years.
This post is the first in a new series called "Preparing for Halley." It aims to describe U.S. efforts to launch a spacecraft to Comet Halley in 1985-1986. The series is timed to coincide with Comet Halley's aphelion passage late in 2023, after which it will be inbound for its 2061 apparition. Other posts on comet exploration relevant to Comet Halley missions in 1985-1986 can be found by following the "More Information" links below.
"A Rendezvous with Halley's Comet in 1985-1986," H. F. Michielsen, Journal of Spacecraft and Rockets, Volume 5, Number 3, March 1968, pp. 328-334; paper presented at the AIAA Guidance, Control, and Flight Dynamics Conference in Huntsville, Alabama, 14 August 1967.