Drifting through the Oort Cloud, in the distant reaches of our Solar System, exist icy relics we call comets. Occasionally, from the gravitational disturbance of a planet or nearby star, a comet will fall from this lonely dark region towards the Sun.
Originally published in the Canadian Space Gazette, Spring 1998
When its path takes it within the orbit of Mars, sunlight is sufficiently intense to sublimate frozen material on this massive dirty snow ball. Gas and dust then rise off the comet and form the coma which is blown away from the nucleus by the solar wind. This is the brilliant stage of a comet's journey we can see from Earth.
The astronauts that manned the Skylab Space Station during 1973 and 1974 conducted some of the earliest spaceborne observations of comets. With the Apollo solar telescope, the astronauts discovered many comets during their closest approaches to the Sun. They even found some that plunged into the Sun and others that were severely fragmented from a near miss. The next craft to study comets was part of an ad hoc mission of opportunity. The International Sun-Earth Explorer 3 (ISEE 3), after completion of its joint NASA ESA primary mission, was put through a series of maneuvers that ejected it from the Earth-Moon system on a trajectory to intersect comet Giacobini-Zinner. At this point the robotic craft was renamed International Cometary Explorer (ICE). On September 11, 1985, ICE passed through the plasma tail of Comet Giacobini-Zinner and after assisting in the International Halley Watch, ICE became the first space probe to investigate two comets.
The International Halley Watch was a global program employing spacecraft from the European Space Agency's (ESA) member countries, Japan, the Soviet Union and the United States. The International Halley Watch seized a rare opportunity to shadow Halley's comet during the brief part of its 76 year orbit when it is nearest to the Sun.
The Soviet Union contributed the twin Vega probes. Launched on December 15 and 21, 1984, they flew an extensive mission past Venus and on to the great comet. On approach to Venus, Vega 1 and 2 each released a lander and a package that inflated a balloon upon reaching the venusian atmosphere. The landers studied the atmosphere during descent and the crust upon impact with the surface. 50 km above, in the middle of Venus's turbulent three-tiered cloud system, the two balloons drifted with their suite of scientific equipment. Even higher above, the twin Vega spacecraft raced by. After using the planet's gravity to alter their trajectory, Vega 1 and 2 flew through the coma of Halley's Comet on March 6 and 9, 1986, respectively.
From the fledgling space program of Japan came Sakigake and Suisei. The University of Tokyo's Institute of Space and Aeronautical Science (ISAS) ran these early missions. The Suisei probe, also know as Planet-A, brought a UV imager and an instrument to measure the solar wind. Its closest encounter was at 151,000 km during March 8, 1986 on the sunward side of the comet. Sakigake investigated plasma features and interplanetary magnetic fields. It encountered the comet at a distance of 7 million km on March 11, 1986 also on the sunward side.
The European Space Agency's addition to the fleet was Giotto. Following its launch on July 2, 1985, the interplanetary spacecraft shot by Halley's nucleus on March 13, 1986 at a distance of just 596 km. During its pass through the thick of the comet, the Kevlar armored craft was pelted by high speed particles destroying its camera and silencing its radio for 20 minutes. It limped out to return valuable data to Earth including photographs of its approach to the nucleus and information on the composition of the coma. After a period of intentional hibernation, the craft flew by Comet Grigg-Skjellerup on July 10, 1992.
Ever since the first launch of NASA's Space Shuttle, astronauts have used the orbiter as a platform for observing comets. Most recently, astronauts followed the progress of comets Hyakutake and Hale-Bopp from Earth orbit. Similar research was done aboard the Soviet Salyut series of orbital labs and today cosmonauts occasionally study comets from on board the Mir Space Station. During the Great Comet Crash of 1994, fragments of Comet Shoemaker-Levy 9 plunged into Jupiter's upper atmosphere and exploded. Spaceborne witnesses to this exceptional incident were the Galileo spacecraft and Hubble Space Telescope. The Galileo spacecraft watched as its destination was scarred by the dead comet. The probe imaged explosions erupting out into space after the fragments penetrated the gas giant. The Hubble Space Telescope documented not only the disaster as it unfolded but it later recorded evidence of the impacts disappearing into the stormy Jovian atmosphere. Another joint NASA ESA mission, the Solar & Heliospheric Observatory (SOHO), plays a major role in detecting comets. SOHO constantly observes the Sun from its halo orbit around Lagrangian point L1, a balancing point between the gravitational tugs of the Earth and the Sun. Launched on December 2, 1995, this observatory, like the astronauts on Skylab, reveals comets as they swing close to the Sun.
