Image: A series of photographs of comet Hartl
6 Intimate Comet Encounters
Feb. 14, 2011 will go down in history as the Valentine's Day when a comet was visited a second time. Comet Tempel 1 has now played host to two different NASA spacecraft; Deep Impact in 2005 and Stardust-NExT in 2011. This amazing scientific feat comes hot on the heels of another cometary encounter only a few months ago. The NASA mission called EPOXI flew past comet Hartley 2 on Nov. 4, 2010 coming within 700 kilometers (435 miles) of the icy body. Both Stardust-NExT and EPOXI (formerly known as Deep Impact) are recycled comet missions and both have seen Tempel 1 up-close. EPOXI and Stardust-NExT may be the first two missions to be recycled for two comet flybys, but they certainly are not the first mission to rendezvous with these mysterious "dirty snowballs." So far, with the help of our robotic space explorers, humanity has had a close-up look at six cometary nuclei in the aim of unraveling their secrets. Let's take a look at each encounter with imagery from other space probes.
Image: Giotto's view of Halley's nucleus (ESA
Unquestionably the most famous comet in history, Halley's Comet was a prime target for space agencies in 1986 during its 75- to 76-year orbit through the inner solar system. Comet science is still a developing field, but in 1986, very little was known about the composition of these interplanetary vagabonds. In October of that year, the 15-kilometer-long Halley's Comet was visited by the European Space Agency's Giotto mission. The half-ton probe came within 600 kilometers (373 miles) of the comet's nucleus, taking the first photographs of the outgassing vapor from discrete areas of the surface producing its tail and coma (the gas surrounding the nucleus). It was this mission that confirmed the "dirty snowball" theory of cometary composition: a mix of volatile ices and dust. However, Giotto was only able to get so close to the famous comet with the help of the "Halley Armada," a number of international spacecraft all tasked with observing this rare event. Giotto captured the closest imagery, but two Russia/France probes (Vega 1 and 2) and two Japanese craft (Suisei and Sakigake) observed from afar.
Image: Comet Borrelly just before Deep Space
At roughly half the size of Halley's comet, Comet Borrelly was found to have similar attributes to its famous cousin. The nucleus was also potato-shaped and blackened. Outgassing vapor was also observed coming from cracks in the nucleus crust where volatiles were exposed to sunlight, sublimating ices into space. NASA's Deep Space 1 probe flew past the comet with a close approach of 3,417 kilometers on Sept. 22, 2001.
Image: A Stardust image of Wild 2 during its
Comet Wild 2 -- pronounced "Vilt" after its Swiss discoverer Paul Wild who spotted it in 1978 -- underwent a dramatic alteration in 1974. It is calculated that due to a close pass of Jupiter in 1974, the 5 kilometer-wide comet now orbits the sun every 6 years as opposed to its leisurely 43 years before the gas giant bullied it. The orbital modification meant that Wild 2 was an ideal target for NASA's Stardust mission to lock onto. On Jan. 4, 2004, the Stardust probe gave chase, getting so close to the comet that it was able to collect particles from Wild 2's coma. This image was taken at a distance of less than 240 kilometers (149 miles). The Stardust sample return canister came back to Earth safely, landing in Utah on Jan. 15, 2006. The microscopic particles captured from the comet continue to provide a valuable insight into the organic compounds comets contain. Interestingly, the Stardust spacecraft has been granted a mission extension (dubbed New Exploration of Tempel 1 -- NExT). In 2011 it will rendezvous with comet Tempel 1 -- the scene of NASA's 2005 Deep Impact mission -- to analyze the crater that Deep Impact's impactor left behind on the cometary surface.
Image: The view from Deep Impact's impactor b
NASA's Deep Impact mission reached the eight-kilometer-wide (five-mile-wide) comet Tempel 1 in 2005. On July 4, the probe deliberately smashed its impactor into the comet's nucleus, producing a cloud of fine material. A crater -- 100 meters wide (328 feet) by 30 meters (98 feet) deep -- was left behind. A treasure trove of compounds were spotted by the Deep Impact spacecraft and the explosion could be observed from Earth. In 2011, the recycled Stardust-NExT mission visited comet Tempel 1 for the second time.
Image: A close-up of comet Hartley 2 (NASA)
The fifth space probe encounter with a comet happened on Nov. 4, 2010. NASA's recycled Deep Impact probe -- now the EPOXI mission -- visited comet Hartley 2, examining its strange-shaped nucleus. Described as a "peanut" or "chicken drumstick," this comet is an oddity. During its close approach of under 700 kilometers (435 miles), EPOXI photographed the comet's irregular topography: two rough lobes connected by a smooth center. Jets of gas could be seen being ejected from discrete locations. During the Hartley 2 flyby press conference at NASA's Jet Propulsion Laboratory (JPL), mission scientists expressed their surprise that these jets of vapor are being emitted from sun-facing
shaded regions on the comet surface. Needless to say, analysis of the Hartley 2 flyby data will keep scientists busy for some time to come. "This is an exploration moment," remarked Ed Weiler, NASA's Associate Administrator for the Science Mission Directorate, during the conference.
Image: Tempel 1 as seen by Stardust-NExT at c
Most recently, on Feb. 14, 2011, the veteran Stardust-NExT (New Exploration of Tempel) mission made history by visiting a comet for the second time. Comet Tempel 1 was first encountered by NASA's Deep Impact mission in 2005 after smashing the cometary nucleus with an impactor. This second encounter provides scientists with an unprecedented opportunity to study the same comet after six years of orbiting the sun. Preliminary findings suggest Tempel 1 has undergone some erosion during those six years in deep space. Also, the impact crater left behind by Deep Impact was imaged during the Stardust-NExT flyby and it appears to match the size and shape predicted after the 2005 impact. However, the crater appears to be smoother than expected, so further work will need to be done to analyze the 72 photographs taken by this most recent flyby to understand the processes shaping the comet's nucleus.
