Category: Comets and Asteroids

The solar system’s small stuff

What Remains of ISON

Fragmented? Probably.

There seems to be something left of comet ISON, perhaps some large fragments. Dimmer than it was going in, more spread out. But something is generating a coma that shows up in the SOHO LASCO C3 imagery.

ISON SOHO LASCO C3
What remains of comet C/2012S1 ISON after perihelion in the SOHO LASCO C3 imagery

So Long ISON?

It does appear that comet C/2012 S1 ISON has come apart at perihelion. Imagery shows the comet coma dimming and smearing out as if the nucleus has totally disrupted. Even worse, the SDO imagery programmed to cover perihelion very near the sun show nothing. The SDO cameras are very good at this sort of thing, it should show traces even if the nucleus had been stripped of the tail by the solar wind.

So long ISON?

SOHO C2 ISON Animation
SOHO C2 camera view of comet C/2012 S1 ISON apparently disintegrating just before perihelion.

Comet C/2012 S1 ISON at Perihelion

Today comet C/2012 S1 ISON will pass through perihelion, its closest approach to the Sun.

At a mere 1,800,000km (1,100,000miles) this will be a close pass indeed. As perihelion is measured from center to center, the distance is even closer if you consider the 695,500km (432,200mile) radius of the Sun. Subtracting the solar radius you realize the comet will pass a mere 1,100,000km (680,000miles) above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day on Earth. Hot indeed!!

C/2006 P1McNaught
C/2006 P1 McNaught while 5° from the Sun on Jan 14,2007
While the comet is so close to the Sun volatile gasses will be streaming off the comet in huge quantities creating an extremely bright comet. It is fairly likely that the comet will be visible in the daytime. If so, it will appear much as C/2006 P1 McNaught appeared in January 2007 just a few degrees from the Sun, potentially visible to the unaided eye.

At closest approach the comet will be less than a degree from the Sun, difficult to pick out. An observers best bet will be as it approaches and as it moves away from the solar disk. As the comet nears perihelion it will approach the Sun from the west, best seen in the dawn sky. After perihelion it will exit the Sun’s vicinity to the north, favoring northern hemisphere observers.

The comet should be spectacular in the cameras of the dedicated solar observation satellites. Check out the real time views from SOHO or Stereo.

Observing ISON through Perihelion

Plotted below is the path of comet ISON through perihelion. The image is zenith up on the morning of November 28th from the island of Hawai’i. The actual moment of perihelion will be Nov 28.77501UT (18:36UT or 08:36HST).

A few things are notable… The obvious one is how close the comet will get to the Sun. Not just in absolute terms, which is really close. But rather how close it will look to us. The comet will be under 30 arc-minutes from the center of the Sun, recalling that the Sun is about 30 arc-minutes across. The comet will not pass behind the Sun from our point of view. While we may not be able to see it while lost in the solar glare, it will remain in the view of those solar monitoring spacecraft that are near the Earth.

Separation will help in trying to spot the comet during the day. During the 27th, 28th and 29th the comet will be very close to the Sun. On the morning of the 29th the comet will be only 4.5° from the Sun. Best bet to attempt a daytime peek may be on the 30th or later, when the comet will again be more than 7.5° from the Sun. Look for the magnitude estimates and be prepared to give it a look.

ISON at Perihelion
The orbit of comet C/2012 S1 ISON as it passes through perihelion on November 28th, 2013

Spotting Comet C/2012 S1 ISON in the Daytime

Astrophotography is not normally a daytime activity, but there are exceptions. If a comet is bright enough, about magnitude -2 or brighter, it is possible to spot the comet in the middle of the day. Comet C/2012 S1 ISON may very well be visible near the Sun in the middle of the day.

C/2006 P1McNaught
C/2006 P1 McNaught while 5° from the Sun on Jan 14,2007
The comet will pass through perihelion on November 28th. At a mere 1,860,000km (1,150,000miles) this will be a close pass indeed. As perihelion is measured from center to center, the distance is even closer if you consider the 695,500km (432,200mile) radius of the Sun. Subtracting the solar radius you realize the comet will pass a mere 1,165,000km (724,000miles) above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day on Earth.

This sort of solar intensity will cause the comet to emit enormous amounts of gas and dust. It is this cloud of material around the comet, the coma and tail, reflecting the sunlight that makes the comet bright.

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Comet? Where?

There is one question we all have to ask when a beautiful comet graces the skies…

C/2007 N3 Lulin
Comet C/2007 N3 Lulin on the evening of 26 Feb 2009
Where to look?

