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March 13th: I "Heart" Hartley 2!


Date: March 13, 2010

Title: I “Heart” Hartley 2!


Podcaster: Elizabeth Warner, U Maryland

Organization: EPOXI Mission:

Description: The Eastern Seaboard of the US is all too familiar with snow and have probably seen more than enough dirty snowballs! But cometary scientists are eager for more. With the successful completion of the Stardust and Deep Impact missions, NASA had two working spacecraft which they re-purposed. The Stardust spacecraft will fly past comet Tempel 1 (the target of the Deep Impact mission in 2005) in Feb 2011. The Deep Impact spacecraft will fly past comet Hartley 2 in Nov 2010. What do we know about this comet? How does it compare to other comets?

Bio: Elizabeth Warner, a faculty research assistant at the University of Maryland, is the campus observatory coordinator and webmaster for the EPOXI mission. The first mission she worked on was Deep Impact for which she was the Liaison to Amateur Astronomers. In that role, she was able to travel to starparties and talk about the mission. For EPOXI, she is taking advantage of social media like Facebook (become a fan of the ‘EPOXI Mission’) and other internet resources to help get word out about the mission.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Liz Fracek and dedicated to the Museum of Science and Industry in Chicago, Illinois, where I learned most of what I still remember about science.


Hello! I am Elizabeth Warner and I’m the webmaster for the EPOXI Mission.

If you live on the east coast of the United States or most anywhere in the US this winter, then you are probably tired of seeing snow and snowballs! But there are some folks who wish we could get a better look at lots of dirty snowballs!

Who, you ask? Cometary scientists, that’s who! Comets are sometimes referred to as dirty snowballs and there are over 3600 known comets and potentially millions at the outer edges of our solar system. Using several different spacecraft, we’ve only captured a handful of glimpses. But in November, we’ll add another one to our observed list.

You may remember the Deep Impact mission which launched in January 2005 and encountered comet Tempel 1 in July 2005. The spacecraft actually had two components: an impactor and a flyby. The impactor was released into the path of Tempel 1 and collided with its surface, excavating a crater, as the flyby flew past taking pictures and spectra. The flyby spacecraft is still in orbit and in 2007, was given a new mission, EPOXI. This new mission combines two tasks: to observe giant extrasolar planets transiting their parent stars, aptly named EPOCh or Extrasolar Planet Observation and Characterization; and to investigate another comet with the Deep Impact spacecraft, thus the name DIXI for Deep Impact Extended Investigation.

The DIXI team had originally proposed comet Boethin as the target, but observations from Mauna Kea and other observatories failed to find it (and it is now believed to have completely disintegrated), so the team is going to its backup comet, Hartley 2.

Comet Hartley 2 or 103P/Hartley 2 was discovered 24 years ago on March 15th 1986 by Malcolm Hartley in Siding Spring, Australia. It was discovered after perihelion at about magnitude 18. Early calculations of its orbit showed that it had had a close approach to Jupiter in 1982 and had an orbital period of just over 6 years which means that it took about 6 years for this comet to go around the sun once. The comet was observed during subsequent apparitions in 1991, 1997, and 2004. The upcoming apparition in fall of 2010 will be very favorable, meaning that we should have a very good view of the comet as it gets close to the sun. The comet reaches perihelion on October 28th. A week earlier on October 20th, the comet will only be 0.12 astronomical units (AU) from Earth – that is just over 17.5 million kilometers or 10.8 million miles. Wow! Very close to us at practically the same time it is closest to the sun! Favorable indeed! The comet is expected to reach about magnitude 5, meaning that under clear and dark skies, it could be visible without telescopic aid.

What do we know about this comet?

From the observations over the years, we know that it had a close approach to Jupiter in 1982, and again in 1993 when it passed a mere 0.37 astronomical units from Jupiter. That close approach changed the comet’s orbital period from 6.26 to 6.39 years and its perihelion distance from 0.95 to 1.058 astronomical units, from just inside the Earth’s orbit to just outside. The aphelion, or far point, is just outside of Jupiter’s orbit. Comets are typically classified according to their orbital period or how long it takes them to go around the Sun. Short period comets take less than 200 years. Long period comets take longer. In addition, the short period comets are further classified as either Jupiter-family comets which have orbits of less than 20 years or Halley-family comets, which have orbits between 20 and 200 years. Therefore, comet Hartley 2, with an orbital period of about 6.5 years, is a short period comet, and more specifically a Jupiter-family comet.

