Comet Pan-STARRS is here! Catch a glimpse tonight at twilight — it’s kind of a big deal

During the next few weeks, Comet Pan-STARRS will grace our skies as a naked-eye comet. As its name indicates, astronomers discovered this comet on June 6, 2011, using the Panoramic Survey Telescope and Rapid Response System, a planned array of telescopes in Hawaii. This array’s primary mission is to detect near-Earth objects (comets and asteroids with orbits near Earth’s) that pose a risk of impact. Of the four telescopes planned for the array, only the first, PS1, is yet operational. This is the telescope used to discover the comet.

Viewers south of the equator have been observing Pan-STARRS since February, as the geometry of its orbit has favored southern observers until now. However, as Pan-STARRS approached perihelion late on March 9, it also comes up through the plane of Earth’s orbit, making it visible from right here in Houston during mid-March 2013.

via Discover Magazine
Image via Discover Magazine

What are comets?

Comets are made of ice and dust and are often called ‘dirty snowballs.’ They are believed to be left over from the formation of the solar system. Long-period comets such as this one originate in the Oort Cloud, located about 50,000 times as far from the Sun as the Earth.  As comets approach the sun, ice changes into gas and the dust embedded within the ice is released. A cloud of particles expands out to form a coma around the comet’s solid nucleus. This coma may be 100,000 miles across. Radiation pressure of sunlight and the powerful solar wind sweep gases and dust away from the comet’s head into a tail spreading millions of miles behind the comet and pointed away from the Sun. Comets have bluish gas tails and yellowish dust tails.

Where should I look?

Face to the west during late evening twilight, which is about 8 p.m. once Daylight Saving Time begins. On March 12, a thin crescent Moon will be to the lower right of the comet. Pan-STARRS will shift towards the north (to the right as you face west) each night. This shift towards the north means that Pan-STARRS always sets soon after the Sun and is visible only during evening twilight. You will therefore need a clear, unobstructed horizon to the west when observing right at dusk. Note that by the time Pan-STARRS appears in our sky, it is already receding from the Sun, and thus getting a little dimmer each day. Most likely, Pan-STARRS will fade from view by April. How long can you follow it?

What should I look for?

Look for a fuzzy spot, not a single point of light. Observers south of the equator have reported the comet’s total brightness as a little brighter than the Big Dipper’s stars. However, the comet always appears low to the horizon and in twilight; the thicker atmosphere and brighter background will make it dimmer. While Pan-STARRS is visible, the Moon goes from crescent to full, which will dim the comet. Past comets, such as Hale-Bopp in 1997, have shown long, extended tails.

However, Australian observers in early March reported a compact appearance for the comet, with a tail visible only up to two or three degrees from the head of the comet. (Your fist held at arm’s length blocks 10 degrees.) This appearance concentrates more of the comet’s brightness in a small area of sky, making it easier to see a against a twilight sky. Comet tails have a fainter look, comparable in brightness to the Milky Way band. Binoculars may help you locate the tail if you can’t make it out with the naked eye in the twilight.

Will I ever see this again?

No one has ever seen Pan-STARRS before, and no one alive today will see Pan-STARRS again.  By all indications, this comet has traveled from the Oort Cloud into the inner solar system for the first time ever.  And once it’s gone, it won’t return for another 110,000 years.  However, Pan-STARRS could be just the ‘warm-up act’ for a much bigger and brighter comet in November and December 2013 — comet ISON.

Comets are notoriously unpredictable, though; there’s no telling if ISON will meet expectations.  It’s better to take advantage of clear skies forecast for this week and look for Pan-STARRS. When you see it, you’ll be looking at one of the oldest, most pristine objects of our solar system.

See Comet Hartley 2!

Perhaps you were able to observe Comet McNaught this past June and July.  In case you missed that comet, however, another brighter than average comet has approached the Earth this October. Comet Hartley 2 is now visible in binoculars, and could be a naked-eye object around the time of its closest approach to Earth on October 20. As its name indicates, this is the second comet found by Malcolm Hartley. He discovered this comet in 1986 at the Siding Spring Observatory in Australia. Comet Hartley 2 is a small object only about 1.5km across. It orbits the sun once in about 6.5 years, attaining a maximum (aphelion) distance from the sun of about 5.87 AU (beyond Jupiter’s orbit) and a minimum (perihelion) distance of about 1.05 AU (just beyond Earth’s orbit). This orbit places it in the Jupiter family of comets, which orbit in the same direction as the planets with periods of less than 20 years.   In fact, there is evidence that interaction with Jupiter has shortened Hartley 2′s orbital period from over 9 years to the present 6.5.

In October 2010, Hartley 2 comes to perihelion while Earth is on the same side of the sun. This brings Hartley 2 within 11.2 million km of the Earth, close enough to make it visible in our skies—possibly even to the naked eye.  Many comets come inside the Earth’s orbit as they approach the sun, which means we look more or less in the sun’s direction when seeing them at their brightest.  Hartley 2, on the other hand, has a perihelion just outside Earth’s orbit, so Earth is passing roughly between the sun and the comet this month.  Therefore, we can observe it while looking away from the sun in our sky. 

