Inner Solar System
From PS1wiki
ISS Science Server
see: Moving Object Processing System (MOPS)
Definitions and Acronyms
In alphabetical order.
Amors Solar system objects (asteroids and comets) with p>1.017AU.
Apollos Solar system objects (asteroids and comets) with q<1.017AU and a>1.0AU.
Atens Solar system objects (asteroids and comets) with a<1.0AU.
IEO (Interior to Earth's Object) Solar system objects (asteroids and comets) with aphelion (q) less than 0.987AU.
MBO (Main Belt Objects) Solar system objects (asteroids and comets) with p>1.3AU (not NEOs) and extending out to about a=4.5AU (beyond the Hilda region near 4.0AU).
MIS (Miscellaneous)
MOID (Minimum Orbital Intersection Distance) The minimum distance between two orbits. Usually in reference to the MOID with Earth but possibly in reference to other planets where it should be explicitly called out as such.
NEO (Near Earth Objects) Solar system objects (asteroids and comets) with perihelion (p) 1.3AU.
PHO (Potentially Hazardous Objects) Solar system objects (asteroids and comets) with MOID<0.05AU
TRO (Jupiter Trojans) Solar system objects (asteroids and comets) in a 1:1 resonance with Jupiter.
VEMT (Venus, Earth and Mars Trojans) Solar system objects in a 1:1 resonance with Venus, Earth or Mars.
ALG (Algorithms) Algorithmic research to improve MOPS operations.
People
Key Project Lead: Robert Jedicke, IfA
Key Project Email List: iss_at_ps1sc.org, or perferably use ss_at_ps1sc.org for both Inner and Outer Solar System Key Projects.
Lead Email: jedicke_at_ifa.hawaii.edu
Members
Member institutions in alphabetical order. Members in alphabetical order.
Harvard-Smithsonian Center for Astrophysics (CfA, Cambridge, MA)
Tommy Grav - Short- and long-period comets, photometry, rotational and phase light curves.
Dan Green - comets
Matt Holman
Pavlos Protopapas
Kyle Smalley MPC orbits
Tim Spahr MPC orbits
Gareth Williams
Institute for Astronomy, University of Hawaii
Bobby Bus spectroscopic followup characterization
Ken Chambers
Mikael Granvik NEOs
Jim Heasley
Henry Hsieh MBCs
Robert Jedicke Key Project Lead MOPS Manager
David Jewitt KBOs, Comets
Nick Kaiser NEO hazard assessment
Jan Kleyna MBC
Pedro Lacerda, Lightcurves
Mike Maberry NEO imaging
Joe Masiero, MB light curves, NEAs for polarimetric followup/albedo measurements
Karen Meech
Jeff Morgan
David Tholen NEOs, KBOs
Alan Tokunaga, NEO followup with the Infrared Telescope Facility
Richard Wainscoat - NEOs, hazard assessment, search for Mars satellites, comets, unusual objects
Mark Willman MB
Johns Hopkins University (JHU, Baltimore, MD)
Andy Rivkin asteroid spectroscopy
Hal Weaver fragmenting minor bodies, SFD and rotation characteristics of comet nucleii
Paul Feldman fragmenting minor bodies
Casey Lisse fragmenting minor bodies, SFD and rotation characteristics of comet nucleii
Andy Cheng fragmenting minor bodies
United Kingdom Institutions
QUB = Queens University Belfast
Damian Christian
Sam Duddy
Alan Fitzsimmons, small NEO colours and spectra, dormant comets (low-Tj NEOs)
Carlos Frenk
Henry Hsieh
Institute of Astronomy, National Central University
Shinsuke Abe
Wen-Ping Chen
Ying-Tung Chen
Wing Ip
Daisuke Kinoshita
H. C. Lin
ZhongYi Lin
Wendy Tseng
Chan-Kao, Chang (Rex)
External
Josef Durech, Helsinki, sparse lightcurves
Mikko Kaasalainen, Helsinki, sparse lightcurves
Andrea Milani, AstDys, orbit determination and proper elements
Bob McMillan, U. Arizona, Spacewatch, astrometric and photometric followup
Lynn Jones LSST/UW
Meetings
PS1 Solar System Science Meetings
ISS; 2nd Tuesday of every month @ 1:30pm HST (23:30pm UT)
in the IfA's Moon Room
Polycom: 128.171.2.215
Telecon: N/A
OSS; 4th Tuesday of every month @ 1:30pm HST (23:30pm UT)
in the CfA's Tea Room
Polycom: 131.142.144.53
Solar System Science Meeting minutes (after 2008 01 22)
Solar System Science Meeting minutes (prior to 2008 01 22)
MOPS Meetings
2:00pm HST (00:00 UT) every Monday at Pan-STARRS HQ, Manoa Information Center
Polycom: 128.171.177.136
Telecon: N/A
Science Questions
How did the solar system form?
- signatures of the primordial distribution of objects in the MB?
- in remant populations like the Hungarias?
- where/when did differentiation occur?
- how big did objects have to be before they differentiated?
- is there a helioentric distance dependence to differentiation?
- signatures of the late heavy bombardment in the Main Belt?
- where is/was the snow line?
How is the solar system continuing to evolve?
- what are the inter-relationships between the various small body population?
- what are collision rates between objects?
- what are the dynamical transfer mechanisms within and between populations?
- how important is the Yarkovsky effect?
- does YORP modify the spin rate or pole orientation of asteroids?
What are the physical properties of asteroids and comets?
- how strong are they and how does strength vary with heliocentric distance, size or other parameters?
- are they monolithic or rubble piles?
- what physical processes contribute to space weathering?
- do the physical properties of comets evolve in time?
Where was/is the water?
- how much water is currently stored in the small bodies?
- where was the snow line during solar system formation?
- where is it now?
- can water-ice survive inside asteroids?
What is the impact rate on the Earth
- is the NEO population in steady state?
- are there asteroid 'streams' that produce increased impact risk at some times of year?
- are there asteroid families?
- where do the NEOs come from?
- what is the size distribution of NEOs?
- what is their diameter distribution?
- what is the impact energy distribution?
Research Topics
Near Earth Objects (NEO)
NEOs are those objects with perihelion (p) 1.3AU.
Subclassifications of the NEOs are the
- PHOs with MOID<0.05AU,
- Amors with p>1.017AU,
- Apollos with q<1.p17AU and a>1.0au,
- Atens with a<1.0AU and
- IEOs with aphelion (q) less than 0.987AU.
