PS1SC Blog

The new @PS1NEOwatch feed tweets when PS1 finds a new Near Earth Object

From today onwards you can see tweets of new Near Earth Objects identified by Pan-STARRS1. Follow @PS1NEOwatch for updates of new PS1 NEOs. If you want to know more about how PS1 finds asteroids then why not check out the following blog posts.

MOPS: Finding things that go bump in the night where Larry describes how advanced software helps Pan-STARRS identify rocks that could come very close to the Earth.

School students find hundreds of potential new asteroids with PS1 where Will Burgett outlines work being done by school students across the globe to identify new asteroids.

As part of last month’s consortium meeting in Hawai`i, astronomers got the chance to visit the telescope that’s been keeping them busy over the last few years. Here’s what they saw on the trip….

The Pan-STARRS1 Telescope (right). The left-hand dome is currently being cleared for the construction of a second Pan-STARRS telescope. Credit: Douglas Finkbeiner

The power behind Pan-STARRS - world's largest digital camera. Credit: Douglas Finkbeiner

No this isn't a trampoline used by astronomers when the weather is bad. This screen is used to calibrate the PS1 telescope. Credit: Douglas Finkbeiner

The primary mirror of the PS1 telescope. Credit: Douglas Finkbeiner

Some Belfast supernova-hunters pose inside the dome. Credit: Douglas Finkbeiner

Telescope Manager Jeff points out the small telescope attached to PS1 that's used to monitor the transparency of the night sky. Credit: Douglas Finkbeiner

In the Telescope control room at IfA Maui the visting astronomers where shown how PS1 observations are scheduled and carried out. Credit:Douglas Finkbeiner

And here's the happy (and slightly windswept) group. Credit: Laura Fiorentino

Stephan's Quintet, a beautiful group of galaxies in Pegasus. Credit: Nigel Metcalfe/PS1SC

This month we are in the constellation of Pegasus looking at one of the most famous groups of galaxies in the sky. Stephan’s Quintet is an arrangement of five spectacular galaxies. Four of these are a physically associated group while one (the largest in the image NGC 7320) actually lies much closer. Note the galaxy with too nuclei, this is actually two galaxies in the process of colliding.

Recently, astronomers have discovered a new class of very bright—and very mysterious—stellar explosions. These events rank as the most luminous supernovae known, more than ten times brighter than the normal supernova explosions which mark the deaths of massive stars. Curiously, these explosions show no evidence for the two most common elements in the universe: hydrogen or helium. Instead, their spectra show the much rarer elements (in the universe on average—although not on earth) of carbon, oxygen, and magnesium. In addition, the colors of these supernovae are very blue, peaking at ultraviolet wavelengths where our eyes are not sensitive and the Earth’s atmosphere blocks radiation.

Images taken before (left) and after (right) explosion, for the two ultra-luminous SNe recently discovered by Pan-STARRS1, PS1-10ky and PS1-10awh.

While this class of explosions was first recognized in the local universe by the Palomar Transient Factory, Pan-STARRS1 has been identifying them at significant cosmological distances and redshifts. We just completed a study of two of these “ultra-luminous” supernovae at a redshift of ~1, which means that these supernovae exploded when the universe was just half the age it is today. It also means they are very far away—you can imagine that they must be very luminous explosions indeed, if we can detect the death of a single star at distances of 18 billion light years! Conveniently, when we observe these sources at such large distances, their spectra get shifted towards redder wavelengths by the cosmological expansion of the universe, so that the ultraviolet peak of their spectra is shifted to visible wavelengths that are visible through the atmosphere. This helps us discover them at these great distances, and also allows us to study the part of their spectra where most of the energy comes out. Pan-STARRS1 is continuing to find approximately one of these rare explosions each month, greatly expanding the number known and allowing us to trace how this class of extremely bright explosions evolves over cosmic time. We note that most of these ultra-luminous supernovae discovered by Pan-STARRS1 have been “orphans”, without a clear host galaxy; targeting orphaned supernovae often turns up particularly interesting transients, as described in this other blog post.

Artist’s impression of a supernova powered by an extra “engine”—perhaps a magnetar (Courtesy of NASA/D. Berry).