 Halley’s Comet with tail

Comets are made of ice and dust and are often called ‘dirty snowballs.’  We believe they are left over from the formation of the solar system.  As comets approach the sun, ice changes into gas and dust embedded in the ice is released.  A cloud of particles expands out to form a coma around the comet’s solid nucleus. This coma may be a hundred thousand miles across. Radiation pressure of sunlight and the powerful solar wind sweep gases and dust away from the comet’s head, forming tails pointed away from the sun.  Comets have bluish gas tails and yellowish dust tails.   Since Earth is passing more or less between the sun and Hartley 2, however, its tails will be mostly oriented away from us and foreshortened from our point of view. 

Hartley 2′s coma is now quite large in our sky, so you should look for a fuzzy area, perhaps bigger than the full moon, not a single point of light. The total brightness of the comet is about that of the dimmest stars visible, so the farther you can get from city lights, the better.  The large coma means that Hartley 2′s brightness is diffused over a large area, and therefore the comet may look dimmer than its total brightness would suggest.  Averted vision, which involves looking slightly away from the comet’s actual position, may help you see Hartley 2 if it is at the threshold of visibility.   If you choose a viewing site far from big cities and a night with no moon, you may see the comet with the unaided eye.  The extended coma has a soft, diffuse look comparable to the Milky Way band.  

October 20, the day of closest approach to Earth, has a large waning gibbous moon approaching its full phase on October 22.  Before the full moon, it may be easier to spot Hartley 2 at dawn, after the moon has set.  Or, you can look towards the end of the month, with the moon is at Last Quarter.   Here  is a chart showing Hartley 2′s position through November 3, 2010.  (Note that the dates given are in Universal Time, which corresponds to the previous evening for us.)  Of the constellations shown, Perseus rises in the northeast at dusk in mid-October, while Auriga comes up at about 9:30, also in the northeast.  Gemini, the Twins, rises closer to midnight. Keep in mind that predictions for a comet’s brightness are just that–predictions.  many comets appear significantly brighter or dimmer than expected. 

 Tempel 1 as photographed by Deep Impact

Amateur astronomers who get out and observe this comet won’t be alone in observing Hartley 2.  NASA has retargeted its Deep Impact spacecraft to fly past Hartley 2 on November 4, 2010.  Back in 2005, NASA used Deep Impact to study comet Tempel 1.  In that mission, scientists released a probe to impact Tempel 1 and study the material released.  Deep Impact will simply fly by Hartley 2, however, taking advantage of this opportunity to study yet another comet up close.

With its short orbital period, Hartley 2 should return for its next perihelion near April 20, 2017.  Earth, however, will not be on the same side of the sun as Hartley 2 in 2017, so the comet will be much dimmer in our sky.  Hopefully, our skies will cooperate, and Hartley 2 will brighten as expected or even more, and we’ll all get to appreciate a fascinating sight in the fall 2010 sky.

Early Risers: You’re In For A Treat! June brings Blazing Comet & Lunar Eclipse

Are you an early riser and up before the crack of dawn?  If so, I encourage you to look up as you pick up that morning paper as there are two special treats in the June 2010 morning sky.

Comet McNaught
Creative Commons License photo credit: c.j.b

In January 2007, a brilliant comet, known as Comet McNaught dazzled observers in the Southern Hemisphere. Houstonians missed out on it, though, because of cloudy weather in our area during the brief time that comet was well placed for us. Now, in 2010, a different comet McNaught is becoming visible in our sky at dawn.

Robert H. McNaught, an astronomer at the Australian National Observatory, discovered this comet on September 9, 2009, using a telescope at Australia’s Sliding Spring Observatory.  McNaught, a prolific discoverer of comets, has discovered 44 comets (including this one) and is a co-discoverer of 12 others, for a total of 56.  This comet’s formal designation is C/2009 R1, where ‘C’ indicates a long period comet and ‘R’ indicates the time of year it was discovered.

Comet McNaught, though, is more than a ‘long-period’ comet.  Astronomers have determined that its eccentricity is greater than 1, meaning that its orbit has the shape of a hyperbola.  A hyperbolic orbit is the trajectory of a comet that passes near the sun once and never returns.  Once McNaught recedes from view, we’ll never see it again.

A hyperbolic orbit also means that McNaught has never been in the inner solar system before.  This challenges astronomers who want to predict how it will behave and just how bright it will become in our skies.  Already, McNaught is brighter than expected; many expect McNaught to become a naked-eye object by month’s end, especially for those able to observe at a dark site far from light pollution.  McNaught is now easily observable in binoculars.