The Population of Asteroids Interior to Earth's Orbit (Tholen, Kinoshita)
Existing NEO surveys have concentrated on the opposition region, thus creating a bias against the discovery of objects that are more readily found interior to the Earth's orbit. To more accurately assess the unbiased impact hazard, this population needs to be characterized more thoroughly.
Hazard Assessment (Kaiser, Wainscoat, Jedicke, Kinoshita, Milani, Granvik, Abe)
One of the most important early products of PS1 will be improvements in the assessments of what level of hazard NEOs pose. This will be critically important for future funding. Of all the things that PS1 does, this may the most visible, and most easily understood by the general public.
A self-consistent program of "asteroid imapct prevention" requires five steps.
1) Detection : observations of NEOs (in particular of Potentially Hazardous Asteroids (PHA)) have to be collected. This is part of the main business of PS1, which is expected to increase at least tenfold the rate of detection of NEOs. To show efficiency in this is critical for both the public perception and the funding of PS1.
2) Orbit Determination : the observations belonging to the same physical object have to be assembled (a process called identification), the corresponding orbits have to be computed and quality control has to be applied to the results (statistics of the observation residuals, removal of discordant identifications). Then the already known objects are identified and the discoveries of new ones have to be made public. This is the main business of the MOPS subsystem, and is also critical for PR and funding.
3) Impact Monitoring : given a known object with the possible orbits constrained by the available observations, it is necessary to compute whether it could impact the Earth in the near future. This task is presently performed, covering the next 80-100 years,q by the two automated Impact Monitoring Software Robots, CLOMON2, part of the NEOdyS information system at the Universities of Pisa and Valladolid [1] and SENTRY at JPL NEO Program Office [2]. These two data processing systems operate ONLY on public data, thus the NEO observations and orbits must be made public before (or at least simultaneously) passing the data to NEODyS and JPL. This implies a very stringent requirement of timely data release; any unjustified delay woul have a negative PR and funding effect which could offset the benefits from the enormous contributions by PS1 to steps 1 and 2.
4) Follow up : If a NEO is found to have a Virtual Impactor (VI), that is a possibility of imapct with the Earth in th next 80-100 years, not contradicted by the presently available observations, then new observations need to be gathered to establish whether this impact is really going to happen. PS1 will to some extent perform automated follow up of its own discovery, e.g., with the observing nights in the same area at the next lunation. The cases not covered by this need to be addressed by specific agreement for targeted follow up with other telescopes. Physical observations should also be included, in particular to assess the size and mass of the threatening object, its possible binary nature, etc..
5) Deflection : If the follow up observations, possibly including radar observations and/or spacecraft data, confirm the reality of the future impact, it is necessay to have available the technology and the know how to slightly change the orbit of the dangerous object, enough to avoid the predicted collision (and with care to avoid other ones). This is beyond the scope of the PS1 project, although it is possible that a really impacting object (most likley a very small one) is discovered by PS1, thus starting also some deflection project.
One short comment on the difference between NEO impact risk and Near Earth Asteroids (NEA) impact risk. According to the current (uncertain) models, comets could represent ~10% of the impact risk. While short periodic comets are handled by PS1 in exactly the same way as asteroids for steps 1 and 2, there are already difficulties in step 3 due to non-gravitational perturbations. Thus the determination of the main parameters of the non-gravitational effects on the orbit of comets needs to be part of the PS1 research program, for hazard assessment on top of the obvious scientific interest. Long periodic comets also contribute to the impact risk, although their total contribution is very poorly constrained. A better assessment of the long periodic comets impact risk can be obtained thanks to the much larger statistics and much more accurate orbits resulting from PS1.
Size-frequency, orbit distribution, source populations and production rates (Jedicke, Wainscoat, Kinoshita, Granvik)
We will determine the bias corrected size and orbit distribution of the NEOs by fitting the observed distribution to an observationally biased theoretical distribution of NEO source populations. In doing so we automatically determine the production rate and absolute number contribution from each source region to the NEO population.
[ Bottke et al. (2002)] used 138 NEOs discovered by the { Spacewatch} project to determine their debiased orbit and size distribution. We will re-create and extend their technique using the vastly larger and superior bias determination for the PS1 data set.
The technique involves fitting the observed orbit and absolute magnitude distribution n(a,e,i,H) to the theoretical distribution N(a,e,i,H) modified by the bias B(a,e,i,H). The theoretical orbit distribution is a linear combination (determined in the fit) of orbit distributions from each of many possible NEO source regions Si(a,e,i) (e.g. the ν6 and 3:1 resonances, Jupiter Family Comets). The theoretical H distribution may simply be a power law (10αH) with the same exponent for all sources (i) or we may allow α to be different for each source. Thus, we fit:
| n(a,e,i,H) = B(a,e,i,H)10αH | ∑ | fiSi(a,e,i) |
| i |
| ∑ | fi = 1 |
| i |
for the fi and α.
The difficulty lies in determining B(a,e,i,H) and the Si(a,e,i). The bias is determined using the PS1 bias calculator - essentially a simulation of PS1+MOPS all the way through final orbit determination of the NEOs. The Si(a,e,i) are determined through integrating test particles placed in the source regions until all particles have been either ejected from the solar system or collided with a planet or the Sun. The 'residence time probability' distribution created by following the particles as they traverse the (a,e,i) space provides the steady state distribution Si(a,e,i) of the objects from the source.
Rotation rates, shapes and pole orientations (Jedicke, Kinoshita)
We will measure the bias-corrected rotation rates, shapes and pole orientations for hundreds of NEOs using sparse light curve inversion techniques.
Rotation rates are currently known for only about 100 NEOs. Pole orientations are known for tens while accurate shapes are known for a couple of spacecraft targets and, much less accurately, asteroids that have been observed with stellar occultations, radar and other techniques. The exquisite photometric precision and long term coverage offered by PS1 provides an opportunity to use sparse light curve inversion techniques (e.g. [ Durech et al. 2006], [ Kaasalainen et. al. 199X]) to determine rough rotation rates, shapes and pole orientations for hundreds of NEOs.
The rotation rates, shapes and pole orientations of asteroids are determined by their collision or tidal disruption history, internal structure, surface properties and the [ YORP] effect (a variation on the [ Yarkovsky] effect that has the ability of spinning up or slowing down asteriod rotation rates and can also affect the pole orientation). The large sample of objects provided by PS1 will enable studies that are impossible with today's data.