Like any other solar system object, comets move against the sky. Even worse, when close to the Earth or Sun they can be moving so quickly against the stars that coordinates quickly become out of date. Aiming a telescope using coordinates a day old, or sometimes even only an hour old will result in a view of empty sky. A few stars perhaps, but no comet.

You need a table of coordinated calculated for regular time intervals, an ephemeris. Alternately you need a set of coordinates calculated for the exact time you will be looking.

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Preparing for Comet C/2012 S1 ISON

Astronomers, professional and amateur alike, are getting ready for comet C/2012 S1 ISON, possibly the highlight of a year that has already seen several good comets.

Discovered in September 2012 by Vitali Nevski and Artyom Novichonok, of the ISON project, a group of telescopes dedicated to discovering and tracking solar system objects. The comet was then an 18th magnitude object in the outer solar system, an impressive find for a small telescope.

Comet C/2012 S1 ISON
An image of comet C/2012 S1 ISON acquired by the Hubble Space Telescope on October 9th, 2013, credit: NASA, ESA, and the Hubble Heritage Team
When astronomers first calculated the comet’s orbit they found a surprise. The comet will pass close to the Sun. Not just close, but extremely close! On November 28th the comet will pass perihelion, its closest approach to the Sun. At a mere 1,860,000km (1,150,000miles) this will be a close pass indeed. As perihelion is measured from center to center, the distance is even closer if you consider the 695,500km (432,200mile) radius of the Sun. Subtracting the solar radius you realize the comet will pass a mere 1,165,000km (724,000miles) above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day on Earth.

The next surprise was hinted at by the orbital calculations. The orbital solution indicated a nearly hyperbolic path, suggesting that this was a new comet, one that had not visited the inner solar system before. This possibility was strengthened by later observations of the comet.

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Taking Comet Photos with Keck

After our comet ISON observing session I have been asked what it is like to take a comet photograph with Keck. An apt question as all of the recent observers were taking spectra, not photos.

The answer? Not very pretty.

A few years back Greg Wirth and I took some frames of comet Hartley 2 with Keck 2. When I processed the frames into a color photo the results were less than impressive.

Comets are big. While the nucleus is quite small, we do not see the nucleus even with the enormous power of a 10 meter telescope. It is hidden in the coma and quite dark, the average nucleus is a shade of dark gray equivalent to charcoal.

The coma and tail are very extended, much larger than the field of view of the telescope, thus the entire frame is inside the coma. The photo of Hartley 2 Greg and I took was no exception.. The image is notable for its complete lack of any interesting structure. There are no jets, shells or other inner coma detail visible. The tail is simply a general brightening to the southwest (lower right in this image).

Small telescopes, in the hands of amateurs, are going to produce the prettiest images of comets. With fields of view measured in degrees, not arcminutes, the comet is going to be seen in all its glory.

103P/Hartley 2
Comet 103P Hartley 2 with Keck2 and DEIMOS 6Oct2010 @ 7:27UT, 3x60s, 3x60s and 3x120s with standard BVR astrometric filters, credit: Cooper/Wirth/W.M. Keck Observatory

The Portents of ISON

Comets have long been regarded as omens of extraordinary portent. Novae, eclipses, conjunctions, anything seen in the sky can serve in this role, but bright comets have always held a special awe.

C/2007N3 Lulin
Comet C/2007 N3 Lulin on the morning of 22 Feb 2009
Even today those who seek signs are quick to seize upon any unusual astronomical event. A few months ago it was meteors, events over Chelyabinsk grabbing the attention. With ISON entering the inner solar system the attention shifts.

Bright comets are invariably seen as omens by some. History is littered with stories of comets and prophesies. This has not changed, there are many recent examples… Hale-Bopp, Hyakutake, Elenin, each has been used to make all manner of wild predictions. Comet C/2012 S1 ISON certainly fits the role. It will most likely be quite bright, easily visible without optical aid. It may even be visible in the daytime.

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Asteroid 2008 QE2 Ephemeris

Planning to make a go at photographing asteroid 2008 QE2 as it passes the Earth. Before I can do that a little planning is in order. Minor questions… Is it above the horizon near close approach, in my nighttime sky? It turns out this object will be available for observation.

The table below is a precision ephemeris generated by the NASA HORIZONS web interface for Mauna Kea on the island of Hawai’i. Listed are celestial coordinates, time, local elevation and azimuth, magnitude and distance. The marks after the time column indicate daylight (*), twilight (A) and moonlight (M).

From the data it is clear that the evening of May 30th (May 31st UT) allows observation from Hawai’i, at an elevation of ~46° a few hours after dark. Nicely placed and nicely timed for observation. Close approach is actually the next day, but the difference is minor. The next evening allows a second chance at the observation while the asteroid is still close.