Inner Solar System

You’ve seen the Hollywood movie version, an astronomer looks through the eyepiece of a telescope (it’s always got an eyepiece), scribbles some calculations on a piece of paper and, with a look of horror flashing across his face, realizes that this is the one, the deadly asteroid that’s going to hit the Earth. In reality discovering asteroids that may threaten the Earth is nowhere near as sensational. But it is somewhat more complicated than and equally as fascinating as the movie version.

Pan-STARRS1’s 7.0 square degree field of view makes it an excellent tool for finding objects moving around our own solar system. Part of PS1’s mission is to discover and catalog these hazardous near-Earth objects (NEOs) and their even more dangerous cousins, potentially-hazardous objects (PHOs). NEOs have orbits that bring them within 0.3 AU (about 45m km) of Earth’s orbit, while PHOs have orbital paths that bring them within 0.05 AU (~7.5m km) of Earth’s orbit and are at least 150m in diameter, large enough to cause extensive damage if one were to collide with the Earth.

To cope with the volume of asteroid data that PS1 and an eventual Pan-STARRS 4 (PS4) would need to handle, the Pan-STARRS project devised its own asteroid-finding software, called MOPS: the Moving Object Processing System. MOPS has been under development for about 6 years, and has proven adept at finding NEOs in Pan-STARRS data and in managing its own catalog of newly discovered and known asteroids beside NEOs so that PS1 scientists can do solar system science.

ASTEROIDS

Discovery observations of 2011 ST3. The objects motion between two the two images can clearly be seen.

By far the largest population of asteroids known lie in the Main Belt between Mars and Jupiter. There are currently about 500,000 known Main Belt Objects (MBOs), a number that increases by a few thousand each month. Occasionally an MBO travels close enough to Jupiter that Jupiter alters the MBO’s orbit so that the MBO transitions to a different orbit. This can be a much more elliptical orbit that sends the MBO well into the inner solar system. If this new orbit brings the MBO close to the Earth’s orbit, it is classified as an NEO. Asteroids range from as large as 950 km in diameter for Ceres (the first asteroid discovered) down to as small as a bus or even a basketball. The smaller asteroids are much more numerous though — while a 1-km asteroid might hit the Earth every million years, a rock the size of a basketball collides with the Earth about once a day.

HOW MOPS FIND ASTEROIDS

Asteroids are first discovered as star-like dots moving between astronomical images taken at the same place on the sky. On short time scales, say less than a day, most asteroids move in a fairly straight line. MOPS uses special spatial-searching software to detect asteroid candidates by playing a large game of dot-to-dot with the millions of star-like sources found in PS1 imagery. PS1′s image processing pipeline (IPP) automatically removes stars, which aren’t moving, so MOPS has the job of trying to find straight-moving combinations of sources in the remaining “transient data” catalogs. We call these nightly associations of asteroid detections tracklets. A large part of the task of finding tracklets is dealing with false sources — star-like image features that come from image artifacts, cosmic rays and random fluctuations in the pixel data. Each night, MOPS scans its transient catalogs for asteroid candidate trackelts, and an IfA scientist confirms real asteroids in each nightly list of candidates.

PS1′s survey designed so that asteroids can be discovered while meeting other science objectives. For example, the PS1 “3π” all-sky survey always obtains images in pairs, so that we can tell if a star-like source is in fact an asteroid because we see it moving between two or more images. About 85% of PS1′s survey time can be used to discover asteroids.

ASTEROID ORBITS

From a single night of observations MOPS cannot determine the complete orbital description of an asteroid. Note that while an asteroid has a straightforward elliptical motion through the solar system, its motion on the sky can be rather complicated due to projection effects. Also, from initial observations we cannot tell how far away an asteroid is from us — we only know its brightness, which can vary according to size and distance. So a faint asteroid might be small or far away; we can’t tell at first.

In order to compute a full six-parameter orbit which describes an asteroid’s motion through the solar system, we need multiple nights of observations of an object, then employ a computational procedure called orbit determination. PS1 uses software provided by NASA’s Jet Propulsion Laboratory — the same software used to guide spacecraft through the rings of Saturn! — and the OrbFit Consortium to fit orbits of solar system bodies to PS1 observations.