This is a chart from Sky and Telescope, showing the path of Comet McNaught against the background stars.  Keep in mind that in June, the stars in this map rise in the northeast just before dawn.  McNaught continues to approach the sun until reaching perihelion on July 2, so we expect it to brighten until that date.  Unfortunately, a comet near perihelion is generally also close to the sun in our sky, and this comet is no exception.  Therefore, McNaught will also get harder to see as it brightens towards the end of the month.   After perihelion, McNaught is poorly placed for observers in the Northern Hemisphere.

17-08-2008 lunar eclipes
Creative Commons License photo credit: emrank

If you’re looking for the comet on Saturday morning, June 26, you might as well turn around and watch the moon set in partial eclipse.  Since the Moon is not precisely aligned with the Earth this time, it will not enter fully into the Earth’s shadow; it goes a little less than halfway in instead.  Still, from 5:17 a.m. until moonset at 6:25 a.m., you’ll notice a chunk of the moon’s upper right side missing.  (Actually, its the northern limb of the Moon that passes through the shadow.  The Moon’s northern limb is on the right as the Moon sets.)  The Moon is only about 10 degrees high when the eclipse starts, so you’ll need a southwest horizon clear of tall trees and buildings.  Note that the eclipse is still in progress at moonset; we will see less than half of it.  Folks far to our west will see a much longer event.

Watch out for that space boulder!

Thomas D. Jones, PhD is a veteran NASA astronaut, scientist, speaker, author, and consultant. He holds a doctorate in planetary sciences, and in more than eleven years with NASA, flew on four space shuttle missions to Earth orbit. In 2001, Dr. Jones led three spacewalks to install the centerpiece of the International Space Station, the American Destiny Laboratory. He has been privileged to spend fifty-three days working and living in space.

Many of you may remember when Dr. Jones spoke here in May 2008 on spacewalking. He’ll be back on Tuesday, Nov. 17 with an all new lecture on near-Earth objects, potential impacts, the search for alien life, and the formation of planets.

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500-m-wide NEO Itokawa, imaged by the
Japanese Hayabusa probe in 2005 (JAXA)

On November 6, we had a close encounter with a near-Earth object, 2009 VA (a NEO is a near-Earth object, including both asteroids and dormant comets). The space boulder, a 7-meter-diameter asteroid, streaked by at a distance of only 14,000 km, well inside the orbits of our geosynchronous satellites. NASA’s Jet Propulsion Lab estimates that we have two such encounters each year, on average, with objects of this size. About every five years, Earth is struck by such a body, but objects this small burn up in the atmosphere, resulting in a fireball and the release of several kilotons of energy (TNT equivalent).

The close pass of 2009 VA surprised some news outlets, which speculated on why the small asteroid had not been detected sooner by astronomers (The University of Arizona’s Catalina Sky Survey picked up 2009 VA about 15 hours before the closest approach). The answer is that these small cosmic rocks are so numerous, and so difficult to observe, that we only discover them at random. NASA runs a search program, Spaceguard, to detect larger objects, 1 km and up, that may pose a civilization-ending threat to Earth. So far about 85% of those objects have been found; none pose an immediate threat to Earth, but may in future decades.

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Jet Propulsion Lab depiction
of recent close pass by 2009 VA

Impacts of small objects like 2009 VA create only sky-high fireworks, no harm to us here on the ground. But the Tunguska impact in Siberia a century ago devastated 2,000 square km of Siberian forest. That airburst of about 5 megatons (Mt) of TNT equivalent was caused by an object 30-40 m in diameter; large enough to level a city center. Such an object strikes us every few hundred years. The last one was a century ago; the next one to come along may hit us tomorrow. With current telescopes, we have only a small chance of seeing such an object before it strikes Earth.

Congress has asked NASA to look into what it would cost to search systematically for NEOs down to 140 m in diameter; if we found most of those objects, we would have greater confidence that no “city-buster” NEO is headed for an imminent collision with a populated area on Earth. A report to NASA on the prospects of detecting and even deflecting such potentially hazardous NEOs is due out by year’s end from the National Research Council.

Impact, or cosmic bombardment, is a process that has been altering the faces of the planets since the dawn of the solar system 4.6 billion years ago. Impacts by giant comets and asteroids have changed the course of life on Earth, possibly ending the reign of dinosaurs 65 million years ago, and possibly causing other mass extinctions through Earth’s long history. We now have the technology to both detect damaging NEOs heading for Earth, and with proper warning, to nudge them out of the way. What we lack is the international will to take action should a hazardous NEO be found on a collision course with Earth. The Association of Space Explorers is working with the United Nations to draft such a NEO decision-making agreement.

At the Houston Museum of Natural Science on Tuesday, Nov. 17, I will be speaking about impact and the other processes that shape the worlds of the solar system, in a talk called Planetology. My talk will discuss these processes — tectonics, volcanism, erosion, for example — and our search for life and “other Earths” across the galaxy. Please join me for the lecture that evening at 6:30 p.m., or turn the pages of Planetology, written by me and noted planetary geologist Ellen Stofan. After the talk, I’ll be answering questions and signing copies of the book.

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