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.
The Eta Aquarids can be a spectacular meteor shower. It peaks each year around early May and in the case of this year's shower, the peak is expected in the morning of May 6, but meteors from this shower can be seen between April 19 and May 28.
We see the meteors when the Earth plows through the orbit of Halley’s Comet which means they are related to the Orionids shower in October, whose members are also the remnants of the same comet. The visibility of the shower tends to be a little better for observers around tropical and southern latitudes, but the Eta Aquarids can be seen further into the Northern Hemisphere and a good display can still be enjoyed.
It may seem a little strange that the shower is better from one part of the globe than the other but the reason is pretty easy to understand and it lies in the location of the radiant and the time of sunrise.
The radiant is the point in the sky that all meteors from a given shower seem to appear and the constellation from which this point is centered is how the shower gets its name. In the case of the Eta Aquarids, the radiant lies very close to the star Eta Aquarii in the constellation of Aquarius.
At any given time, the radiant for the Eta Aquarids (or any other point in the sky for that matter) rises at the same local time for one location as it does for any other location. However, sunrise happens a little later in the southern hemisphere during May than it does in the northern hemisphere. This means that Aquarius has risen higher in the pre-dawn sky giving Southern Hemisphere observers a better chance of seeing meteors from this shower.
Spotting meteors is a bit of a tricky business though and I have generally found that it is best to keep your gaze moving around, but focused on a point about 40 degrees away from the radiant.
A really easy way to estimate 40 degrees is to stretch out your arm and, while keeping the palm of your hand facing away from you, stretch out your fingers. The distance from small finger to thumb is about 25 degrees so two of these will give you a rough estimate of 40 degrees.
Looking directly at the radiant means that any meteors appearing at that point in the sky will be heading straight for you and will burn up high in the atmosphere. However, as they will be heading toward you, any trail left by them will be very short and difficult to spot.
By far the best way to observe meteor showers is to get a few hours sleep before midnight then get up, wrap up warm and make yourself comfortable on a sun lounger or other comfortable seat, lie back and wait. You may be lucky and see one straight away but you may be there for some time so making yourself warm and comfortable is essential so you can stay the distance.
You don't want to be cold and uncomfortable after half an hour and pack up only to find that others saw a wonderful display a few hours later. You do not need any other special equipment for meteor shower observing -- not even a telescope or binoculars -- so this is something anyone can enjoy.
If the weather is kind, then the astronomical forecast for this year's Eta Aquarids display is good. The moon will have set by the time the peak is upon us so the chances of seeing more meteors is higher.
Make sure your eyes are dark-adapted too as that can limit what you can see, but if you are lucky and patient then during the peak you might see as many as 50 in one hour. Good luck!