At this time the actual distribution of asteroid spin rates is unknown. The expectation is that for a relaxed set of asteroids the 'normalized' spin rates should have a Maxwellian distribution. This distribution is observed for the largest asteroids but the distributions for moderate and small asteroids are skewed. It is unlikely that the skew is due to observational selection effects but no bias-corrected rotation survey exists. The non-maxwellian distribution is more likely due to some dynamical process (YORP) that modifies the natural distribution.
Many NEOs appear to be in unusual spin states e.g. spinning at rates near their breakup limit or in complex rotation. A large and unbiased survey of the NEO rotation statistics will provide the basis for understanding these observations.
NEO Families (Jedicke, Kinoshita, Milani)
We will use the large sample of NEOs detected by PS1 to search for genetically related families of NEOs.
There are no confirmed NEO families. [ Drummond (199X)] reported the discovery of dozens of families while searching for over-densities in the distribution of NEO orbital elements but [ Fu et al. (2005)] showed that most likely none of those associations were real. On the other hand, [ Fu et al. (2005)] identified a technique for discovering and confirming NEO families. The technique has not yet been applied on the real population of NEOs and it must be applied to NEO discoveries with PS1.
The discovery of NEO families will be interesting because of their implications for understanding the collisional environment within the solar system and, more practically, the implication for increased probability of an asteroid impact with the Earth. If a family producing event occured with a parent body on an Earth-crossing orbit it could significantly increase the risk of an impact.
Another technique uses proper elements, which can be computed for NEOs although they have very different properties with respect to the main belt case [3]; in particular the stability of the proper elements is limited to a much shorter time span [4]. The main application of proper elements for NEOs is to look for parent bodies of meteor streams, although there are also implications for impact risk assessment.
Many Earth approaching asteroids appear to be binary based on radar (e.g. [ Ostro et al.]) or photometric methods (e.g. [ Pravec et al.]) and the members of the pair are often in strange rotation states. It is possible that these observations can be explained by spin-up and tidal disruption of the asteroids on previous close passages by the Earth. If this is the case, we should see evidence of tidally disrupted families in the NEO population once PS1 discovers a large fraction of the objects down to 300m in diameter. The measurement of the member's rotation rates, shapes and pole orientations will be useful to determine the effects of tidal disruptions. e.g. in determining the mechanical strength of asteroids.
NEO Characterization (Tokunaga, Kinoshita, Abe)
Using the IRTF for characterization of NEOs.
The 3-m NASA Infrared Telescope Facility (IRTF) is a dedicated telescope for mission support and planetary science research. It is likely that substantial amounts of time on the IRTF can be committed to NEO characterization-- determination of surface composition, albedo, size, and rotation state.
The primary instrument being used for characterization of NEOs is SpeX, a 1-5 micron moderate resolution spectrograph.
Consideration is being given to the construction of new instruments to support NEO characterization. These include: an efficient polarimeter to obtain albedo and size estimates, a multicolor photometer for the 0.4-2.5 micron spectra range for taxonomic studies, and a visible wavelength spectrograph to complement SpeX.
Construction of the instruments for NEO followup is under discussion. An instrument we are pursuing is a polarimeter that gets all of the Stokes parameters in a single exposure. An instrument that has already been built to do this is described by:
Pernechele, C., Giro, E., Gantinel, D. (2003). Device for Optical Linear Polarization Measurements with a Single Exposure, SPIE 4843, 156-163.
Followup work may be propsed through the regular time allocation process (see the IRTF web site above). Note that there is an MIT-IRTF NEO survey underway using SpeX at 0.8-2.5 micron. A web site describing this can be found at the IRTF web site and also at SMASS.
All spectra obtained under this program are made public within a few days of acquiring the data.
For a detailed summary of following up opportunities see NEO Characterization with the IRTF
Extreme lightcurve objects (Lacerda, Kinoshita)
We will search for extreme lightcurves in the NEO population using sparse sampled data.
These data will be used to select candidates for follow-up observations. Detailed rotational properties (lightcurve period and range) will be measured during follow-up observations and will be used to derive the spin period, as well as limits on the shape and density for individual bodies. This project focuses on extreme, high range lightcurves, likely due to contact binaries. The fraction of extreme lightcurves also constrains the proportion of this type of binaries among the population, which was can used to test formation and destruction theories. It can also serve as a useful comparison between classes of bodies.
Search for asteroidal parent bodies of meteor streams (Kinoshita, Abe)
There are some meteor streams whose parent bodies are thought to be asteroidal objects. The Geminid meteor stream and an asteroid (3200) Phaethon is an example. The fragmentation of asteroids may also be the source of dust particles and those ejected dust particles form meteor streams. The fragmentation may be caused by thermal stress or collisions. The study of fresh surface of identified fragments provide us good opportunities to study the degree of space-weathering. I try to search and identify asteroidal parent bodies of meteor streams. This work will lead to better understanding of fragmentation process, space-weathering, internal structure of asteroids, and formation of meteor streams.
Meteor/Meteorite associations compared with families in MBO (Abe, Kinoshita)
Generalizations about meteor and meteorite parent bodies and dynamic evolution of them are incomplete. Most of meteors have been treated as cometary origin, while meteorites are usually thought to be associated with asteroids. It is generally expected that the mineralogy of the asteroids that are most likely to hit Earth should reflect those of the most common meteorites. None of meteorite is associated with known solar system small bodies by their orbits so far. Spectroscopic observations of more than 400 NEAs in visible wavelengths show that 65% of NEAs have S- and Q-type spectral properties that are thought to be the source of LL chondrite meteorites. However, only 8% of all meteorite falls are classified as LL chondrite type, that is in most debate. We are going to identify meteor/meteorite association bodies and their possible sources in the Main Belt region, e.g., Flora, Gefion families.
Trojans of Venus, Earth and Mars (VEM)
VEM are those objects in a 1:1 resonance with a planet.
Discovery and Population Studies (Jedicke, Wainscoat)
The origin and evolution of trojans of the inner planets is must be different from that of Jupiter. Neither is understood well.
The typical lifetime of inner-planet trojans trapped in a secular resonance is 1Myr for Venus, Earth and Mars trojans (Brasser & Lehto, 2002).
PS1 is not anticipated to survey close enough to the Sun to discover Venus trojans.