FINDING NEOS

Discovering NEOs is even more challenging because they can be found all over the sky, often moving quickly. Unlike main-belt asteroids, which are mostly a similar distance from the sun (2-3 AU) and lie in the plane of the solar system, causing them to appear in a “stripe” on the sky, NEOs can be whizzing by quite close to us and can therefore be projected anywhere on the sky. Repeated PS1 detections of these objects can be quite far apart, and making the dot-to-dot associations more difficult. Because PS1′s survey is largely preprogrammed, PS1 cannot always “chase” fast-moving NEOs to obtain repeated observations. So when we discover a candidate NEO tracklet, we submit the observations to the IAU Minor Planet Center, which maintains lists of NEO candidates that need additional observations. PS1, with its wide field, excels at finding initial observations of new NEOs, but prompt follow-up requires worldwide teamwork and cooperation.

Diagram of the solar system seen from above, showing orbits of NEOs discovered by PS1. Click for large version.

PS1 DISCOVERIES

To date PS1 has discovered 85 new NEOs, two comets, and about 4000 main-belt asteroids. PS1 has also submitted observations for over 200,000 known asteroids — nearly half of all known asteroids! This is an important contribution because PS1′s position measurements are so precise that they substantially improve the accuracy of orbits for known asteroids, allowing us to know their positions even better. Here are some highlights of PS1 discoveries:

2010 ST3. PS1′s first NEO discovery from September 2010.

2011 BT15. An especially hazardous NEO, since we cannot yet rule out an impact in the future between years 2037-2110. JPL maintains a list of still-worrisome asteroids at their risk page.

C/2011 L4. Long-period comet on its way toward the sun from the icy reaches of the outer solar system. This object should be visible to the naked eye in early 2013. PS1SC scientist Richard Wainscoat has more information about C/2011 L4 in another blog post.

OTHER SOLAR SYSTEM SCIENCE

There’s alot more to the solar system than just NEOs and MBOs though. PS1SC scientist Darin Raggozine posted a great summary of outer solar system research, and there’s currently research into newly discovered main-belt comets, “contact binary” asteroids that are fused together, and asteroid impacts. When there’s exciting news to report you can be sure to find it on the PS1SC blog.

One of the many exciting aspects of being involved in the Pan-STARRS PS1 survey is the potential to utilize its data products for education and public outreach purposes (EPO). Members of the PS1SC have recently completed some extremely successful work with the International Astronomical Search Collaboration to allow high school students in Germany, Texas, and Hawaii an opportunity to use images collected by PS1 to make asteroid discoveries. This first “pilot project” of 10 schools will be expanded to about 30 schools in a second “campaign” to be conducted during Spring 2011, and eventually the expansion could reach several hundred or even a thousand schools (thousands of students). And because of the vast amount of data produced by the wide field PS1 images, it requires only a handful of images to support such a large number of schools. For me the most gratifying part of bringing asteroid searches into the classroom is the high enthusiasm expressed by both the students and their teachers for participating in this program.

For the past few months we have teamed up the Pan-STARRS 1 telescope, designed to become one of the world’s most powerful asteroid hunters, with school students from the USA and Germany to discover and study asteroids – clumps of rock, between a couple and a few hundred kilometers in size, that cruise through our Solar System. At the close of the campaign, which was coordinated by the International Astronomical Search Collaboration, the students can look back on exciting eight weeks of asteroid search, which included the confirmation of four “Near-Earth Objects” (asteroids passing relatively close to Earth) and the discovery of what could turn out to be more than 170 previously undiscovered asteroids.

As the 1.8 meter (71-inch) Pan-STARRS 1 telescope (PS1), one of the most powerful current survey telescopes, scans the night sky, its 1400 Megapixel digital camera takes more than 500 exposures per night. Between October 25 and December 21, 2010, a small fraction of these data found its way into classrooms in the USA and in Germany, where high-school students have used it to track known asteroids, and also to discover candidate objects that could be previously unknown asteroids. When Hawaiian skies were overcast, schools also received data taken with a telescope operated by the Astronomical Research Institute (ARI) in Westfield, Illinois.

