Skull-like dead comet buzzes past Earth as a Halloween treat

Monday, November 2, 2015, 12:04 PM - Talk about a Halloween treat from space! "Spooky" asteroid 2015 TB145, which safely flew past Earth and the Moon on Saturday, not only bore a resemblance to a giant skull, but it was likely what astronomers call a dead comet.

On October 31, 2015, at just after 1 p.m. EDT, a roughly 600 metre-wide asteroid buzzed past Earth and the Moon, travelling at a relative speed of about 126,000 kilometres per hour - "unusually" fast, according to NASA.

This asteroid, known as 2015 TB145, but nicknamed "Spooky" by some, was first spotted on October 10, 2015 by the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS), located on Hawaii's Mount Haleakala. After its discovery, astronomers quickly determined that the track of "Spooky" would bright it on a rather close pass by Earth - at least "close" relative to the size of the solar system, that is.

At the time of closest approach, 2015 TB145 came to within about 486,800 kilometres of our planet - about 1.27 Lunar Distances (LD) away, or a little over 100,000 kilometres farther than the orbit of the Moon. About two and a half hours prior to that, it made an even closer pass by the Moon, at a distance of about 286,100 km (around 0.75 LD).

Trajectory of 2015 TB145 as it passed Earth and the Moon on October 31, 2015. Credit: Celestia, with labels by author.

Was this a frightening encounter? Should we have been concerned? Vaguely anxious?

No. Not really.

NASA and the ESA classified 2015 TB145 as a "Near-Earth Object," and it was also entered on the list of "Potentially Hazardous Asteroids", joining all other asteroid of a certain brightness and size that come within 7.5 million km of Earth. However, since 2015 TB145 is flying past us at a distance further away than the Moon's orbit, there was no danger from it. This was just another harmless pass by a space rock, and it completely missed both Earth and the Moon.

Also, despite some reports which were hyping this asteroid's path as "skimming" past the Moon - and even though the above image may seem to support that idea - that perspective view doesn't show the whole story.

Since it is difficult to represent the three dimensions of space in a two-dimensional image, below are views of the asteroid's path from both above the Earth-Moon system and from the side.

Two views of 2015 TB145's Halloween flyby. The green line is the asteroid's path. Credits: Image - Celestia, Data - NASA/JPL-Caltech. Compiled, with edits, by author.

As seen in the two panels of the image, although the "above" view makes it seem as though the asteroid passed between Earth and the Moon, and rather close to the Moon at that, the "side" view reveals that the asteroid was well-above both Earth and the Moon as it flew past on Saturday. This was, indeed, a clean miss.

Some facts about 2015 TB145:

• NASA scientists believe that this is likely a dead comet. This idea was first suggested due to 2015 TB145's unusual orbit, which takes it on a long elliptical journey from within the obit of Mercury well "below" the plain of the solar system, on a roughly 3-year journey. After taking infrared imagery of the object with NASA's Infrared Telescope Facility (IRTF) on Mauna Kea, however, astronomers became even more convinced of this possibility. Comets are normally quite active this close to the Sun, shedding ice and volatile gases into space to form a coma and long tails of debris. Since 2015 TB145 was not producing a coma or tails, but exhibited other characteristics of a comet, it was very likely a "dead comet" - one that has lost most or all of its ice and volatile gases, and now behaves as an asteroid. According to NASA:
  "We found that the object reflects about six percent of the light it receives from the sun," said Vishnu Reddy, a research scientist at the Planetary Science Institute, Tucson, Arizona. "That is similar to fresh asphalt, and while here on Earth we think that is pretty dark, it is brighter than a typical comet which reflects only 3 to 5 percent of the light. That suggests it could be cometary in origin –- but as there is no coma evident, the conclusion is it is a dead comet."

• NASA currently lists this object with a "condition code" of 2. An asteroid's "condition code" is a scale of 0-9, with 0 meaning it has a well-defined orbit and 9 meaning that the orbit is still highly uncertain. 2015 TB145 started off near the "uncertain" end of the scale, first with a condition code of 7, then 6, and now it has been upgraded to a condition code of 2. Thus, while there were still significant errors in the plot of the asteroid's orbit before, which would affect its orbit in the long-term future - years or decades from now - with more recent observations they have been able to lock down its track with greater certainty.