No 'traditional' Earth Trojans are known. The PS1 sweet spot survey will cover about half of the area of the sky where Earth Trojans would be located, with the half at solar elongation < 60 degrees missed.
Satellites of Planets
Objects that orbit the terrestrial planets and Jupiter.
Satellites of the Earth (Wainscoat)
We will discover objects orbiting the Earth and assess whether they are natural or artificial. We will run complementary numerical simulations on capturing objects into Earth orbit.
We will discover objects orbiting the Earth. The difficulty will lie in determining whether or not they are natural or artificial. We are, of course, most interested in the natural objects since the discovery of even a single natural object (besides the Moon) in Earth orbit will be of tremendous scientific interest.
We will need to identify geocentric orbit determination software. We can compare the derived orbit to the list of orbits of published artificial satellites but this is probably not sufficient as there are many unpublished and even unknown articial objects orbiting the Earth. We may obtain colors from PS1 observations and from targetted followup at other partner and external member sites. If the object is bright enough we will attempt spectroscopic followup in the visibile and NIR.
The number and sizes of such objects are of great interest, along with their likely origin, the stability of their orbits, and the impact threat that they pose. If an object id identified it will provide an unprecedented opportunity for a space mission. If the object is small enough it could be returned intact without having to pass through the atmosphere.
We will determine likely orbits for captured natural objects by integrating approaches of NEOs by the Earth-Moon system and determining which (if any) get captured). The types of orbits identified in these simulations may be used to guide the search for natural Earth satellites.
Satellites of Mars (Wainscoat)
If Mars has any small satellites, PS1 is well suited to detecting them.
Satellites of Jupiter (Wainscoat, Grav)
If Jupiter has any undiscovered small satellites, PS1 is well suited to detecting them. In addition the Pan-STARRS system will be well suited to observations of known irregular satellites of Jupiter to help determine their dynamical and physical properties (especially their phase functions).
Main Belt Objects (MBO)
MBO are those objects with p>1.3AU (not NEOs) and extending out to about a=4.5AU (beyond the Hilda region near 4.0AU).
Size-Frequency Distribution (Jedicke, Ip, Kinoshita)
We will measure the size-frequency distribution (SFD - i.e. the number of objects as a function of size or absolute magnitude) of MBO. Modelling of the evolution of the SFD will provide
- the primordial SFD after accretion in the solar system had completed,
- the strength of MB objects as a function of their size,
- MB collision rates as a function of time,
- constraints on the family production rate.
Previous and recent MB surveys (e.g. [], [], [], []) rarely detect objects smaller than 1 km in diameter and this is true only in the inner edge of belt. Deep surveys for the smallest objects are limited by the small number of objects they detect. Shallow surveys that cover a wide area of sky are similarly limited . Almost all the surveys have been limited by their inability to determine accurate distances and therefore absolute magnitudes (H) for the objects.
PS1 will be the first deep wide-field asteroid survey. Furthermore it will have well characterized detection efficiency allowing accurate compensation for observational selection effects. The PS1 detection limit is about 750m diameters in the inner region of the MB for objects with a mean S/C albedo. Since S-class asteroids have a higher albedo, the size detection limit is actually smaller for these intrinsically brighter objects.
The size-frequency distribution (SFD) of MBO is determined by the original SFD of the objects in the primordial disk after accretion processes had been halted (through dynamical pumping of their orbits to high eccentricity and inclination by gravitational interactions with Jupiter and Saturn) and by their ongoing size evolution due to collisions (cratering, catastrophic disruption). Thus, their existing SFD is a relic of processes that may be modelled in order to determine the MB's history and continuing evolution.
PS1 will provide the first deep high-statistics wide-field asteroid survey that will provide the best measurement of the MB SFD after correcting for observational selection effects. Since all the objects will be identified and tracked by the same quality-controlled system we will be able to, for the first time, accurately determine the orbital parameters (and therefore distance) and absolutel magnitude for a vast number of objects.
The modelling process provides information about the bulk strength of the MBO as a function of their size and the physics of impacts between large objects. The modelling has been used to explain 'bumps' in the SFD as either
- a relic of a primordial bump in the SFD or
- due to the Poynting-Robertson drag removal of the smallest dust particles which then causes a cascade in the size distribution to larger sizes or
- due to a size-dependent asteroid strength or
- a combination of these effects.
The latest hydrodynamic simulations (e.g. []) and modelling of MBO evolution (e.g. []) suggest that asteroids about 200m in diameter are the weakest in terms of energy required to catastrophically disrupt them per unit mass. Since these objects are weak they should be easy to disrupt and there should be relatively fewer of them in the MB than expected by extrapolating to small sizes from larger objects. The reduced number and strength of objects in this size range also causes a ripple effect to a wavy size distribution at large sizes.
Real-time detections of collisions (Jedicke, Wainscoat, WP Chen, Abe)
We will measure or set a limit on the collision rate of MB objects too small to detect directly with PS1. We will do this by searching for signatures of the transient dust clouds produced in the catastrophic collision of two objects that are otherwise too small to detect OR by detecting transient increases in the brightness of asteroids. This will allow us to
- test whether the SFD measured for the larger MB objects can be extrapolated to smaller sizes,
- test and refine collision models,
- understand the physical structure of asteroids
The catastrophic disruption rate for 10m diameter asteroids in the MB is expected to be about 10 − X per year. However, since there are so many objects of this size, we expect there to be roughly one catastrophic disruption of an object this size every day. The PS1 3π survey covers an area about 60o wide on the ecliptic 3 times every lunation within about a couple weeks. Thus, we expect to image about one catastrophic disruption of an object 10m in diameter every week.
As the dust cloud from a catastrophic disruption expands its apparent brightness increases as long as the optical depth τ > 1 after which the clouds brightness will decrease. A 10m diameter asteroid's disruption could create a dust cloud 1 km in diameter which would have the apparent brightness of a 1 km diameter asteroid (easily detected by PS1).
The difficulty lies in knowing the expansion rate of the dust cloud and therefore determining how long the cloud is visible to PS1. If the cloud is visible for many days to a week we might detect the expanding dust cloud on each of three nights during a lunation. The brightness of the cloud could vary dramatically from night-to-night and it will be impossible to precover or attribute the object. If the dust cloud does not last that long it is possible that we will detect bright but 'orphaned' tracklets that are impossible to link to other tracklets.