In 1998, the first of a series of missions will launch to take us right to the comets themselves again. Not only will these missions take us through a comet as in the International Halley Watch, but some will return dust from the coma to Earth and others will take us right onto the surface of a comet's nucleus.
First in line for launch is the Deep Space 1 (DS1) mission. DS1, the first craft from NASA's New Millennium Program (NMP), will launch on a Delta II rocket from Cape Canaveral on July 1, 1998. The NMP "focuses on testing high-risk, advanced technologies in space with low-cost, rapid-development flights." DS1's tour includes a pass by Asteroid 3352 McAuliffe in January, 1999, Mars in April, 2000, and finally Comet West-Kohoutek-Ikemura in June of 2000. DS1 will pioneer Solar Electric Propulsion (SEP). In this form of propulsion, xenon ions are electrostatically accelerated and shot out in a beam behind the craft. The electricity for this propulsion comes from DS1's high power solar concentrator arrays which have never been tested in space before. This spacecraft will also feature autonomous navigation; DS1's computer will free up controllers on Earth by navigating the craft on its own. It will determine its position in space through triangulation of reference asteroids imaged against a background of stars. DS1 will pioneer many other technologies, in addition to those aforementioned, that space probes of the next century will depend on.
Comet Wild-2 has been marked for exploration, too. In February of 1999, the fourth mission of NASA's Discovery Program, Stardust, will leave Earth and after a completing a long looping trajectory, will arrive at Wild-2 in January, 2004. On approach to the comet, Stardust will analyze the frequency of particles striking the spacecraft, determine the composition of the particles and photograph the oncoming comet. During its relatively slow 6.1 km/s flyby, the craft will expose a container of aerogel to collect and preserve the cometary dust it plows through. During another phase of the mission, aerogel will gather interstellar dust grains. In 2006, a reentry capsule holding the aerogel will return to Earth and parachute down to Utah where waiting helicopters will snag it.
Contour, the sixth mission selected for the Discovery Program, will liftoff in June, 2002. It will conduct an extensive survey during three encounters with these icy travelers. It will first flyby Comet Encke in November, 2003, then Schwassmann-Wachmann 3 in June, 2006 and later Comet d'Arrest in August, 2008. The primary mission goal is to assess the diversity of comet nuclei. Contour will utilize a camera to image the nuclei with a resolution of 4 m per pixel, 25 times better than Giotto's survey of Halley's Comet. With a mass spectrometer and dust analyzer, Contour will scrutinize the gas and particles streaming from these comets' nuclei. This mission is designed to be flexible enough that should an unexpected comet such as Hyakutake show up, it could change course and rendezvous.
The NMP's Deep Space 4 promises to be one of the most ambitious projects of the next decade. Following its launch in 2003, the craft will match orbits and rendezvous with Comet Tempel-1 in 2005. Over the following months, it will build a detailed image and compositional map of Tempel-1. The mission planners will then choose a suitable landing site to anchor the drilling and sampling vehicle. After a careful touchdown, the lander will drill and analyze samples acquired from various depths in the nucleus. Following a few days on the surface, the 100 kg lander will scoop some material off the nucleus, stow it, and leave the comet to dock with the part of DS4 waiting in orbit. Once docked, the frozen material will be transferred to a sample return capsule and the spacecraft will arrive at Earth in 2010.
The European Space Agency will also participate in the comet assault of the next decade with their Rosetta spacecraft. It will launch on the Ariane 5 rocket in January, 2003 and pass two asteroids on the way to a rendezvous with its primary objective, Comet Wirtanen. Rosetta is by far the most massive of any comet rendezvous craft. It will depend on Mars for a gravity assist in July, 2005 and Earth for another in November, 2005. The first asteroid it will pass is 3840 Mimistrobell in September, 2006. A brief study of asteroid 2703 Rodari will follow in May of 2008. During the long periods of time between targets, Rosetta will hibernate, using a technique ESA first pioneered on Giotto. It will enter an elliptical polar orbit around the nucleus of the comet in 2011. The ROLAND lander will later be harpooned into the surface in August, 2012. Here it will inspect Wirtanen's nucleus with a entourage of instruments almost all of which are unprecedented by earlier comet probes. The mission will draw to a close in October, 2013.
Over the next fifteen years, our understanding of comets and therefore our understanding of the early history of the Solar System will increase greatly. During the early history of our planet, they intensely bombarded Earth providing us with oceans of water and rare organic compounds. By taking our investigations to the heart of the comets and then returning comet matter to Earth's laboratories, we will unveil a phenomenon that has mystified humans for thousands of years.
John P. Kavanagh is a freelance space exploration writer