Typically in a supernova explosion, only ~10% of the energy of the explosion is transformed into light. The rest of the energy goes into heat and the motion of expansion. We think it’s possible to get ten times more light out of a supernova in two ways: either all of the explosion’s energy is transformed into light, or there is an extra source of energy that boosts the total explosion energy of the supernova. The first popular explanation for these ultra-luminous explosions touches on point (a): the supernova blast interacts with a dense envelope of material that surrounds the exploding star, and this strong interaction makes the blast wave dump all of its energy into radiation in one relatively rapid burst. The second popular explanation exploits possibility (b): when the star explodes, it leaves behind a rapidly-spinning, highly-magnetic remnant called a magnetar. This magnetar then rapidly spins down, and transfers the rotational energy that it is losing to the supernova explosion. In this way, the magnetar is an “engine” that gives the supernova extra power. As of today, both of these explanations have strengths, and both have weakneses. In the months to come, as we discover more of thse powerful explosions, we will better understand which, if either, of these explanations is most likely—or additional possible explanations will surface. However, for the moment, the cause of thse extremeley energetic explosions remains a mystery.

Comets and asteroids have classically been considered to be two distinctly different types of objects. Both are considered “small solar system bodies”, too small to be considered planets, but large enough to be tracked individually as they travel through the solar system. Asteroids are typically thought of as inert chunks of rock or metal that are mostly found on roughly circular, flat orbits (compared to those of the major planets) in the main asteroid belt between the orbits of Mars and Jupiter, where we believed they formed (and where we therefore believe they’ve been since the formation of the solar system). Comets, on the other hand, are thought of as “dirty snowballs” that travel along often highly elongated orbits that take them from the cold, distant outer solar system to the warm inner solar system where we usually observe them. They are believed to originate in the one of two distant reservoirs of frozen, icy bodies: the Kuiper Belt just outside the orbit of Neptune, and the far more distant Oort Cloud. Occasionally, a collision or the slight gravitational tug of a passing star sends one of these bodies Sun-ward into the inner solar system, where the object’s ice heats up and sublimates (turns from solid to gas), ejecting gas and dust which we observe as the familiar fuzzy haze that
surrounds the core, or nucleus, of the comet, and often also in the form of a cometary tail.

Comets have been able to preserve their icy content over the 4.6 billion year life of the solar system because they have spent most of their lives stored in the cold outer solar system beyond the orbit of Neptune. Meanwhile, if asteroids ever contained ice (and there is evidence that indicates some did once contain ice, albeit in the distant past), they are believed to be mostly baked dry by now by the much higher temperatures in the main asteroid belt. Recent research has been challenging this traditionally-held picture though.

The first main-belt comet, an object that had an orbit like a main-belt asteroid but had the appearance of a comet, was discovered in 1996. The thought that an object orbiting so close to the Sun could still have enough surviving ice to power cometary activity, however, was initially so disturbing to astronomers that many believed that what they had witnessed was the result of an impact tossing dust up into space. Observations six years later, however, showed that cometary activity had returned. This discovery all but ruled out the impact hypothesis for driving the activity since two random impacts on the same asteroid would be required in an
extraordinarily short period of time. Comets, however, routinely exhibit recurrent activity, as temperature changes as their orbits take them closer to and then farther away from the Sun make them warmer and then colder, turning sublimation on and off in predictable ways. Main-belt comets have much more circular orbits and as such do not go through temperature swings as severe as other comets, and so we suspect that their activity may be instead be controlled by seasonal effects caused by the tilt of their rotational poles (as compared to their orbits) in the exact same way that seasons with widely varying temperatures are caused on Earth.

Main-belt comets have much to tell us about the true composition of the asteroid belt, which in turn will help us to understand the formation of our own solar system, and therefore the conditions that might need to be present for similar solar systems to form around other stars. In the case of Earth in particular, main-belt comets may be the key to understanding a particularly vexing problem, that of discovering the origin of our water. Due to its close proximity to the Sun, the Earth is thought to have been too warm to be able to accumulate much water as it was forming, and likely accumulated most of its water from impacts from objects from colder parts of the solar system. Comets from the outer solar system were once considered good candidates for playing this role as water deliverers, but recent studies have suggested that main-belt objects may have played a much  larger role than previously thought. The discovery that ice still remains in the asteroid belt in main-belt comets gives us a present-day opportunity to probe this potential ancient water source, and as such, is of great interest to astronomer.

The Main Belt Comet La Sagra imaged with PS1. studying the origin of water, and therefore of life, on Earth.