It may also be possible to detect the collision of small objects into larger objects that are easily detected by PS1. By continuously monitoring many objects over the PS1 operational lifetime we can search for unusual and unrepeated brightening of asteroids as a signature of a recent collision.
With a sufficient number of collisions we may determine the collision rate of these objects. The rate at which the dust clouds brighten and fade will provide details on the physical structure of the asteroids. Color measurements or detailed spectroscopic followup of the dust clouds will provide information of the dust properties.
Asteroid Families (Milani)
Both the dynamical structure of the Main Belt and the collisional history of MBOs cannot be studied from the distribution of osculating orbital elements, because they are changed over short and medium time scales (from a few years to a few hundred thousands years) by planetary perturbations. The needed information is contained in proper elements, which can be computed as a function of osculating elements by complex procedures, including analytic, semianalytic and numerical algorithms. They are stable for timescales of millions of years, at an accuracy of the order of 0.001, thus allowing to detect the signature of dynamical long term evolution (secular resonances, Yarkovsky effect).
The concentrations of asteroids in the proper elements space may indicate the common origin from the collisional diruption of a single parent body. This needs to be confirmed by the statistical significance of the clustering Zappala' et al. 1994 and by consistent spectral properties Ivezic et al. 2007, Figure 6. Only after this confirmation the clumps of objects can be considered asteroid families. The outstanding success of the family classifications in providing a consistent collisional history and the first successes in identifying young families Milani and Farinella 1994 make this a primary science goal for the a survey capable of increasing by an order of magnitude the set of MBA discovered and with accurate, multiopposition orbits.
Currently the primary source for proper elements of MBA is the AstDyS online system [5] (the data are also mirrored in the NASA DPS system); the algorithms are described in: for analytic proper elements in Milani and Knezevic 1990 Milani and Knezevic 1992Milani and Knezevic 1994, for synhtetic proper elements Knezevic and Milani 2000 Knezevic and Milani 2003; for a review Knezevic et al. 2002. The proper elements of the MBA discovered by the PS1 survey need to be computed as soon as a multiopposition orbit is available. Athough the algorithms are well established, the computaional task is significant and techniques of parallel processing need to be used.
Once proper elements are available, the classification of MBA in the already recognized families needs to use algorithms different from the ones used for the taxonomy (i.e., the definition of new families). This because the number density of MBA in proper elements space may become, with PS1 dicoveries, too high to avoid the phenomenon of chaining, resulting in dificulty in separating nearby families. These algorithms are being developed, but the real test will be the PS1 output.
New families, with smaller parent body, can be identified and added to the known classification. This done is using the proper elements, and should not be confused with the search for very small and comparatively recent families, which can use also osculating elements (see the next subsection).
New Young Families (Jedicke, Kinoshita, Abe)
We will identify new extremely young asteroid families. As new MB objects are identified we will automatically run software that determines whether other objects with similar orbits exist. The number of new young families constrains models of the SFD. The families themselves provide excellent followup targets for space weathering and physical studies.'
The field of asteroid family studies has provided a great deal of information about the asteroids collision history, rates and strengths. The number of asteroid families provides constraints on models of the MB's collisional evolution. Most of the known MB families contain hundreds or thousands of asteroids and are the remnants of massive collisions that took place billions of years ago. Their size and orbit distribution provides insight into the physical mechanisms at play in catastrophic collisions far beyond the scale of any created by humans.
More recently, new small asteroid families have been detected that are only hundred of thousands or millions of years old (e.g. [ Nesvorny et al. 200X], [ Nesvorny et al., 200X]). The size distribution of the parent bodies of these young small families provide tight constraints on the current collision rate within the MB. The size distribution of the objects within a family provides insight into the physical impact and fracturing processes. The orbit distribution of the small families provides information on the collision rate as a function of position in the belt while the differential orbit distribution within a family can provide physical information about the asteroids. e.g. Yarkovsky effect. In order to make useful statements about either SFD or the orbit distribution of young families we require a good understanding of the observational selection effects inherent in the survey.
MOPS already provides utilities for determining the orbital similarity between two objects for the process of orbit identification. We currently use the D-criterion ([ Drummond, 199X]) but other techniques are also available ([ Nesvorny et al., 2006]). As new objects are discovered or orbits are updated (due to precovery, attribution, identification) we will run software that compares each orbit to every other one and identify clusters in the osculating (or perhaps proper) element space. Once a statistically significant cluster of objects is identified we will determine the probability that the association is real through 1) statistical studies of the nearby orbital element phase space and 2) backwards integrating the orbits of all the objects to determine if their orbit elements converge at some time in the recent past.
We will determine the actual orbit and size distribution of small asteroid families from the observed distribution using MOPS's ability to characterize the observational selection effects for the survey.
Space Weathering Rate determination (Jedicke, Kinoshita, Abe)
We will obtain followup spectra of asteroids in the young families identified above using the large telescopes on Mauna Kea and elsewhere around the world. We will determine their spectral type and, for those families composed of S- or C-type asteroids, use the spectra to determine the rate of space weathering. The young family members will have relatively unweathered surfaces compared to the older families.
The S-type asteroids are thought to be the parent bodies of the ordinary chondrite meteorites. This is due to dynamical arguments that suggest that the most frequent type of meteorite should originate from that part of the solar system with the highest probability of delivering meteorites to the Earth. The ν6 asteroid resonance defines and eats away at the dense inner edge of the MB and it is very efficient at delivering asteroids to the Earth. The asteroids closest to the ν6 resonance are predominantly S-types. The problem was that the surface spectra of freshly cut ordinary chondrites (Q-type spectra) are quite different from the remote spectra for the S-type asteroids. The S-type asteroid spectra have lower albedo, are redder, and have reduced 1 and 2 micron band depths (usually associated with olivine and pyroxene). The proposed solution to the inconsistency was 'space weathering' due to exposure of the S asteroid's surface for billions of years to micrometeorite impacts, solar radiation, solar wind and cosmic rays.
While 'space weathering' has been observed to occur on lunar soil and rocks, and while there was good evidence for similar weathering occuring on S-type asteroids (a continuum of spectra between S and Q type), the rate of space weathering was only recently determined by [ Nesornvy et al. 2004]. Furthermore, this group reported the first detection of space weathering in C-type asteroids.
One of the problems with the determination of the space weathering rate is that while there are a large number of family members in old families there are few known members in young families. The determination of the space weathering rate is therefore skewed towards the older families without an anchor for the young surfaces. As PS1 discovers new young families we will obtain spectra for the family members to provide the anchor for the freshest asteroid surfaces.