Let’s step back a moment. A comet is often described as a dirty snowball because we see water and other volatiles in the spectra of comets. The main part of the comet is the nucleus and it is usually very small, just a few kilometers to a few tens of kilometers in diameter. When the comet is further from the sun, only the nucleus exists—and because it is so tiny, it is extremely faint and hard to observe. However, as the comet approaches the sun, the volatiles are heated and start to sublimate, forming a cloud of dust and gas around the nucleus called the coma. The coma can be thousands and thousands of kilometers in diameter. The coma can also be affected by the solar wind and motion of the comet, creating two tails: a dust tail and a gas tail. When we see a comet, we are usually observing the coma and tails. Photographs of bright comets like Hyakutake, Hale-Bopp, and more recently McNaught show only the coma and tails. The nucleus is simply too small to see. In fact, the only detailed images of comet nuclei have been taken by spacecraft. The Giotto mission flew past comet Halley in 1986. Deep Space 1, a technology mission, flew past comet Borrelly in late September 2001. Stardust encountered and collected dust from comet Wild 2 in January 2004. And the Deep Impact mission with its two-part spacecraft slammed into Tempel 1 in July 2005. Only four comets! And they all look very different! Another mission called CONTOUR or Comet Nucleus Tour, launched from Florida on July 3rd, 2002. CONTOUR’s objective was to study two very different comets, Encke and Schwassmann-Wachmann-3, as they made their periodic visits to the inner solar system. Unfortunately, the spacecraft broke apart six weeks after launch during a planned maneuver to propel it out of Earth’s orbit and to its targets. The loss of CONTOUR means two fewer samples. And with over 3600 known comets in the Solar System (as of November 2009), we could have used more samples. So, the Stardust spacecraft and the Deep Impact Flyby spacecraft were given new missions. Stardust will fly past Tempel 1 to see how that comet has changed since its encounter with Deep Impact as part of the Stardust-NExT Mission. And the DI Flyby will go to Hartley 2. We’ll be getting a tour of comet nuclei after all!

And why do we want to see these or any comets up close? Well, before the Stardust and Deep Impact missions, the prevalent ideas on how and where comets formed were different. Comets were thought to be the frozen remnants left over from the formation of the solar system. Short period comets came from the Kuiper Belt region and long-period comets had their origins in the Oort Cloud, a spherical halo of potentially millions of comet nuclei surrounding the Solar System. But the observations from Stardust and Deep Impact showed that comet origins may not be so simple. They contain particles that have been “cooked” or processed in the inner solar system before being mixed with volatiles in the outer solar system. In addition, Tempel 1 has some intriguing features that simply did not make sense in the previous theories. So the theories on how comets formed are getting rewritten!

But what do we know about Hartley 2, the dirty snowball part?

Not much! The Spitzer Space Telescope made some observations in August 2008 and based on that data, we believe the nucleus of Hartley 2 is about 1.2km in diameter. But before the Deep Impact encounter with Tempel 1, we had photometric observations that led us to estimate that Tempel 1 was more elongated than round. The spacecraft images however showed it to be 7.6 by 4.9 kilometers (4.7 by 3.0 miles), with an average diameter of 6.0 +- 0.2 kilometers (3.7 +- 0.1 miles) or more round than we had estimated. So reconciling derived data from ground-based observations with spacecraft data is an important aspect. It helps us to improve our models because we can only observe most comets from the ground.

Can I observe Hartley 2?

Yes! As previously mentioned, Hartley 2 will be closest to Earth on October 20th with perihelion several days later on October 28th. Some magnitude estimates show it could reach magnitude 5 which certainly brings it easily into view of binoculars and telescopes. The Jet Propulsion Laboratory’s Horizons system, an on-line solar system data and ephemeris computation service, estimates Hartley 2 at about magnitude 18 currently and only reaching magnitude 7.5 in late October. To see the comet now, you’ll need to be an early riser as it is in the morning sky in the constellation of Aquila. Over the coming months, the comet will slowly brighten as it drifts through the constellations Aquarius, Pegasus, Lacerta, Andromeda, and Cassiopeia. As we get closer to its perigee and perihelion dates on October 20th and 28th and to the November 4th EPOXI encounter, it will seem to fly through the sky passing through Perseus, Auriga, Gemini and Monocerous. Observers are encouraged to get images of the comet as well as photometric data. More detailed charts will be available on the Amateur Observers’ Program website. That program was setup during the Deep Impact mission to encourage amateur observers to observe Tempel 1 and to submit their observations. It has been expanded to include targets from other Discovery Program missions like Vesta and Ceres for the Dawn Mission, Tempel 1 again for Stardust-NExT, and of course Hartley 2 for the EPOXI mission.

Even though you may not have heard of comet Hartley 2 before, I hope that after this brief overview, and after you’ve checked out some of the mission websites and try to observe it, that you will also love Hartley 2!


End of podcast:

365 Days of Astronomy
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