Over the Internet, the participating schools received series of astronomical images. Each series included images of one specific region of the sky, taken an hour apart. During this hour, the image of a main belt asteroid (one lying between Mars and Jupiter) would have moved noticeably (in the images in question: about 100 pixels) relative to the distant background stars. The students examined the images for exactly this kind of position change, carefully sorting image artifacts from moving celestial objects, and reported back to the International Astronomical Search Collaboration, whose volunteers then checked the results and arranged for follow-up observations.

Some of the most interesting student observations during the project concerned “Near-Earth Objects” (NEO), asteroids or similar objects whose orbits bring them into the inner Solar System. Some NEOs might turn out to be potential “killer asteroids” that are bound to collide with our home planet; finding these is one main goal of the PS1 telescope. In order to keep track of NEOs, at least two separate observations at different times are required. Katharina Stöckler (age 17), an 11th grade student at Gymnasium Neckargemünd near Heidelberg, explains:

“We obtained a ‘NEO confirmation’ for the asteroid 2010 UR7 – the second observation ever made of that object, which confirmed the asteroid’s existence and gave crucial information about its orbit.”

Three additional such “NEO confirmations” were made during the project; in addition, 64 of the students’ observations amounted to the third or fourth time a specific NEO had been observed. All these observations provide important additional data to scientists studying the motion of NEOs.

In the course of the project, the students also observed 151 candidate objects in the Pan-STARRS data (plus an additional 20 candidates in the ARI/Westfield telescope data) that could be newly discovered main belt asteroids, which orbit the Sun between the orbits of Mars and Jupiter. In one case, students from Benedikt Stattler Gymnasium, a high-school in Bavaria, Germany, discovered 7 such candidate objects in a single night! Before the students’ finds are confirmed as discoveries, however, and assigned provisional designation numbers, they will need to be observed again – for a number of the candidates, this is going to prove impossible; on the other hand, some are likely to turn out to have been previously known, after all. Once a newly found object has been observed over at least a whole orbit (which typically lasts 3 to 6 years), it is assigned a definite numerical identifier, and can also be given a proper name.

IASC director Dr. Patrick Miller, of Hardin-Simmons University in Abilene, Texas, says:

“Pan-STARRS images contain an amazing amount of data, providing students with opportunities for literally hundreds of new discoveries. With this amount of data, we could expand our campaign to a thousand schools a year, and tens of thousands of students, which is very exciting, and is an unbelievable opportunity for high schools and colleges!”

It is incredibly exciting that we can use a state-of-the-art system such as Pan-STARRS to allow students around the world to learn astronomy with real research quality images. We hope we’ve made this a valuable and enjoyable experience for both the students and their teachers. Hopefully this is only the first step in eventually involving as many as a thousand schools around the world.

If you’re interested in your school getting involved please email me at burgett AT ifa.hawaii.edu

More information can be found in this press release:

“Successful hunt for asteroids in the classroom”
Dr. Markus Pössel, Center for Astronomy Education and Outreach
Max-Planck-Institute for Astronomy

The participating international teams of schools were:

1. Luitpold-Gymnasium, Munich, Germany
Ranger High School, Ranger, Texas

2. Christoph-Probst-Gymnasium, Munich, Germany
May High School, May, Texas

3. Benediktinergymnasium Ettal, Germany
Vernon High School, Vernon, Texas

4. Benedikt-Stattler-Gymnasium, Bad Kötzting, Germany
Bullard High School, Bullard, Texas

5. Werdenfels-Gymnasium, Garmisch-Partenkirchen, Germany
Colleyville Heritage High School, Colleyville, Texas

6. St. Anna-Gymnasium, Munich, Germany
El Campo High School, El Campo, Texas

7. Helmholtz-Gymnasium, Heidelberg, Germany
Tarrant County College, Hurst, Texas

8. Gymnasium Neckargemünd, Neckargemünd, Germany
Brookhaven College, Farmers Branch, Texas

9. Lessing-Gymnasium, Lampertheim, Germany
Madisonville High School, Madisonville, Texas, and Baldwin High School, Wailuku, Hawaii

10. Life Science Lab, Heidelberg, Germany
Collin County College, Plano, Texas