It is strange that Lexell’s Comet is not better remembered. Discovered by ace comet-hunter Charles Messier on the night of 14 June 1770, it passed Earth just two weeks later at a distance of 1.4 million miles, closer than any other comet in history. On the evening of 1 July 1770, its nucleus shown as brightly as Jupiter at its brightest, and its silvery coma was five times larger than the full moon.
Lexell’s Comet then drew close to the Sun – that is, it reached perihelion – and was lost in the glare. Messier saw it next in the pre-dawn sky on 4 August. Having moved away from Earth and the Sun, it had become small and faint. Messier observed the comet with difficulty before dawn on 3 October 1770, then lost sight of it.
Comets are today named for their discoverer or discoverers, but in the 18th century it was the mathematicians who computed their orbits who got all the credit. Comet Halley is, for example, named for Edmond Halley, who computed its orbit and determined that what had seemed like a series of individual comets was in fact a single comet that returned again and again. Partly this was because in Comet Halley’s case no one knows who discovered it; records of the comet’s apparitions extend back at least to 240 BCE, but it almost certainly was noticed in Earth’s skies much earlier.
Lexell’s Comet was named for Anders Johan Lexell, who determined that it completed one elliptical orbit about the Sun in 5.6 years. This was for the time a remarkably short period, raising questions as to why it had not been observed before. Lexell wrote that the comet had previously followed a long path with a perihelion close to Jupiter’s orbit, but then had passed Jupiter at a distance of less than two million miles in 1767. The giant planet had, he explained, slowed it and deposited it into its new short-period orbit.
Lexell’s Comet was due to reach perihelion again in 1776, but this occurred on the far side of the Sun as viewed from Earth and so was not observed. Astronomers eagerly awaited its next perihelion in 1781 or 1782, but nothing was seen. Again, Lexell had an explanation: in 1779, as it neared the point in its new orbit where it was farthest from the Sun – its aphelion – the comet had again intersected Jupiter. This time, it had sped up and entered an unknown but probably long-period orbit. It might even have escaped the Sun’s gravitational grip entirely. In any case, Lexell’s Comet has not been seen since and is officially designated “lost.”
The light-show of 1 July 1770 should have ensured that no one forgot Lexell’s Comet, but both its close pass by Earth and its orbit changes soon faded from memory. If they had not, then Michael Minovitch’s discoveries in 1961-1964 might not have shaken the interplanetary mission planning world the way they did.
Minovitch, in 1961 a 25-year-old graduate student at the University of California Los Angeles (UCLA), began his discoveries while working a summer job at the Jet Propulsion Laboratory (JPL) in Pasadena, California. He calculated that a flyby spacecraft which passed behind a planet as it orbited the Sun would in effect be towed by the planet’s gravity, increasing its speed. As the spacecraft departed the planet’s vicinity, it would keep that speed. Conversely, a flyby spacecraft that passed ahead of a planet would be slowed. Minovitch viewed this as a form of propulsion; he called the effect the planet had on the spacecraft “gravity thrust.”
Minovitch determined that a spacecraft could use gravity thrust flybys to travel from world to world indefinitely without use of rocket propulsion. It could even return to the vicinity of Earth, enter a very close solar orbit, or escape the Solar System entirely. In all, he calculated about 200 different planetary-flyby sequences using charts he devised and computers at JPL and UCLA.
Many engineers who learned of Minovitch’s results assumed at first that they violated fundamental physical law. It seemed that the spacecraft would get something for nothing. This was, of course, incorrect: when the spacecraft was slowed, the planet gained a very tiny amount of momentum; when the spacecraft was accelerated, the planet lost a very tiny amount of momentum. Nature thus balanced its books. Minovitch, for his part, was not at first skilled at explaining his discoveries; he seems to have understood the clean elegance of numbers better than he did the fuzzy vagaries of human beings.
Nevertheless, he had his champions. The most important was Maxwell Hunter, who met Minovitch at the American Astronautical Society’s Symposium on the Exploration of Mars (6-7 June 1963). Before joining the professional staff of the National Aeronautics and Space Council (NASC) in January 1962, Hunter had worked at Douglas Aircraft for 18 years. He ended his career there as Chief Engineer for Space Systems. As part of the NASC, he was well placed to promote Minovitch’s discoveries; the advisory body, chaired by Vice President Lyndon Johnson, provided advice directly to President John F. Kennedy.
