The Missions to Comet Halley

Back when I was fresh out of college in the mid-1980s, the buzz in the astronomical community centered on the return of Comet Halley to the inner solar system for the first time since 1910 when my grandparents were just children. Comet Halley, officially designated 1P/Halley by astronomers, is undoubtedly the most famous of all the periodic comets. First observed in ancient times perhaps as early as 466 BC, its 76-year periodicity was first recognized by English astronomer Edmond Halley in 1705. The observation of its subsequent return to the inner solar system in 1759 was considered a triumph of the predictive power of the Newtonian laws of gravitation. With its return to the inner solar system in early 1986 (its first appearance since the beginning of the Space Age), Comet Halley was a tempting target for spacecraft exploration by the spacefaring nations of the Earth.

 

American Mission Proposals

The first serious studies into the requirements for a mission to Comet Halley were begun in the late 1960s. It was quickly recognized that there would be difficulties reaching this famous target. Its 76-year orbit is highly eccentric, coming as close as 0.59 AU (an AU or astronomical unit being the average distance of the Earth from the Sun) at perihelion and swinging out as far as 35.1 AU at aphelion. Unlike all of the planets in our solar system, the orbit of Comet Halley is steeply inclined to the plane of Earth’s orbit known as the ecliptic. At an inclination of 162°, this comet actually travels backwards relative to the planets of our solar system. Launching a spacecraft into a highly eccentric, retrograde orbit to match that of Comet Halley would require enormous amounts of energy far beyond what is practical with conventional chemical propulsion.

One solution to this problem was to intercept Comet Halley as it passed through the ecliptic where the launch energy requirements would be more modest. As it approached perihelion, Comet Halley reached its ascending node (where the comet passed up through the plane of Earth’s orbit or the ecliptic) on November 8, 1985 at a distance of 1.8 AU from the Sun – beyond the orbit of Mars in the asteroid belt. Low energy launch windows for a simple ballistic path from Earth to this point occurred in February and July of 1985. After passing perihelion on February 9, 1986, Comet Halley reached its descending node (where it passed back down through the ecliptic plane) on March 10 at a distance of 0.85 AU from the Sun. Low energy launch windows to this point occurred in July and August of 1985.

While the energy requirements for a descending node encounter with Comet Halley were slightly less owing to the fact that the spacecraft would be placed into a heliocentric orbit not too unlike Earth’s, the relative velocity of encounters at either point would be in excess of 60 kilometers per second. Even though there were higher energy trajectories out of the ecliptic plane that could decrease the encounter speeds somewhat, it was the general consensus of the American science community that first-rate science could not be performed at such high velocities. Some sort of low-speed rendezvous was considered to be the only viable option given what the assumed state of space technology and interplanetary navigation would be in the 1980s.

Undeterred, American engineers and scientists began to look into alternatives to the simple ballistic encounters. One family of trajectories examined involved using the powerful gravitational field of Jupiter for a gravitational assist. In one proposal, a Saturn V topped with a Centaur as a fourth stage could launch a probe in 1977 or 1978 for a fast, one-year trip to Jupiter where the spacecraft would be flung high out of the ecliptic plane into a retrograde orbit. The spacecraft would then encounter Comet Halley five to eight months before perihelion at a relative velocity of several kilometers per second. Unfortunately the Saturn V was very expensive and its availability uncertain (in fact, the Saturn V would be retired in 1973). The trip time of over seven years was also considered to be far too long given the state of the technology at the time. It was for this same reason that NASA’s decade-long Grand Tour proposal was downsized to the much shorter, four-year mission of Voyager to Jupiter and Saturn (see “Voyager 2: The First Uranus Flyby”).

Another option involved using a probe fitted with ion thrusters. Much more efficient than conventional chemical-based systems, ion propulsion could provide the needed velocity change to rendezvous with Comet Halley. One proposal examined called for the spacecraft to be launched in 1978 into an elongated solar orbit away from the Sun. Solar-powered ion thrusters would then gradually slow the speed of the receding spacecraft and, after almost four years, reverse its direction of travel around the Sun while it was far beyond the orbit of Jupiter. As the spacecraft fell back towards the inner solar system, the ion thrusters would continue to alter the orbit resulting in a low-speed rendezvous with Comet Halley a couple of months before perihelion. But once again the seven-year trip time was considered far too long. Even substituting a nuclear reactor as the power source (which entailed its own development issues) would cut only three years from the flight time. An even more efficient method of propulsion was required to cut down the total mission length.

