NASA JPL
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 calculated their orbits who got all the credit. Comet Halley is, for example, named for Edmund Halley, who had calculated 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.
Lexell’s Comet was named for Anders Johan Lexell, who computed that the 1770 comet 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, but then had passed Jupiter at a distance of less than two million miles in 1767. It had passed ahead of the giant planet, which had 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 orbit where it was farthest from the Sun – its aphelion – it had again intersected Jupiter. This time, it had passed behind the planet, which caused it to speed up and enter an unknown but probably long-period orbit. It might have escaped the Sun’s gravitational grip entirely. In any case, it has not been seen since and is officially designated a “lost” comet.
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 orbital antics soon faded from memory. If they had not, then Michael Minovitch’s discoveries in 1961-1964 probably would not have shaken the interplanetary mission planning world as they did.
Minovitch, in 1961 a 25-year-old graduate student at the University of California Los Angeles (UCLA), began his discoveries during a summer job at the Jet Propulsion Laboratory (JPL) in Pasadena, California. He calculated that a spacecraft that passed behind a planet as it orbited the Sun would be towed by the planet’s gravity, increasing its speed. As it departed the planet’s vicinity, it would thus move faster than when it had arrived at the planet. Conversely, a spacecraft that passed ahead of a planet as it orbited the Sun would be slowed. Minovitch called the velocity change that the planet would impart to the spacecraft “gravity thrust.”
In addition, Minovitch determined that a spacecraft could use these techniques to travel from world to world indefinitely without any use of propulsion. It could even return to the vicinity of Earth if desired, 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 work assumed at first that such maneuvers violated fundamental physical law. It seemed that the spacecraft would get something for nothing. This was, of course, not correct: 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 and enjoyed the clean elegance of numbers far more than he did the 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, ending 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 eventually became the Mariner Venus/Mercury 1973 (MVM 73) mission, 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 the innermost planet. 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 assumed to be 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-powered ion drive “space cruiser” and proudly presented it at a conference attended by about 500 other electric-propulsion engineers. The system was still early in its development, but the JPL engineers hoped that, with sufficient funding, they might develop it for spaceflight by 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 roughly one-ton package comprising a small chemical-propellant propulsion system for small course corrections, star-trackers for precise spacecraft position determination, science instruments, an electricity-generating system, 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, however. In addition to developing small station-keeping electric-propulsion systems for Earth-orbiting satellites, they sought niches where ion drive could outshine gravity-assist. Ironically, given the Lexell’s Comet connection, 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 ion drive supporters. Their efforts to explore Comet Halley using ion drive 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 often has been touted as a once-in-176-years opportunity to visit all of 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 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.
The Venus-Earth gravity-assist trajectories Minovitch calculated came in handy beginning with the loss of the Space Shuttle Challenger (28 January 1986) and subsequent cancellation of the high-energy, Shuttle-launched Centaur G-prime 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 a Space Shuttle payload bay then directly to Jupiter on a Centaur G-prime in May 1986.
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. A final gravity-assist series among the 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, the Cassini Saturn orbiter, the MESSENGER Mercury orbiter, the Rosetta comet rendezvous/lander spacecraft, the Juno Jupiter orbiter, and the New Horizons Pluto flyby spacecraft. These spacecraft have performed gravity-assist flybys of Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Titan. Even the Dawn Vesta/Ceres orbiter mission, which includes solar-electric propulsion, used a gravity-assist Mars flyby on 4 February 2009 to 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://ift.tt/1sr6YQl
New Horizons II (2004-2005) –
http://ift.tt/1Eko4TT
Galileo-Style Uranus Tour (2003) –
http://ift.tt/1sr6YQn
Cometary Explorer (1973) –
http://ift.tt/1Eko6LG
Blueprint for 1970s Planetary Exploration (1968) –
http://ift.tt/1utHblU
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. It is not meant to be in any way discouraging; rather, it is intended to inform and inspire. Comments are encouraged. Off-topic comments might be deleted.
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