The extremely recent discovery of the main-belt comets (first discovered in 1996, but not recognized as a new class of objects until 2006 when the discoveries of two
more were announced) means that we still have much to learn about them. At the moment, just five such objects are known, meaning that at the moment, a high priority
is to discover more so that we can begin to understand the extent and diversity of the population of these strange objects. How many are there in total? Are they
confined to particular parts of the asteroid belt? Luckily for astronomers interested in these questions, this is one area where Pan-STARRS is expected to help. By
surveying the sky repeatedly and being able to detect fainter objects than previous surveys, we expect that Pan-STARRS should be able to discover many more main-belt comets. To do so, we require sophisticated techniques to sift through the mountains of Pan-STARRS data generated each night to automatically select potential comets for further inspection by humans. Since the start of the Pan-STARRS survey, these techniques have been undergoing refinements to optimize their comet-finding effectiveness and have now reached the point where Pan-STARRS has been credited with the discovery of four comets this year (three of them just in the last 2 months including C/2011 L4 (PANSTARRS) ). None of these have so far turned out to be main-belt comets, but as comet discoveries start to become more routine, we hope it’s just a matter of time!

Even before Pan-STARRS makes its first main-belt comet discovery, it is already assisting research on known main-belt comets. A new paper submitted to the Astronomical Journal earlier this week describes a worldwide observational campaign to study the most recently discovered main-belt comet named P/2010 R2 (La Sagra), or P/La Sagra for short. Pan-STARRS actually recorded the first known observations of this object, about a month before its official discovery, but unfortunately, it escaped our detection software at the time and was not found in our data until after it was discovered by others. Nonetheless, early Pan-STARRS observations of the comet played a key role in the monitoring of its activity over the year-long series of observations that we present in this new paper. In particular, these observations show the comet becoming steadily brighter over a period of months, strong evidence for ongoing dust emission, a characteristic signature of cometary activity, and confirmation that this object is indeed a true main-belt comet.

While an exciting start, we of course hope that this paper will not be the last that Pan-STARRS has to say about main-belt comets. Stay tuned…

The globar cluster M2. Credit PS1SC/Nigel Metcalfe

This month our image comes from the constellation of Aquarius. It’s M2, a globular cluster situated 37,000 light-years away. Globular cluster such as these are dense groups of some of the oldest stars known in our Galaxy.

The Ring Nebula, M57. Credit:PS1SC/Nigel Metcalfe

This month’s Pan-STARRS image of the Month is M57, the Ring Nebula. This is a planetary nebula, remains of the death-throws of a star of about the same mass as the Sun. The colour variations are caused by excited low density gas. Different colours indicate different elements or different levels of excitation. The dot in the centre is a white dwarf, the remnant of the now dead star which created the nebula.

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

Potentially Hazardous Object ST3 shown moving between two Pan-STARRS images.

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.

For this month’s PS1 Image of the Month we’re bringing you an unnamed cluster of galaxies found in one of the Pan-STARRS1 Medium Deep fields. While it hasn’t got a name, it’s certainly got pretty galaxies.

For this month’s PS1 Image of the Month we’re bringing you an unnamed cluster of galaxies found in one of the Pan-STARRS1 Medium Deep fields. While it hasn’t got a name, it’s certainly got pretty galaxies.

The history of the solar system deserves investigation because it provides the astronomical backstory for our civilization and suggests processes that might be at work around other stars that could form their own planetary systems. With just the 8 major planets, it would be difficult or impossible to get much information on how everything came to be. The additional insights provided by the properties of small bodies, leftovers of planet formation, provide valuable clues to the processes active in solar system history. For example, Pluto and other objects now known in similar orbits show strong evidence that the outer planets, particularly Neptune, have moved substantially from where they originally formed. A well-characterized survey of the wonderful icy bodies that orbit beyond Jupiter will continue to refine these theories in useful ways.

There is no question that Pan-STARRS 1 (PS1) is a unique telescope that can provide unique results. There have been other surveys of the sky, but none have the deep, complete, multi-color, multi-epoch observations of PS1. Such a telescope and survey allow for a wealth of new investigations in a variety of different fields, including those relevant to the history of the solar system.

The organization of the PS1 Science Consortium has been focused on 12 Key Projects, ranging from the inner solar system to distant galaxies. The Outer Solar System Key Project (PS1OSS) is led by my advisor, Matt Holman at the Harvard-Smithsonian Center for Astrophysics. We’re interested in the small body populations beyond the orbit of Jupiter, including the Kuiper belt and its citizens (like Pluto) and to the farthest reaches of our solar system. Similar searches have been undertaken before but since small bodies are only visible by reflected sunlight (and even though these faraway bodies are much larger than asteroids) they are so faint that only recent astronomical surveys have been the first to really understand what’s out there.