In the mid-1970s, mission planners at the Jet Propulsion Laboratory (JPL) became aware of studies performed under a NASA contract by an engineer at the Battelle Memorial Institute named Jerome Wright. His work showed it was possible to employ a solar sail, which uses the pressure from sunlight for propulsion, to perform a low speed rendezvous with Comet Halley. By 1976 JPL started an in depth study of the proposal. The initial design called for a solar sail about 800 meters on a side supported by four diagonal cross beams carrying a payload of 800 kilograms. Launched by the Space Shuttle around 1981 and potentially assembled with the assistance of astronauts in Earth orbit, the solar sail would spend its first 250 days of flight slowly spiraling closer to the Sun. Using the enhanced light pressure from its closer 60-day solar orbit, the light sail would be used to pump up the inclination of the probe’s orbit over the course of the next nine months until it matched that of Halley. After further maneuvers to shape the probe’s orbit, a low speed rendezvous with the comet could then take place in early 1986.

Because of the unknowns associated with deploying such a large structure in space and the questionable availability of the Space Shuttle, in early 1977 the square solar sail design was replaced by the heliogyro which had been invented by Richard MacNeal and John Hedgepath a decade earlier. Consisting of a set of 12-kilometer-long rectangular sails arranged like the blades of a helicopter, the blades of this solar sail concept did not require a rigid structure but used the centrifugal force from slowly spinning to maintain rigidity. And since the individual blades would be deployed using the centrifugal force to unreel them from a set of storage drums, the heliogyro concept would be easier to set up in space with no assistance from an astronaut. But even this innovative concept had too many unknowns and it was doubtful it could be developed in time for a launch in 1981. In September 1977 NASA officially abandoned the solar sail concept in favor of solar-electric ion propulsion.

It quickly became clear that the estimated $500 million price tag for this project (equivalent to almost $2 billion today) was simply too large given the pressures on NASA’s budget not the least of which included the increasingly delayed and expensive Space Shuttle program. By early 1978 NASA began to examine other options. One way to meet the science objectives of a Halley mission was to rendezvous with a short-period comet that would be easier to reach. One proposal that gained favor was a rendezvous with Comet 10P/Tempel 2 (not to be confused with the periodic comet 9P/Tempel visited by Deep Impact in 2005 and the NExT mission in 2010) using solar-electric propulsion.

The proposed 2,700-kilogram spacecraft would be launched by the Space Shuttle in late July 1985 and sent into interplanetary space using the all-solid IUS upper stage. Once on its way, the probe would deploy its huge solar arrays and start thrusting using a set of six mercury-fueled ion engines. Around November of 1985, the probe would make a distant encounter with Comet Halley when it was 1.53 AU from the Sun. Ten days before closest approach, a small probe supplied by ESA (European Space Agency) would be deployed to make a closer inspection of Comet Halley. With a mass of 150 to 250 kilograms, the spin-stabilized European probe would have been based on the successful ISEE 2 spacecraft design and would pass only 1,500 kilometers from Halley’s nucleus while the American mothership flew by at a safer distance of 130,000 kilometers. The American spacecraft would then continue its flight and rendezvous with Comet Temple-2 in July of 1988.

In the end, even this mission was never funded because of its high costs and the need to fund other planetary missions with higher priority like Jupiter Orbiter-Probe mission later known as Galileo. While less expensive options for a fast flyby of Comet Halley like HIM (Halley Intercept Mission) which would have employed Voyager technology and, later, other comets would be studied in the years and decades to follow, by 1980 it was clear that the United States — the undisputed leader in planetary exploration for almost two decades — would not launch a dedicated mission to Comet Halley. The consensus of the American scientific community was that a more affordable fast flyby was unacceptable and scientifically inadequate. While it was not realized at the time, the infamous hiatus in American planetary missions had already started because of overly ambitious goals at a time of shrinking budgets (see “The Future That Never Came: Planetary Missions of the 1980s“). Fortunately, other groups of scientists around the world did not share America’s somewhat myopic view.