Hunter described Minovitch’s “unconventional trajectories” in a report to NASC Executive Secretary Edward Welsh in September 1963. The report became the basis for a prominent article in the May 1964 issue of the important trade publication Astronautics & Aeronautics. Hunter permitted Minovitch to review a draft before the article went to publication.
In June 1964, JPL began planning what became Mariner Venus/Mercury 1973 (MVM 73), the first planetary mission to use one of the trajectories Minovitch had calculated. The MVM 73 spacecraft would fly past Venus to slow down and enter a Sun-centered orbit that would take it past Mercury. The flight past Venus was labelled a “gravity-assist flyby” – Minovitch’s “gravity thrust” moniker never caught on.
At nearly the same time, high-energy propulsion systems, which had been deemed essential for travel to worlds beyond Venus and Mars, rapidly lost support. As described in my last post, the leader among these systems was electric (ion) propulsion.
In 1962, JPL engineers had prepared a preliminary design for an automated 10-ton nuclear-electric “space cruiser” and proudly presented it at a conference attended by about 500 other electric-propulsion engineers. It was received with great enthusiasm. The system was still early in its development, but the JPL engineers hoped that, with sufficient funding, they might develop it for interplanetary spaceflights in the 1970s.
By late 1964, however, such brute-force high-energy systems were increasingly seen as needlessly complex and costly (at least for the preliminary reconnaissance of the Solar System). NASA could instead use a relatively small booster rocket to place on an interplanetary trajectory a package comprising a small chemical-propellant propulsion system for course corrections, star-trackers for precise spacecraft position and trajectory determination, science instruments, a computer, an electricity-generating isotopic system or solar arrays, and a radio. By 1962 standards, such a package hardly qualified as a spacecraft, yet it remains the basic form of our proudest interplanetary flyby and orbiter spacecraft to this day.
Electric propulsion supporters were loathe to give up their labors. In addition to developing small station-keeping electric-propulsion systems for Earth-orbiting satellites, they sought planetary exploration niches where electric propulsion could outshine gravity-assist. Ironically, given the adventures of Lexell’s Comet, the most significant niche they identified was comet rendezvous. Before the end of the 1960s, the 1985-1986 Comet Halley apparition became a particularly important target for electric propulsion supporters. Their efforts to explore Comet Halley using electric propulsion will be described in subsequent Beyond Apollo posts.
In the years that followed Mariner 10 (as MVM 73 came to be known), more of Minovitch’s gravity-assist trajectories were put to use. Though often attributed to JPL’s Gary Flandro, among Minovitch’s trajectories was the basic Jupiter-Saturn-Uranus-Neptune path of Voyager 2. This sequence of flybys has been touted as a once-in-176-years opportunity to visit all the outer Solar System planets during a single mission; Minovitch, however, has been quick to point out that this claim is spurious. Jupiter and the rest of the outer planets are each massive enough that their gravity is capable of bending a spacecraft’s path and accelerating it toward any point in the Solar System at any time.
Voyager 2, with a mass at launch of about 1600 pounds, left Earth on 20 August 1977 atop a Titan IIIE rocket. It flew within 350,000 miles of Jupiter’s trailing side on 9 July 1979; within 63,000 miles of Saturn on 25 August 1981; within 51,000 miles of Uranus on 24 January 1986; and within 3100 miles of Neptune on 25 August 1989. In all, its primary mission spanned just over 12 years. It then began its “Interstellar Mission,” which continues to this day. At this writing, Voyager 2 is more than 12 billion miles from the Sun; unless humans catch up with it and reverently bring it home, it will in centuries to come depart the Solar System entirely and wander among the stars.
Minovitch calculated Venus-Earth gravity-assist trajectories; these came in handy beginning with the loss of the Space Shuttle Challenger (28 January 1986) and subsequent cancellation of the Shuttle-launched Centaur-G’ upper stage. The accident and stage cancellation grounded the Galileo Jupiter orbiter and probe mission, which had been set to launch to Earth orbit in May 1986 in a Space Shuttle payload bay then boost directly to Jupiter on a Centaur-G’.