Aside from tracking weather on other planets, some volcanism, and the sun’s own petulant outbursts, our solar system is a pretty staid place for astronomers. Planets and moons follow orbits under the strict laws of gravity.
As first scientifically recognized by Sir Edmund Halley 300 years ago, comets are the biggest wildcard among the planets. Comets come and go unpredictably and can undergo spectacular transformations as they swing close to the blistering sun.
Now falling inexorably toward a nail-biting close encounter with the sun in late November, comet ISON has mesmerized the planetary community. Its journey is being tracked at different points along its orbit by numerous ground and space base telescopes.
What could be better?
Imagine being able to fly formation with the comet’s icy nucleus like a fighter plane escorting the President’s Air Force One jumbo jet. The spacecraft would transmit daily reports of changes in the comet’s effervescent surface. The probe would watch eruptions of water-ice geysers and other unpredictable cryo-fireworks as the icy nucleus warms near the sun.
This 24/7 monitoring can’t be done for comet ISON because it was only discovered a year ago. An interceptor vehicle and accompanying super-booster would need to have been waiting in a clean room ready for setup and launch.
Therefore, a comet rendezvous mission would have to target a short-period comet with a well-known orbit. Such a comet would not be as scientifically exciting as ISON because it would have made many trips around the sun and been “thermally processed” so that it lost a lot of pristine ices.
Nevertheless, an ideal target we could start planning for now is comet Halley.
The first historical footnote of this comet swinging by Earth dates back to 446 BC according to ancient Greek records. Isaac Newton’s friend Edmond Halley first realized that this comet was periodic. Halley’s analysis revealed that the comets of 1531, 1607, and 1682 moved in almost identical orbits and were separated by intervals of roughly 75 years. Using Newton’s laws of gravity he correctly predicted the comet would next appear in 1758. Unfortunately, Halley died in 1742 and so did not live to seen his forecast come true.
Last seen by naked-eye observers in 1986, Halley’s Comet will reach its farthest point from the sun on Dec. 9, 2023, at a distance of 3.3 billion miles. It will then begin to fall back toward the sun like a baseball dropping back to the ground.
The comet will be best seen from Earth in late July 2061 when it sweeps within 50 million miles of our planet.
Though Halley is not as exciting as ISON, its celebrity status might help garner public support and hence political support for a space visit. Regrettably, this didn’t work back in the 1970s when a string of three different Halley space missions proposed by NASA were turned down. The new space shuttle program was using most of NASA’s budget, and space science missions had to fiercely compete for remaining dollars.
A solar-sail propulsion system (pictured here) needed for a Halley probe was rejected. Funding instead went to the Compton Gamma-Ray Observatory (launched in 1991). According to Former JPL Director Bruce Murray (who, sadly, died on Aug. 29), U.S. President Jimmy Carter was fascinated by black holes and so favored funding the CGRO instead of a comet jaunt.
It’s impossible to predict the status of the U.S. space program in the coming decades, but let’s be optimistic and expect that space technology will have advanced to the point where the requirements for a Halley rendezvous mission would be comparatively trivial and use lots of off-the-shelf components. It might easily be an offshoot from any number of robotic missions designed for visits to asteroids.
The big caveat is that it would take a powerful propulsion system to match the comet’s velocity. This, preferably, would be a nuclear rocket which, pound for pound, is much more efficient and brawny than chemical engines. What’s more, Halley is falling in from far below a the plane of the solar system, and so even more energy is needed to go “out-of-ecliptic” to reach it.
No doubt mission designers would put Mother Nature to work by launching the Halley probe out to Jupiter or Saturn. The probe would then rob orbital momentum from either of these massive worlds. This would not only give the craft a boost in speed but reversal of direction to catch up with and pace the sunward falling comet.
In my mission scenario the probe finally reaches the cold, inactive Halley nucleus in mid-2060 at a distance of a few hundred million miles from the sun. After rendezvous, the probe settles into a safe distance from the Manhattan Island-sized nucleus to avoid being sandblasted when dust starts streaming off the surface. The probe flies ahead of the peanut-shaped nucleus, upstream from the solar wind-blown tail of dust and gas.
Onboard, artificial intelligence carries out the mission with minimal input from ground controllers. A wide-angle camera transmits nonstop images of the entire nucleus. Narrow angle cameras and spectrometers seek out active regions in the crust such as fissures or other breaks that allow gases to shoot out like a rocket exhaust.
The probe deploys any number of nanobot subsatellites to skim closer to the comet surface and monitor interesting new activity. Nanobot “spiders” are also deposited on the surface for in-situ chemical analysis and geomorphology studies.
The probe stays alongside the nucleus well after it has skimmed close to the sun and hurtled back toward the frigid outer solar system. Because the probe is nuclear-powered the mission duration can last for decades. The space robot diligently watches the nucleus go into frozen hibernation until its next return in 2134.
It’s impossible to predict what our great great grandchildren of 2134 will think of the comet. Will we have learned enough about comets by then that it no longer interests astronomers?
The only motivation for yet another rendezvous might be to salvage the 2061 probe and bring it back to the Smithsonian National Air & Space museum as a relic of the golden age of solar system exploration.
Image credits: NASA, ESA