Rotation rates, shapes and pole orientations (Jedicke, Kinoshita)
We will measure the bias-corrected rotation rates, shapes and pole orientations for tens of thousands of MB asteroids using sparse light curve inversion techniques.
Rotation rates are currently known for only about 1,000 asteroids. Pole orientations are known for perhaps a few hundred while accurate shapes are known for a handful of spacecraft targets and, much less accurately, asteroids that have been observed with stellar occultations, radar and other techniques. The exquisite photometric precision and long term coverage offered by PS1 provides an opportunity to use sparse light curve inversion techniques (e.g. [ Durech et al. 2006], [ Kaasalainen et. al. 199X]) to determine rough rotation rates, shapes and pole orientations for tens of thousands of MB asteroids.
The rotation rates, shapes and pole orientations of asteroids are determined by their collision history, internal structure, surface properties and the YORP effect (a variation on the Yarkovsky effect that has the ability of spinning up or slowing down asteriod rotation rates and can also affect the pole orientation). There is particular interest in the members of asteroid families where the physics of the catastrophic collision has implications for the spin rates and rotation vectors. The large sample of objects provided by PS1 will enable studies that are impossible with today's data.
At this time the actual distribution of asteroid spin rates is unknown. The expectation is that for a relaxed set of asteroids the 'normalized' spin rates should have a Maxwellian distribution. This distribution is observed for the largest asteroids but the distributions for moderate and small asteroids are skewed. It is unlikely that the skew is due to observational selection effects but no bias-corrected rotation survey exists. The non-maxwellian distribution is more likely due to some dynamical process (YORP) that modifies the natural distribution.
Basaltic Asteroids (Jedicke,Tholen, Ip, Kinoshita, Abe)
We will identify asteroids with potentially basaltic surfaces using PS1 colors and then obtain low resolution (~100-200) spectra of the objects to confirm or deny the identification.
Studies of the worldwide meteorite collection suggest that there are on the order of 60-70 parent bodies that heated sufficiently in the early solar system to undergo mineralogical differentiation. These asteroids were subsequently disrupted in catastrophic collisions, scattering the remant differentiated chunks of the parent body. Thus, we would expect to see the remains of many differentiated asteroids in the MB asteroid sample. The metal cores of these asteroids might be resistant to further catastrophic disruption and could be easily identified (unless they are covered in a stony rubble pile). The mantles and basaltic (magmatic) surfaces of the differentiated asteroids might also be easily identified through their characteristic V-type spectra. The problem is that, up until recently, few unique metal or basaltic asteroids were known. Thus, where are the parent bodies of the meteorites that were differentiated?
We will use the accurate and repeated color measurements of the asteroids to pre-select those candidates with colors that best match metallic or basaltic surfaces. PS1 will be particularly well suited to these measurements (in comparison to the SDSS) because the y-band at 1 micron is well matched to the deep olivine/pyroxene band in basaltic asteroid spectra. Once candidate asteroids are selected we will obtain low resolution (~100-200) spectra of the objects to determine their actual nature. With PS1's accurate magnitude estimates it is almost guaranteed that any object deemed interesting on the basis of color will also turn out to have an interesting spectrum.
Main Belt Comets (MBCs) (Kleyna, Meech,Ip, Kinoshita)
We will search for MB objects with comet-like activity, similar to recently observed by Hsieh and Jewitt (Science 2006, 312 561). The search will yield
- A precise estimate of the prevalence of MBCs
- An estimate of the size of the water reservoir contained in the MB
- Important information about the origin of Earth's water, and the acquisition of water by terrestrial planets in general
Until recently, it was believed that significant reservoirs of comets exist in only two places: the Kuiper belt, and the Oort cloud. Recently, objects with cometary activity were discovered in the MB as well. First, 133P/Elst-Pizzaro was seen to eject dust near perihelion (Elst IAU Circ. 1996 6456 1; Hsieh, Jewitt, & Fernandez AJ 2004 127 2997). Subsequently, P/2005 U1 (Read) was found to be active (Read et al. IAU Circ. 2005 8624 1), and Hsieh and Jewitt (Science 2006, 312 561) found a third active asteroid 1999 RE70 (176P).
All three objects were identified by their tails (see figure), presumably consisting of sublimation ejected dust. Because of the limited lifetime of a sublimating object, it is likely that the activity is a transient phenomenon, probably triggered by collisions. Hence there are probably many dormant objects for each active object.
PS1 promises to observe every asteroid tens of times, providing ample opportunities to detect even transient activity. We propose to detect active MBCs through excess light at large radii. For this purpose, bright transient PS1 detections will have additional photometric information stored, probably in the form of flux bins in polar coordinates. The figure shows the detection of 176P in existing UH 88" images using such a binning scheme - despite the near-invisibility of the tail even in the combined exposure, most of the individual 25 sub-exposures used to create the visual detection image show a 2% flux excess relative to the stars in the image. With the superior background subtraction of PS1, we expect to find MBCs with much lower activity levels than 176P.
Hsieh and Jewitt surveyed 300 asteroids, and found one new active MBC. If we pessimistically assume that PS1 will detect only 10 km or larger MBCs outside 3 AU, it follows that about 150 MBCs will be detected. More realistically, if we consider all asteroids larger than 1 km outside 2 AU as potential MBC candidates, the yield may be on the order of 10,000 MBCs.
High-order Mean Motion Resonances (Holman)
Extreme lightcurve objects (Lacerda)
We will search for extreme lightcurves in the MBA population using sparse sampled data.
These data will be used to select candidates for follow-up observations. Detailed rotational properties (lightcurve period and range) will be measured during follow-up observations and will be used to derive the spin period, as well as limits on the shape and density for individual bodies. This project focuses on extreme, high range lightcurves, likely due to contact binaries. The fraction of extreme lightcurves also constrains the proportion of this type of binaries among the population, which was can used to test formation and destruction theories.
Masses of Asteroids (Tholen, Granvik)
The large increase in the known population of main belt asteroids will cause a corresponding increase in the number of observable close approaches between the most massive asteroids and their smaller breathren. Coupled with the accurate astrometry that the Pan-STARRS pipeline will deliver, it should be possible to use the measured gravitational perturbation of the smaller object caused by the larger object to constrain the mass of the larger object. With sizes derived from occultation, radar, or radiometric methods, the bulk density of the object can be determined. This approach has been used with limited success on the few largest asteroids, but Pan-STARRS should make it possible to extend the work to a larger sample.