The Space Shuttle resumed flights in September 1988. Galileo was launched in the payload bay of Space Shuttle Atlantis (18 October 1989) and boosted from Earth orbit using a solid-propellant Inertial Upper Stage that was incapable of sending it directly to Jupiter. Instead, Galileo flew by Venus (10 February 1990), Earth (8 December 1990), and Earth again (8 December 1992) before it built up enough speed to begin the trek to Jupiter. Galileo reached Jupiter on 7 December 1995. Over the course of 35 Jupiter-centered orbits, it explored the four largest Jovian moons using gravity-assist flybys to speed up and slow down. A final gravity-assist series among Jupiter’s moons caused it to orbit 16 million miles out from Jupiter and then perform a pre-planned death-dive into its atmosphere on 21 September 2003.
Current operational missions that used or will use gravity-assist flybys include (in no particular order) Voyager 1 (which flew by Jupiter and Saturn), the Cassini Saturn orbiter (which carried out a Venus-Venus-Earth-Jupiter sequence of gravity-assist flybys), the MESSENGER Mercury orbiter (Earth-Venus-Venus-Mercury-Mercury-Mercury), the Rosetta comet rendezvous spacecraft and Philae lander (Earth-Mars-Earth-Earth), the Juno Jupiter orbiter (Earth), and the New Horizons Pluto flyby spacecraft (Jupiter). Even the Dawn Vesta/Ceres mission, which relies on solar-electric propulsion, used a gravity-assist Mars flyby on 4 February 2009 to gain speed and reach the Asteroid Belt between Mars and Jupiter.
Previous Installments in the Challenge of the Planets Series
References
“Gravity Propulsion Research at UCLA and JPL, 1962-1964,” R. Dowling, W. Kosmann, M. Minovitch, and R. Ridenoure, History of Rocketry and Astronautics, AAS History Series Volume 20, J. Hunley, Editor, 1997, pp. 27-106.
Comets: A Chronological History of Observation, Science, Myth, and Folklore, D. Yeomans, John Wiley & Sons, New York, 1991.
The Voyager Neptune Travel Guide, C. Kohlhase, editor, NASA JPL, June 1989, pp. 103-106.
“Fast Reconnaissance Missions to the Outer Solar System Utilizing Energy Derived from the Gravitational Field of Jupiter, G. Flandro, Astronautica Acta, Volume 12, Number 4, 1966, pp. 329-337.
Utilizing Large Planetary Perturbations for the Design of Deep Space, Solar Probe, and Out-Of-Ecliptic Trajectories, JPL Technical Report No. 32-849, M. Minovitch, December 1965.
“Future Unmanned Exploration of the Solar System,” M. Hunter, Astronautics & Aeronautics, May 1964, pp. 16-26.
Determination and Characteristics of Ballistic Interplanetary Trajectories Under the Influence of Multiple Planetary Attractions, JPL Technical Report No. 32-464, M. Minovitch, October 1963.
Future Unmanned Exploration of the Solar System, M. Hunter, report to Executive Secretary, National Aeronautics & Space Council, September 1963.
Related Beyond Apollo Posts
Beyond Cassini: Saturn Ring Observer (2006) – http://more-deals.info/2012/03/beyond-cassini-sro-2006/%3C/a%3E%3C/p%3E%3Cp class="paywall">New Horizons II (2004-2005) – http://more-deals.info/2012/05/new-horizons-ii-2004-2005/%3C/a%3E%3C/p%3E%3Cp class="paywall">Galileo-Style Uranus Tour (2003) – http://more-deals.info/2012/05/galileo-style-uranus-tour-2003/%3C/a%3E%3C/p%3E%3Cp class="paywall">Cometary Explorer (1973) – http://more-deals.info/2013/05/cometary-explorer-1973/%3C/a%3E%3C/p%3E%3Cp class="paywall">Blueprint for 1970s Planetary Exploration (1968) – http://more-deals.info/2012/04/blueprint-for-1970s-planetary-exploration-1968/%3C/a%3E%3C/p%3E%3Cp class="paywall">Beyond Apollo chronicles space history through missions and programs that didn’t happen. It is a space history blog, not a blog devoted to current space policy. Comments are encouraged. Off-topic comments might be deleted.