 

Plans for International Missions

After having their piggyback probe to Halley stranded after NASA’s cancellation of the Tempel-2 rendezvous mission, ESA decided to go it alone and launch their first deep space mission to Comet Halley. The Giotto mission, named after the Italian Renaissance artist Giotto di Bondone who included Halley as the star of Bethlehem in his 1304 painting “Adoration of the Magi”, was officially approved by ESA’s science committee on July 8, 1980, despite the criticism of the French. The spin-stabilized design of Giotto was based on the successful GEOS research satellites built by British Aerospace first launched in 1977. Because Giotto would penetrate deep into the dusty coma of Comet Halley, a major modification included the addition of a two-layer Whipple shield to the base of the probe to help protect it from cometary dust particles as large as one gram during its 68 kilometer-per-second flyby. With a mass of 573.7 kilograms at the time of its encounter, Giotto carried ten instruments with a total mass of 60 kilograms to study Comet Halley and its environment including a multicolor camera to provide high-resolution images of the comet nucleus. Tracking of the fast moving nucleus was accomplished using specially designed control software to keep the comet in the camera’s field of view. All data would be transmitted live because it was not expected that Giotto would survive its pass 500 to 1,000 kilometers from Halley’s nucleus.

Originally Giotto was to be launched into a geosynchronous transfer orbit by an Ariane 3 rocket with another commercial payload riding along, and use a MAGE 1S solid rocket motor built into the spacecraft to send it into deep space. However, it proved to be impossible to find a commercial payload that would be available during the limited July 1985 launch window. Eventually Giotto was shifted to an Ariane 1 rocket as its sole payload. Even though the Ariane 1 now had ample power to launch Giotto directly into the required solar orbit without the MAGE 1S kick motor, the original launch profile and solid motor were retained due to the advanced state of development and the lack of sufficient time to make the required changes.

The Giotto mission would not be the only one to Comet Halley. During the 1970s the Soviet Union had mounted a very successful series of missions to the planet Venus and were studying a joint mission with France designated Venera 84. Originally meant to deliver a pair of French-supplied balloons into the Venusian atmosphere, Soviet scientists were hoping to observe Comet Halley using their Venera orbiters just as the Americans planned to do with their Pioneer Venus 2 orbiter, which arrived at Venus in late 1978. As luck would have it, Comet Halley would pass only 40 million kilometers from Venus and provide a better vantage point than Earth for observation.

During the early 1980s an even more daring mission was developed. Instead of the Venera orbiters observing Comet Halley from afar, it was found that it was possible for them to flyby Venus after dropping off their payloads and continue on to encounter Comet Halley in early March 1986. As a result of the change in mission, the French balloon payload had to be downsized and the Veneras would no longer be placed into orbit to support them. In the end the French decided to walk away from the balloon program, leaving the Soviets to build their own balloon payload instead.

In April 1982 the Soviet Union publically announced their plans to send a pair of modified Venera spacecraft, designated 5VK, to Comet Halley which they named VEGA (the Russian acronym for “Venera-Halley” where the “H”, which does not exist in the Cyrillic alphabet, is usually transliterated as a “G”). With a mission length of about 15 months, it would be the longest interplanetary mission attempted by the Soviet Union. In addition to the Venera lander and smaller balloon payload, the pair of VEGAs would carry an impressive 240 kilograms of instruments including those supplied by 13 other countries from around the world.

