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.
We think a wayward comet is the leading culprit in the extinction of the dinosaurs 65 million years ago. But has a comet ever killed anyone? The Tunguska atomic bomb-sized blast in 1908 could have, if the comet had exploded over a city rather than desolate Siberia.
More recently, comet Hale-Bopp can be indirectly blamed for the 1997 mass suicide of 38 members of a New Age religious cult called Heavens Gate. UFO buffs, eager to call any blob of light seen in the heavens a visitor from space, said that a “Saturn-like” alien spaceship was hiding in the tail of the comet.
The Heavens Gate folks thought the UFO was coming to pick them up and take them to a higher plane of existence. Even if I believed that aliens were doing a bus stop at Earth, I wouldn’t think that dying got you to the front of the boarding line — unless Saint Peter is checking you in. What makes this tragedy even more absurd is just the idea that space aliens would “hide” near an object that lots of astronomers on Earth happened to be looking at.
This “spaceship” didn’t exist of course, but was a misinterpretation of a background star in an amateur astronomical photo. The photo was publicized on a conspiracy/UFO radio show. This was bolstered by claims of a so-called remote viewing expert who said he could “visualize” the spacecraft.
This behavior is common in true believers. In 2011, UFO enthusiasts misinterpreted star trails in a photograph of comet Elenin as being two distinct rows of eight smaller circular shaped spaceships from what they reported as “as yet unidentified extraterrestrial civilization.”
“Elenin is fast approaching our sun is under ‘intelligent control’ and heralds a warning to all humanity of a great global catastrophe soon to come,” one UFO site announced.
Now that Comet ISON is heading for a close encounter with the sun in late November, the Internet is again abuzz with UFO ghost tales, not to mention doomsday theories, mysticism, and miscellaneous strange ideas that go bump in the night.
For starters let’s look at the UFO allegations. A selection of pictures form the Hubble Space Telescope data archive were interpreted by some UFO sleuths as an image of a spaceship, not a diminutive icy nucleus of a primeval body.
Among the online speculation:
“Cloaked and seen engines”
“The winged planet”
“Klingon battle formation”
“This isn’t how they wanted us to see it!”
In reality the comet is streaked because Hubble wasn’t tracking it in the particular series of five exposures originally posted on a comet blog. The moving comet left a trail in the exposures and appeared to follow an arc due to the fact that Hubble is also moving through space in Earth orbit. Similar trails are commonly seen in other Hubble fixed target observations of background stars where a moving asteroid strays into the field of view and can trace out a “C” or “S” shape on the camera detector.
A comprehensive explanation of this ISON image can be seen below.
Comets Gone Wild
In 2011, a number of cosmic disasters were predicted to accompany the appearance of comet Elenin. It supposedly caused earthquakes, shifted Earth’s rotation axis, melted ice on Mars, and triggered a storm on Saturn. In reality, the comet was so fragile it disintegrated on its way to the sun and never reached naked-eye visibility.
Just a year after Elenin we had the 2012 Mayan Calendar alleged apocalypse come and go without the predicted devastating earthquakes, solar flares, rogue planets, magnetic field reversal, or death rays form the galaxy’s black hole. No one has ever apologized for making these outrageous and science illiterate prognostications, but instead cashed-in on dozens of Mayan doomsday book titles.
During that time I got an e-mail from a self-employed young mother who was so fearful the world was going to end she was deeply depressed. ”I make Christmas plans and find myself (embarrassingly) adding ‘that is, if the world hasn’t ended by then.’” She felt I gave her life back when I reassured her that all the 2012 doomsday predictions were just a bunch of baloney.
Like a board game on a closet shelf, these scenarios are all being pulled out, dusted off, and put back on the table for comet ISON.
From one Internet site: “Dust from ISON will fly at the sun, activating massive solar storm activity which would in effect destroy Earth’s electric grid.” Not to be left out in the cold, remote viewers are also predicting a flyby of a “large massive Space Body” that will trigger huge solar flares.
Reality check: There is no evidence or theoretical mechanism where sun-grazing comets cause solar flares and coronal mass ejections. Though some bright comets have been seen approaching the sun immediately before flares, it is purely coincidental because comets are completely insignificant in size compared the sun.
Like Linus’ imaginary Great Pumpkin character in the Peanuts cartoon strip, the imaginary rogue planet called Nibiru is also rising once again out of the patch of goofy ideas. “Comet ISON could in fact be Nibiru,” asserts one self-appointed expert. “Government Agencies are keeping this information under wraps. ISON is sort of a code name given by the Powers That Be. So that the public will remain: clueless, controllable and calm.” Another site predicts, “ISON, as it sweeps past Mars, will yank the planet from its orbit and send it crashing into the Earth, bringing an end to world as we know it.”
ISON’s icy nucleus is no more than a few miles across. Its gravitational pull is as wimpy as it gets. The notion of it disrupting a planet is as nonsensical as imagining a mosquito denting the fender of a speeding truck. If you have any worries about this simply take a look at the animated computer plot (above) of ISON’s trajectory, the planets continue solidly along their orbits without any gravitational deflection by the visiting comet.
The reality is simply that the Internet is a bottomless pit for nurturing screwball astronomical ideas and paranoid conspiracy theories.
In the coming months ISON may become a naked eye object as it swings past the sun. The fragile nucleus may or may not survive being blowtorched and tugged on by the sun’s gravitational field.
If it does survive it will be flung out of the solar system along its multi-million year orbit. But the soothsayers won’t be paying attention. To them, imaginary space disasters are like buses. If you miss one, another will be following straight after.
In addition to Comet ISON, mark your calendars for Oct. 19, 2014 when comet C/2013 A1 is predicted to skirt perhaps as close as 25,000 miles to the planet Mars. It would be ultra-cool for scientists if the comet plowed into Mars. We’d have a reenactment of what commonly happens to the inner planets in the solar system’s formative years.
Soothsays will have a field day with this comet. Once again, nothing’s going to happen to Earth. But our Mars rovers might be in for a bona fide doomsday event!
Image credit: NASA, Majdanak observatory / A. Sergeyev and A. Novichonok