• Radar imagery (see below) has confirmed that it's quite big! As of 3 p.m. ET on Friday, October 30, imagery and updates issued by the Arecibo Observatory put the size of 2015 TB145 at 600 metres wide. This is quite a bit larger than the original estimate of 470 m, and is at the high-end of the range of estimates given after the asteroid was discovered. Their observations also found that it rotates on its axis once every 5 hours.

• Why haven't we seen this asteroid before, given how big it is? Tracking the asteroid back as its orbit became more clear, it seems that we've had bad timing and bad luck in spotting 2015 TB145 in the past. The asteroid appears to orbital the Sun every 3 years, 25 days, 13 hours and 36 minutes. While it may seem that it should come around and be visible to us every 3 years, that extra time - 25 days, 13 hours and so on - really matters. Based on that, it's never in the same place in the Earth's sky each time it comes around to cross our orbit, and often it's lost in the glare of the daytime or dawn/dusk sky. Also, due to the shape of its orbit, only a telescope in the far reaches of the southern hemisphere could hope to catch it in the deepest, darkest part of the night sky. Even then, it is usually much farther away at that time, making it more difficult to spot. Of the half-dozen closer encounters we appear to have had with 2015 TB145 going back to the year 1920, this year appears to be the first time that the encounter has been close enough to make spotting the asteroid easier.

• Remarkably, this "Halloween" visitor actually looks like a skull!! It's really only pareidolia at work here - the human tendency to see faces and other familiar sites in random objects and scenery - however in the first animation of radar imagery collected by the Arecibo Observatory in Puerto Rico (both shown below), it really does bear a resemblance to a skull!

Great scientific opportunity

Far from scary, though, the passing of 2015 TB145 was an excellent opportunity for the scientific community.

During this flyby NASA aimed its Goldstone radio antennas at the asteroid, to obtain detailed radar images of this unusually fast, possibly cometary object.

"The flyby presents a truly outstanding scientific opportunity to study the physical properties of this object," they wrote in their planning notes.

The Goldstone astronomers labeled 2015 TB145 as "one of the best radar targets of the year" for their observations. The images that have already returned, via the Arecibo Observatory (enhanced, inverted animations above, and originals below), are certainly living up to that potential.

The very first radar images from the Arecibo Observatory, taken October 30, 2015, at a resolution of 7.5 metres per pixel. Credits: NAIC-Arecibo/NSF

According to NASA, more radar images are due out from this encounter soon!

First Look at "Spooky"

Astronomers around the world had been checking in on 2015 TB145 on a nightly basis since it was discovered, and astronomers with the Slooh Community Observatory had been watching along with the rest. Below is an animation they recorded from their telescope in the Canary Islands, on the night of August 22, 2015. The asteroid starts off in the centre of the image (circled in red) and then slowly tracks towards the upper left.

Watch a replay of "Spooky" 2015 TB145's flyby!

Slooh astronomers Paul Cox and Bob Berman presented a live show on Saturday afternoon, just as this Halloween visitor flew past the Earth and the Moon. Along with Dr. Mark B. Boslough, an expert in asteroid impacts and planetary defense, and other guests, they tracked the asteroid with telescopes, discussed the dangers inherent with near-Earth asteroids, and talked about why it took us so long to detect 2015 TB145.

Even though the show is over now, watch the replay of the Slooh event in the embedded stream below:

Pieces of 2015 TB145?

Over the weekend, several reports of bright fireballs showed up on the internet - in Poland, in Thailand and even in parts of the United States.

Watch Below: A bright fireball meteor flashes through the sky over Chwaszczyno, Poland on Oct 31, 2015.

Was this a tiny fragment of 2015 TB145? Very likely, the answer to that question is "no."

It is possible for asteroids to have little "tag-a-longs" flying along the same trajectory as them - either from impacts or perhaps, in this case, some remnant of this dead comet's past activity. With the track of the asteroid, though, any tiny bits of it that encountered Earth's atmosphere would have been travelling from west to east across the sky. The American Meteor Society has pieced together the track of the fireball from eyewitness accounts, and it was found to be travelling roughly southeast to northwest. Reports from Thailand have the fireball seen there also travelling west.

That trajectory makes the annual northern Taurid meteor shower a far more likely origin of these fireballs.

Sources: NASA/JPL-Caltech | NASA Goldstone | Arecibo Observatory | Slooh | American Meteor Society | Bangkok Post

Related Video: Watch as asteroid 2015 TB145 flies past Earth in this animation of images captured on October 30, by Slooh's observatories in the Canary Islands.