The outer solar system hosts a variety of interesting icy bodies with a range of sizes. PS1 will be sensitive to objects as small as tens of kilometers miles in diameter for the closest objects up to Jupiter-sized objects (if there are any) well beyond the edge of the known population at about 100 times the distance from the Sun to the Earth (i.e., 100 AU). These objects are usually classified by the way they orbit the Sun: Centaurs are in chaotic orbits pushed around by the giant planets (and will become comets), Trojans are in stable orbits that lead or trail the giant planets, and Kuiper belt objects (KBOs or Trans-Neptunian Objects or TNOs) are in mostly stable orbits beyond Neptune. There are also objects in very distant orbits like Sedna; we don’t know how they got there, but finding more of them will teach us something interesting about how the solar system formed.

The vast majority of solar system objects are too small to appear different from a single point of light, even for the best telescopes in the world. How will PS1 distinguish a solar system object from every other point of light out there? The Greeks knew that some stars didn’t stay in fixed relative locations and these wandering stars were called “planetes” from the Greek word for wanderer. In a combination of the motion of the Earth and the motion of the small body, these objects move relative to the fixed background of stars.

My favorite illustration of the first effect, known as parallax, is to stick out your arm with your thumb up and to close one eye and then the other. Your thumb appears to move relative to the background. Bringing your thumb closer causes larger apparent motion. In the same way, the motion of the Earth in its orbit provides different vantage points which are like looking through one eye and then the other and objects in our solar system move appreciably compared to the more plentiful stars.

Motion of Pluto between two different Pan-STARRS1 images. Credit: PS1SC

This image shows this effect on Pluto as observed by PS1 and how it changes position in a 15 minute interval, due to parallax.

Both the inner and outer solar system key projects of PS1 look for moving points of light to identify solar system objects. This is more challenging than it might seem at first glance, especially if you imagine a crowded field of stars. The sheer amount of PS1 data makes it impossible to imagine a human-eye search, so automated routines have been developed to find these objects. The inner solar system team uses the Moving Objects Processing System or MOPS, but a more specific algorithm can be used in the outer solar system since these objects move much more slowly (mostly because they are more distant, but also because their orbital motion around the Sun is slower in accordance with Kepler’s Third Law). To really identify an object and understand its motion around the Sun, it has to be identified by its motion, usually during one night, and then a giant and complex connect-the-dots algorithm is employed to eliminate false positives and track the motion of real solar systems objects over weeks and months and years.

Now, the PS1OSS survey is not the first survey of the Kuiper belt. Other surveys have been done, but PS1OSS has some advantages that other surveys do not. With a location in Maui, PS1 can see both North and South of the Earth’s orbit, covering virtually the entire region where solar system objects appear on the sky. The multi-epoch observations by PS1 allow us to track the orbits properly. (Some past surveys did not have the giant field of view that we have and so lost some objects; we will find these and link their orbits together using data from both surveys.) One aspect I’m really excited about is the precise color observations that PS1 obtains… by using different filters (glass screens that only allow certain colors to pass through), we’ll be able to identify the color of many outer solar system objects. This has been done for small groups of objects, but the comprehensive and precise color survey of PS1 will be new and exciting.

Finally, the all-sky nature of the survey will ensure that we didn’t miss anything significant. Past surveys have not been as complete as PS1 will be. The best example of a near-complete outer solar system survey is that of Mike Brown at Caltech (my former advisor) who discovered dwarf planets Eris (the object larger than Pluto), Haumea, and Makemake and other interesting objects like Sedna. Mike’s survey covered most of the sky, but there’s still a good chance that 1-3 dwarf planets were missed and that we’ll be the first to catch these intriguing worlds. We may also find another Sedna-like object, which would be a true boon to interpreting how the planets formed.  And we’ll have a well-characterized survey that can help us take an accurate census of just what is out there.

The PS1OSS Key Project is hard at work identifying objects, determining their orbits, measuring colors, and generally putting together the full picture of the outer solar system. The orbits and distribution of these small bodies are the fingerprint of processes at work in the history of our solar system. We’re all looking forward to the results of the unique PS1 survey in our interpretation of that history.