The 5VK spacecraft, with a mass of 4,920 kilograms, were modified versions of the very successful second-generation Venera built by NPO Lavochkin (see “Venera 9 and 10 to Venus“). In addition to shielding to protect the spacecraft from dust impacts, the 5VK also sported larger solar panels and carried an increased load of propellants as well as attitude control gas. Another major addition to the 5VK was a Czech-built pointable scan platform that carried, along with other optical instruments, a CCD-based television system jointly developed by the Soviet Union, Hungary and France. A tracking system would allow the platform to stay pointed at the comet despite the high encounter velocity and the uncertain position of the nucleus relative to the spacecraft. There would be an unprecedented level of international cooperation on this mission. Given the uncertainty of the position of Halley’s nucleus, the VEGA spacecraft would serve as pathfinders for ESA’s Giotto which was suppose to fly much closer to the nucleus.

While the Soviet VEGA spacecraft were the largest sent to Comet Halley and the European Giotto mission would pass the closest, very early on Japan decided that they also could attempt a much more modest but still scientifically useful mission to Comet Halley as well. During the 1970s Japanese scientists and engineers at ISAS (Institute of Space and Aeronautical Science—one of the precursors of the Japanese space agency JAXA established in 2003) began studies for a Halley probe launched using their Mu-3S all-solid, three-stage launch vehicle that was capable of orbiting a 300-kilogram payload. In 1979 the Japanese Halley mission was approved with six years to complete the project.

Early on it was decided to launch two spacecraft: the Planet-A probe that would make the close pass of the comet and the MS-T5 (Mu Satellite – Test 5) technology demonstrator launched seven months earlier.  The MS-T5 would test the modified launch vehicle and the probe design as well as allow distant observations of the interplanetary environment upstream of the comet itself. The launch vehicle was an upgraded Mu-3SII rocket that used a pair of strap-on boosters to increase the payload to low Earth orbit to 770 kilograms or send up to 150 kilograms on a direct ascent escape trajectory.

The two spin-stabilized spacecraft, built by Nippon Electronics Corporation, were nearly identical and capable of carrying 12 kilograms of instruments. MS-T5 had a launch mass of 138.1 kilograms and carried three instruments to characterize the solar wind. It was intended that MS-T5 would pass 5 million kilometers upstream of Comet Halley near the time of its sister probe’s encounter. Planet A had a launch mass of 139.5 kilograms and carried an electrostatic analyzer to study charged particles and a UV imager based on an instrument flown earlier on the Kyokko (originally known as the Exos-A) mission launched in 1978. Because Planet-A carried no dust shield in order to save mass, it was planned to fly 200,000 kilometers from the nucleus where less dust was expected. Because of various launch constraints including those imposed by the powerful Japanese fishing lobby, Planet-A would be launched in August 1985 to make its closet approach near Halley’s descending node, like the ESA and Soviet spacecraft.

 

The Missions

The first spacecraft launched to Comet Halley were the Soviet Vega probes. Vega 1 and 2 were launched from the Baikonour Cosmodrome in Soviet Kazakhstan at the end of 1984 using a pair of Proton 8K82K rockets on December 15 and 21, respectively. Vega 1 reached Venus first on June 11, 1985. As Vega 1 passed 39,000 kilometers from Venus, the lander successfully deployed the Soviet balloon payload in the Venusian atmosphere on its way to a landing on Raskala Planitia near local midnight. Four days later Vega 2 passed 24,500 kilometers from Venus while its lander also successfully deployed a balloon as it descended to a nighttime landing 1,500 kilometers southeast of Vega 1 in Aphrodite Terra. With their missions at Venus successfully completed, the two Vega spacecraft were on their way to encounter Comet Halley in nine months.

Next out of the gate was the Japanese MS-T5 mission launched from the Kagoshima Space Center on January 7, 1985. After it was successfully launched on a direct-ascent trajectory into interplanetary space, MS-T5 was renamed Sakigake (“Pioneer” in Japanese). The launch had been delayed for three days because of ground equipment problems. As a result, the miss distance from Comet Halley had been increased by almost 3 million kilometers. Course corrections performed on January 10 and February 14 decreased the miss distance to about 7 million kilometers. After a series of engineering tests, all of Sakigake’s instruments were turned on by the end of February 1985. The success of this technology demonstrator paved the way for the launch of Planet-A later that summer.

