It's been an historic experience watching C/2011 W3, otherwise known as Comet Lovejoy, plunge through the Sun's multi-million degree corona. The comet defied expectations by not only surviving its encounter, but also by quickly re-growing a tail and becoming bright enough to be observed with the naked eye, providing you live in the Southern Hemisphere. An unprecedented number of observations were made by six Sun-observing spacecraft, making Comet Lovejoy's perihelion passage one of the most memorable solar events in recent memory.
A multitude of data poured in during the days leading up to the comet's perihelion, so I figured the best way to recap the full story would be a timeline format. Before we do that, let's take a quick look at the six spacecraft that watched the event unfold from front-row seats, sending us the images and video that made this such an incredible event. It's worth noting that none of these spacecraft were designed specifically for comet hunting; it just happens to be a wonderful by-product of their sun-observing capabilities.
- SOHO: NASA/ESA's Solar and Heliospheric Observatory. SOHO studies the Sun from core to outer corona, and has inadvertently discovered over 2,000 comets due to the coronagraph its uses to block out the Sun's direct glare.
- STEREO: NASA's twin-spacecraft Solar Terrestrial Relations Observatory. STEREO provides stereoscopic views of the Sun to better understand coronal mass ejections.
- SDO: NASA's Solar Dynamics Observatory. SDO records solar activity to understand how it affects life on Earth.
- Proba-2: ESA's Project for OnBoard Autonomy. Proba is a technology demonstration spacecraft that includes solar observation and space weather experiments.
- Hinode: JAXA/NASA/ESA's satellite to study the Sun's magnetic energy cycles. Hinode is the Japanese word for sunrise.
Now that you've been introduced to the spacecraft, I'll refer to them in acronym form for brevity throughout the rest of the article. One more thing I'll mention before we get started: here are links to two prior Comet Lovejoy articles on the Planetary Society Blog: Sungrazing with Lovejoy's Comet and Comet Lovejoy entered SOHO's LASCO C3 field of view this morning! Now, onto the timeline and pretty pictures!
Terry Lovejoy, an Australian amateur astronomer, discovers the comet as he processes 200 image sets of Earth's southern sky that he snapped with his C8 Schmidt-Cassegrain telescope. At first, he cannot conclusively determine whether he sees a genuine comet, or an image-processing glitch.
Lovejoy is able to image the comet again, noting it is moving 3 degrees per day (your clenched fist, held outward, covers about 10 degrees). Subsequent collaboration with other astronomers confirms that Lovejoy has discovered a Kreutz sungrazer, a type of comet that passes extremely close to the Sun during perihelion. Lovejoy's find marks the first time a sungrazer had been discovered from the ground in over 40 years.
November 30 - December 9
The comet is imaged by several ground-based astronomers as it passes through the southern constellations Lupus and Scorpius. Most astronomers predict that it will not survive its encounter with the Sun.
December 10, 19:29 UT
Comet Lovejoy appears in STEREO-B's Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) Heliospheric Imager (HI). This video contains two time-lapse compilations. The first shows the comet from STEREO-B's HI1 during Dec. 10-16, and the second shows the comet in STEREO-A's HI1 from Dec. 16-21. The saturation line the comet passes through in STEREO-B is Mercury. In STEREO-A's view, the small saturation line is Mercury and the large one is Jupiter. Also note the ion tail coming from the comet in the second compilation!
December 14, 02:06 UT
The comet appears in SOHO's Large Angle and Spectrometric Coronagraph (LASCO) C3 camera, which has a field of view of 32 solar radii.
December 15, 18:00 UT
The comet appears in SOHO's LASCO C2 camera, which has a field of view of 6 solar radii. This video shows the full time-lapse of the comet in both the C3 and C2 cameras. The ion tail is visible in C2 prior to the comet entering the corona, and in C3 after it exits, marking a first for a LASCO-imaged Kreutz sungrazer. The dark spot in the center of the compilations is the coronagraph, which blocks out the Sun's direct glare for better coronal viewing. Notice how the comet loses its tail after it enters the corona, and quickly begins to grow a new one upon emerging. Also, you can see the original material from the tail lingering through most of December 16.
The comet enters the Sun's corona and disappears. It is expected to break apart within minutes as it reaches perihelion, due to the Sun's intense energy and tidal forces. This video shows the view from three different spacecraft. First, check out the view from SDO's Atmospheric Imaging Assembly (AIA): Comet Lovejoy has a wiggly tail! It is hypothesized -- but not yet proven -- that the tail material being shed is somehow interacting with the magnetic loops inside the solar corona, creating uneven motion. This is a perfect example of what an unprecedented opportunity for discovery Comet Lovejoy's Sun passage has provided; new observations were made that are not yet fully understood.
Next, Hinode snaps two images of the comet via its Solar Optical Telescope (SOT). The image of the Sun is superimposed for reference. The two images of the comet were taken 30 seconds apart, during which time it travelled a whopping 15,000 kilometers!
Thirdly, Proba-2 sees the comet entering and exiting the corona via its Sun Watcher Active Pixel (SWAP) instrument.
December 16, 00:37 UT
According to SDO estimates, the comet reaches perihelion, a mere 140,000 kilometers above the Sun's surface.
December 16, 01:17 UT
NASA's SDO team reports via Twitter that, defying predictions, the comet has survived perihelion, and was imaged exiting the Sun's corona. Here's my favorite movie of it emerging, as seen by STEREO-A's SECCHI Extreme Ultraviolet Imager (EUVI).
And here's another amazing video of it, as seen through STEREO-B's SECCHI Coronal and Heliospheric Investigation (COR-1) cameras. This one is incredible because you can actually watch the tail get blown off by the solar wind, continuing onward in the same vector while the comet emerges in a different direction! The comet's tail almost immediately starts to grow again.
The comet is seen in SOHO's LASCO C2 camera again after it emerges from the instrument's coronagraph.
December 16, 03:30 UT
The comet is seen in SOHO's LASCO C3 camera again after it emerges from the instrument's coronagraph.
December 16, 05:12 UT
Last image of the comet in SOHO's LASCO C2 camera.
December 18, 08:42 UT
Last image of the comet in SOHO's LASCO C3 camera.
Having survived its spectacular encounter with the Sun, Comet Lovejoy is now cruising through the constellation Scorpius. The brave snowball continues to amaze, remaining bright enough to be captured by ground-based photographers in the Southern Hemisphere. Check out this amazing time-lapse video by photographer Colin Legg, whom captured the comet at dawn on December 21 near Perth, Australia:
A special thanks is in order to all of the various spacecraft teams that worked overtime to process the data that made all of the images and videos of Comet Lovejoy possible. I also want to personally thank Karl Battams, the curator of the excellent Sungrazing Comets website, operated by the U.S. Naval Research Laboratory. Karl's timely posts were incredibly helpful references when I was navigating the SOHO, SDO and STEREO data sets, and he rushed through a final image set from STEREO-A's SECCHI HI instrument as I finished this article. Finally, a big congratulations to the comet's namesake, Terry Lovejoy, and a thanks to all of the astronomers and scientists -- both amateur and professional -- that make events like this fun to watch.