Phase-brightness relation of asteroids (Kinoshita, Grav, Durech)
The PS1 survey has typical observation interval of a week. This time interval is too sparse to construct rotational lightcurves of asteroids with the classical method, but it is suitable to construct the phase-brightness relation of asteroids. The phase-brightness relation is useful to classify asteroids into sub-groups (ex. C-type, S-type, M-type, etc.). There will be problems for irregularly shaped asteroids with unknown rotation period, since they exhibit brightness variation due to the rotation of the body. This program concentrate to deal with asteroids with small lightcurve amplitude, and try to establish typical phase-brightness relations of asteroids for each sub-groups. The results of this work will be used for taxonomic studies. Also, it helps to construct size-frequency distributions for each sub-groups after classifications of objects.
The phase-curves in different filters can also be used to constrain the colors of the individual objects.
Hilda Asteroids (Abe)
The Hilda-type asteroids reside in the 3:2 mean motion resonance with Jupiter, while that of Jovian Trojan is in the 1:1. There is substantial evidence that Jupiter Family Comets (JFCs) have evolved from the trans-Neptunian region. Since JFCs have very unstable orbits caused by strong perturbations of Jupiter, dynamical integrations suggested that Hilda asteroids are another continuous source of JFCs' supply. On the other, planetary migration in the early stage of the solar system had a major influence on the final architecture of giant planets and strongly influenced structure of small-body populations in the solar system. Nice model proposed that the population of Jupiter Trojan asteroids was destabilized and repopulated during this phase. Within the Nice model, the same probably occurs for populations of asteroids in the 3/2(Hilda group) resonances. In addition, only about 2 dozen asteroids were found in the region between Hildas (a=3.7-4.2 AU) and Jovian Trojan (a=4.97-5.4 AU), which is in the 4:3 mean motion resonance with Jupiter. All of their diameters are smaller than 10 km except for 279 Thule which diameter is estimated at 127 km (H=8.57, D-taxonomic classes). Search for additional Thule members are well worth doing, too.
Jupiter Trojans (TRO)
Jupiter trojans are those objects in a 1:1 resonance with Jupiter.
Family Identification and Studies (Jedicke, Grav, Milani)
We will calculate the proper elements for the Jupiter trojan asteroids and use the Hierarchical Clustering Method for identifying asteroid families.
Families produced in main belt asteroid collisions have provided interesting constraints on the formation and evolution of the belt. The size distribution of the fragments provides information about the strength and internal physical structure of asteroids. Their orbit distribution provides information about gravitational and non-gravitational (e.g. Yarkovsky) forces acting on the objects. Studies of families produced amongst the Jupiter trojan asteroids will extend these results to this population. Additionally, about 20% of the fragments in these collisions are ejected onto unstable orbits that soon become undistinguishable from the short period comets (SPC or JFC) (Marzari et al., 1995). Thus, studies of the production rates of Jupiter trojan families ties into the production rate of the short period comets.
There is only one known/proposed Jupiter trojan asteroid family(Milani 1993). The data sample provided by PS1 should provide tens of thousands of new and improved Trojan asteroid orbits suitable for the determination of proper elements and then for family identification. We will use the techniques of Knezevic et al., 2002 to determine proper elements and the Hierarchical Clustering Method (HCM, e. g. Cellino & Bendjoya, 1998) to identify the families; see Milani 1993 for an early attempt at this procedure, with results limited by the small number statistics.
Size and Orbit Distribution (Holman, Jedicke, Grav)
We will determine the unbiased population and orbital distribution of Jupiter Trojans.
This is essential for a careful comparison to the populations of Saturn, Uranus, and Neptune Trojans.
Jovian Trojans as a source population of Irregular Satellites (Grav)
Using the dynamical, color and phase properties determined by the PS1 observations for the Jovian Trojans we will explore the possibility that these objects are a source population for the Jovian irregular satellites.
Colors and phase light curves (Grav, Kinoshita, Durech)
The observations of the Trojan asteroids at multiple epochs will make it possible to determine accurate phase light curve information for a large number of their brightest members. We will explore the possibility of using the phase light curves to derive an independent taxonomy of the Trojans that can be used in searching for links among objects within and outside the population.
Extreme lightcurve objects (Lacerda, Grav)
We will search for extreme lightcurves in the Jovian Trojan population using sparse sampled data.
These data will be used to select candidates for follow-up observations. Detailed rotational properties (lightcurve period and range) will be measured during follow-up observations and will be used to derive the spin period, as well as limits on the shape and density for individual bodies. This project focuses on extreme, high range lightcurves, likely due to contact binaries. The fraction of extreme lightcurves also constrains the proportion of this type of binaries among the population, which was can used to test formation and destruction theories. It can also serve as a useful comparison between classes of bodies.
Short Period Comets (SPC)
These are also called the periodic comets and can be divided into the Jupiter family comets and the Halley family comets.
Demographics and Origin of the Short Period Comets (Grav, Wainscoat, Ip, ZhongYi Lin)
Fragmenting Short Period Comets (Weaver, Lisse, Feldman, Cheng, Grav, Ip, ZhongYi Lin)
By studying a randomly selected sample of 49 comets, Chen and Jewitt (1994) estimated that Jupiter family comets (JFC) typically fragment roughly once per century. The JFCs apparently suffer many of these splitting events over their lifetimes until they either become dormant (from volatile depletion or mantle build-up), or finally suffer a catastrophic fragmentation event. Based on a comparison of the observed and predicted number of dormant JFCs, Levison et al. (2002) concluded that the catastrophic fragmentation rate for JFCs was substantially smaller than for Oort cloud comets. Nevertheless, they estimated that up to 93% of the JFCs might eventually disappear from catastrophic fragmentation.
A Pan-STARRS survey of the entire JFC population, including objects newly-discovered by the survey, will provide the data that can narrow the considerable uncertainties in our current estimates of the JFC disruption rates, so that we can accurately assess the role played by fragmentation events on the evolution of JFCs.
A new population of short-period comets has recently been discovered (Hsieh & Jewitt 2006), the main belt comets (MBCs), so-called because they reside in the asteroid belt. Many more MBCs are likely to be discovered by Pan-STARRS, and their investigation will be led by a team from the University of Hawaii. Our group plans to work together with the Hawaii team to determine what role fragmentation plays in the evolution of the MBCs, and how that compares to fragmentation processing of the other minor bodies in the solar system.