The next mission off the pad was the European Giotto mission. It was launched from Kourou in French Guiana on July 2, 1985 on Flight V14 of the Ariane. After spending 32 hours in a 198.5 by 32,000 kilometer parking orbit inclined 7 degrees to the equator, Giotto’s MAGE 1S kick motor ignited for a 55-second burn to send the probe into solar orbit. The first course correction was made on August 26 to move Gitto’s initial aim point to within 4,000 kilometers of Halley’s nucleus.

The last dedicated mission to be sent to Comet Halley was the Japanese Planet-A probe. It was successfully launched on August 18, 1985 and subsequently renamed Suisei (Japanese for “Comet”). The launch of the Mu-3SII proved to be so accurate with a miss distance of only 210,000 kilometers that a course correction planned for August 22 was cancelled. A course correction on November 14 moved Suisei’s aim point to about 151,000 kilometers on the sunward side of Halley’s nucleus.

The first of the international armada to reach Comet Halley was the Soviet’s Vega 1. It passed 8,890 kilometers from Halley’s nucleus at a relative velocity of 79.2 kilometers per second at 7:20:06 UT on March 6, 1986. Near closest approach Vega 1 was pummeled by up to 4,000 dust particles each second as it returned ghostly images of the 15-kilometer-long peanut-shaped nucleus. Vega 1 survived the dangerous encounter and successfully transmitted about 800 images and other data, but two instruments had been disabled and the output from the unprotected solar arrays was cut by 55%.

Next up was the Japanese Suisei, which had been observing Comet Halley with its UV imager since mid-November 1985. It passed at a much safer range of 151,000 kilometers at 13:06 UT on March 8 where it secured useful data on the properties of the comet’s extended cloud of hydrogen. Just 18 hours later, Vega 2 made its dangerous plunge towards the nucleus. Its path through Halley’s coma afforded a less obscured view of the nucleus compared to Vega 1. Although the main processor controlling the scan platform failed 32 minutes before closet approach (forcing a switch to a less capable backup system), Vega 2 survived its 76.8 kilometer-per-second encounter with Halley at 7:20:00 UT on March 9 at a range of just 8,030 kilometers. Vega 2 had several instruments lost or partially disabled during the encounter and lost 80% of the power from the solar panels, although this was later revised to only a 50% loss. In total the two Vega spacecraft returned 1,500 images and a mountain of other data on Comet Halley.

After Sakigake made its distant 6.99-million-kilometer pass by Comet Halley at 4:18 UT on March 11, the last spacecraft in the international armada was Giotto. Data from the Soviet Vega probes had pinned down the position of Halley’s nucleus to within 75 kilometers at a 99.7% confidence level—a 20-fold improvement over what was provided by Earth-based observations alone. With such an accurate fix, on March 11 Giotto project scientist decide to attempt a 500-kilometer pass by the nucleus and performed a final course correction. Giotto made its closest approach on March 14 at 00:03:02 UT at a range of 605 kilometers. It returned 2,112 images of the comet and provided the clearest views we have of Halley’s nucleus. More images would have been returned except that a hard hit by a large dust particle just 16 seconds before closest approach knocked the spinning Giotto’s antenna out of alignment with the Earth. While full contact with the probe was restored 32 minutes later after the impact-induced wobble was dampened, several instruments were damaged including the camera, whose baffle was severely mauled, rendering it unusable.

Despite the dangers, all the members of the Halley armada had survived their encounter with the famous comet. Contrary to the earlier expectations of the American scientific community, the data returned from these missions was very useful in beginning to unravel the properties of comets. Despite there being no dedicated American mission, the United States did make some contributions. The US provided much needed tacking coverage using NASA’s Deep Space Network. Individual scientists participated in international science teams as well as provided instruments for Giotto and Vega. In addition to observations of Comet Halley made by Pioneer Venus Orbiter in early February 1986 at a range of 40 million kilometers, two other American spacecraft made distant observations of the interplanetary environment upstream of the comet. Pioneer 7, launched into solar orbit in 1966 to monitor the solar wind, passed 12.3 million kilometers from Halley on March 20 by sheer luck. The ICE spacecraft, which encountered the short-period comet 21P/Giacobini-Zinner on September 11, 1985 for the first ever comet encounter of a spacecraft, passed 31 million kilometers from Halley on March 28 (see “ICE: The First Comet Flyby”).

 

Subsequent encounters

After the encounters with Comet Halley, further plans were considered for the various spacecraft of the armada. Soviet scientists considered redirecting Vega 2 for a distant 6-million-kilometer pass by the near Earth asteroid 2101 Adonis in 1987. Unfortunately, contact with Vega 2 was lost on March 24, 1987. Contact with Vega 1 had been lost two months earlier when it ran out of attitude control gas on January 30. Despite this, VEGA would prove to be the most successful interplanetary mission ever mounted by the Soviet Union.

Japanese scientists also had high hopes for their Sakigake and Suisei probes. There were plans to redirect Sakigake towards an encounter with Comet Giacobini-Zinner in 1998, but there proved to be insufficient propellant for that mission. Contact was maintained until November 15, 1995, although its signal beacon continued to be received until January 7, 1999. Scientists also hoped to guide Suisei towards a distant encounter with Comet 55P/Tempel-Tuttle on February 28, 1998, followed by a close flyby of Comet Giacobini-Zinner on November 24, 1998. Suisei changed course in early April 1987 for a planned Earth gravity assist at a range of 60,000 kilometers on August 20, 1992. Unfortunately Suisei’s hydrazine propellant was depleted on February 22, 1991, ending hopes for further encounters.

By far ESA’s Giotto had the most productive life after its encounter with Comet Halley. Giotto adjusted its course to flyby the Earth at a range of 16,300 kilometers on July 2, 1990, and subsequently passed about 200 kilometers from Comet 26P/Grigg-Skjellerup on July 10, 1992. Despite the loss of its camera, Giotto returned important data on this comet. Giotto was placed into hibernation 13 days later, ending spacecraft operations. There was insufficient propellant left for any extensive maneuvers and Giotto was not reactivated when it flew past the Earth at a range of 219,000 kilometers on July 1, 1999. With this, the last of the Halley armada had ended its mission.

 

Follow Drew Ex Machina on Facebook.

 

Related Video

Here is a brief video produced by ESA showing Giotto’s approach to the nucleus of Comet Halley using the probe’s actual images.

 

 

Here is another (much longer) video looking back at the Giotto mission entitled “Giotto: Encounter with Halley”.

 

 

Related Reading

“ICE: The First Comet Flyby”, Drew Ex Machina, September 11, 2015 [Post]

“The Fate of Comet Halley”, Drew Ex Machina, October 2, 2014 [Post]

“The Future That Never Came: Planetary Missions of the 1980s – II”, Drew Ex Machina, December 1, 2014 [Post]

 

General References

J. Kelly Beatty, “The High Road to Halley”, Sky & Telescope, Vol. 71, No. 3, pp 244–245, March 1986

R.M. Bonnet, “History of the Giotto Mission”, Space Chronicle: JBIS, Vol. 55, Suppl. 1, pp 5–11, 2002

Louis Friedman, Starsailing: Solar Sails and Interstellar Travel, John Wiley & Sons, 1988

Brian Harvey, Russian Planetary Exploration: History, Development, Legacy and Prospects, Springer-Praxis, 2007

Jeffrey M. Lenorovitz, “Giotto Redirected to Fly Past Earth After Returning Data on Halley’s”, Aviation Week & Space Technology, Vol. 124, No. 12, pp 22–23, March 24, 1986

Yasunori Matogawa, “Giotto: Historical Encounters for Japan”, Space Chronicle: JBIS, Vol. 55, Suppl. 1, pp 31–33, 2002

Robert M. Powers, Planetary Encounters: The Future of Unmanned Spaceflight (Revised), Warner Books, 1979

Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System Part 2: Hiatus and Renewal 1983–1996, Springer-Praxis, 2009

Andrew Wilson, Solar System Log, Jane’s Publishing, 1987

“New Vega Flyby”, Aviation Week & Space Technology, Vol. 124, No. 12, p 22, March 24, 1986