The above describes only part of the JHU program to investigate the role played by fragmentation among the minor bodies of the solar system. If you are interested in reading more details about our program, please grab our white paper, in either MS Word or PDF format, at Weaver-JHU-PanSTARRS.
Dead Comets (Wainscoat)
There are likely many old/dead/inactive comet nuclei. These will have relatively small diameters and low albedos, and so will be very hard to detect unless they are close to the Earth; they would be expected to have cometlike orbits, rather than asteroid-like orbits, and therfore would be more likely discovered in the opposition region observations than in the sweet spots. They may representa an additional hazard.
Algorithms (ALG)
Includes linking and orbit determination alogrithmic research to improve MOPS operations.
Miscellaneous (MIS)
Variations in the Zodiacal Light (Morgan)
We will look for spatial, color, and temporal variations in the Zodiacal light using both PS1 and ISP data.
The PS1 observations offer a unique look at large scale structures in the solar system. One of the largest known solar system structures is the Zodiacal light. For many years there have been anomolous observations of color changes, shape changes, and correlated variations of the Zodiacal light with solar cycles and seasonal variations. Some observations have implied that the Zodiacal light is dimmed when in the lunar shadow. All of these observations will benefit greatly from the all-sky, multi-band temporal coverage provided by the PS1. Studies of these large scale structures will also provide useful benefits towards understanding photometry of all extended objects observed with PS1 and may be helpful in understanding terrestrial sources of variations in the night sky brightness.
Wide Binaries in the Inner Solar System (Tholen, Holman)
Binary systems allow for the opportunity to measure the mass of the system via Kepler's third law. Although the binary systems that have been discovered in the inner Solar System are not wide enough to permit direct observation from ground-based telescopes, it is possible that Pan-STARRS will discover some that are. The astrometric data will permit orbit solutions to be performed and system masses to be determined. Coupled with size estimates from other sources (Spitzer or other ground-based telescopes), it should be possible to constrain the densities of these systems. Once enough systems have had orbit determinations, it will also be possible to investigate whether there are dynamical similarities that could constrain formation mechanisms.
Asteroids in comet-like orbits (Kinoshita)
Currently about 30 asteroids are known to have Tisserand invariant smaller than 2.0, and they are called as Damocloids. These objects are possible dormant and/or extinct cometary nuclei. Nuclei of active comets are difficult to observe because of their optically thick coma. Damocloids are good targets to study the surface of cometary nuclei analogues. Also, the orbital study of this class of objects lead to the orbital evolution of Halley-type comets. I work on astrometry, multi-color photometry, phase-brightness relation, and lightcurve of Damocloids.
Current Projects
Observing Proposals
Submitted Observing Proposals
| PI | Institution | Time Period | Telescope | Instrument | Technique | Requested | Awarded | Comments |
|---|---|---|---|---|---|---|---|---|
| Fitzsimmons & Hsieh | Belfast | Apr 08 - Aug 08 | NTT | EFOSC2 | Optical Spectroscopy | 3 nights | 3 nights | |
| Fitzsimmons & Hsieh | Belfast | Aug 08 - Jan 09 | WHT | ISIS | Optical Spectroscopy | 2 nights | 2 nights | |
| Jedicke & Wainscoat | IfA | Aug 08 - Feb 09 | Keck 2 | NIRC2/OSIRIS+LGS | NIR Spectroscopy & LGS AO | 2 nights | ||
| Jedicke & Wainscoat | IfA | Aug 08 - Feb 09 | Gemini | NIRI | NIR Spectroscopy | 40 hours (queue) | ||
| Jedicke & Wainscoat | IfA | Aug 08 - Feb 09 | Gemini | GMOS | Imaging | 4 hours (queue) | ||
| Jedicke & Wainscoat | IfA | Aug 08 - Feb 09 | 2.2m | Tek 2048/SNIFS | Imaging & IFU spectroscopy | 10 nights |
Scheduled Observing Runs
| PI | Institution | Dates | Telescope | Instrument | Technique | ISS/OSS/Both | Comments |
|---|---|---|---|---|---|---|---|
| Fitzsimmons & Hsieh | Belfast | 10-12 May 2008 | NTT | EFOSC2 | Optical spect. | ISS | 3 nights clear |
| Jedicke & Wainscoat | IfA | 25, 27-28 June 2008 | 2.2m | Tek 2048/SNIFS | Imaging & IFU spect. | Both | |
| Jedicke & Wainscoat | IfA | 23, 25, 27 July 2008 | 2.2m | Tek 2048/SNIFS | Imaging & IFU spect. | Both | |
| Fitzsimmons & Hsieh | Belfast | 7-8 Oct 2008 | WHT | ISIS | Optical spect. | ISS | 2 nights clear |
| Jedicke & Wainscoat | IfA | Gemini | NIRI | NIR Spect. | Both | ||
| Jedicke & Wainscoat | IfA | Gemini | GMOS | Imaging | Both | ||
| Jedicke & Wainscoat | IfA | from July 26 to... | Keck 2 | NIRC2/OSIRIS+LGS | NIR Spect. & LGS AO | Both |
Observing Considerations
Keck AO
Research/Grant Proposals
Research Papers
Asteroid Models From Pan-STARRS Photometry
Durech et al. 2006
Summary of work on light curve inversion of sparse long-term photometric datasets.
The Next Decade Solar System Discovery with PanSTARRS
Jedicke et al. 2006
High level summary of PS1, IPP and MOPS for IAU Symposium 236 on NEOs. A. Milani Ed.
A Multiple Tree Algorithm for the Efficient Association of Asteroid Observations
Kubica et al. 2005
Description of the algorithm used for linking asteroid detections.
Efficiently Identifying Tracks in Continuous Timed Data
Kubica et al. 2005
Description of the algorithm used for linking asteroid detections.
Variable KD-Tree Algorithms for Spatial Pattern Search and Discovery
Kubica et al. 2005
Description of the algorithm used for linking asteroid detections.
Unbiased Orbit Determination for the Next Generation Asteroid and Comet Surveys
Milani et al. 2005
Discussion of new techniques developed by Milani et al. for the purpose of linking detections and determining orbits for Pan-STARRS and other next generation sky surveys.
Reports