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The Solar System: Small Bodies, Big Consequences
This E-News article is based on Chapter 5 of the LSST Science Book: The Solar System. This chapter describes science investigations of the Solar System that LSST will make possible. Authors of Chapter 5 are:
LSST will bring “close-to-home” objects into focus as no other telescope has. Whether searching for asteroids that might one day collide with the Earth, or finding new information about planet formation and evolution, LSST will provide users with unparalleled access to the millions of small objects that lie within our own Solar System—remnants of the primordial solar nebula that allow us to decipher its early history.
LSST’s ability to reach faint magnitude limits in a short time will capture data on objects as small as hundreds of meters. Although the predicted total mass of all small objects in the Solar System is only about equal to that of the Earth, the importance of these small objects lies not in total mass, but in the statistical ensembles of orbits and physical properties of the great number of objects LSST will catalog. With these numbers, much more work can be done to create a more thorough picture of the Solar System’s evolution, to find potentially deadly visitors as Congress mandates, and to find targets for prospective missions.
While astronomers have been studying asteroids since discovering the first in 1801, to date scientists have cataloged only about 300,000 Main Belt Asteroids (MBAs). The Minor Planet Center lists close to 10,000 Near Earth Objects (NEOs) with about 1,000 classified as Potentially Hazardous Asteroids (PHAs)—asteroids of most concern for potential collisions with Earth. Other types of small bodies number in the thousands. With LSST, we expect to detect and characterize millions of objects throughout the Solar System.
Orbital dynamics, spatial distribution, and physical properties are known for thousands of these objects, but this information represents a small percentage of all there is to learn and is biased to a selection of larger objects. LSST’s fainter flux limit will allow it to probe the Main Belt for objects as small as 100 m and to detect objects down to diameters of 400 km as far away as 100 AU.
Even without considering the deep-drilling fields, the number of objects that LSST will detect is stunning. A significant percentage will have several hundred detections each, allowing rough studies of their physical parameters. As astronomer Lynne Jones says, “the sheer volume of data—orbits, colors, and lightcurves of millions of objects—will be revolutionary for studying the smallest members of our Solar System both individually and as whole populations, so that we can really start to understand the formation and evolution of the entire Solar System.”
New Views of the Solar System
By compiling catalogs with accurate orbits, LSST will help planetary astronomers understand the history of the Solar System. Recent theories of planetary evolution suggest migration and chaotic rearrangements have had significant effects. The Nice model proposes that all giant planets formed at less than 14 AU from the Sun in a solar nebula truncated at 30 AU. Angular momentum exchange among planets and small bodies then caused migrations to current positions. Other theories include slow migration of giant planets and perturbations from rogue planetary embryos, large planetesimals or passing stars. LSST’s vastly increased sample size will permit much stronger statistical tests to evaluate these models. As LSST adds measurements of color to the orbital data, observers can explore sub-populations and groups in more depth, including links among groups.
The observed size distributions of MBAs and other small bodies provides one of the most significant constraints on their history. Current data sets are limited to an absolute magnitude of about 15 and asteroid families complicate evaluations. LSST will extend these inventories about 3-4 magnitudes fainter or to bodies approximately 5 times smaller than now visible. More importantly, LSST will be able to detect asteroids over huge amounts of sky. For the first time, observers will be able to scrutinize not only massive numbers of small bodies, but large numbers of bodies only hundreds of meters in diameter. By gathering color information as well as single-band photometry, LSST will able to improve size estimates to uncertainties of 30%-50%, on average.
LSST will detect many binary systems, both in the Main Asteroid Belt and in the Trans-Neptunian region in the outer Solar System. Binary behavior in the Kuiper Belt looks very different from that in the Main Belt, and understanding these differences will allow LSST to constrain ideas of how these systems form, evolve, and survive in the disruptive environment of the early Solar System.
LSST is expected to discover on the order of 10,000 comets with 50 observations or more of each one. These numbers will allow determinations of size, color, and gas-to-dust ratio. These data will allow evaluations of Oort Cloud structure and how physical aging and fading of comets changes the populations over time. LSST astrometry of Long Period Comets when far from the Sun will identify those newly arrived from the Oort Cloud to improve understanding of material unchanged for 4.5 billion years.
Discussion of distinct populations, that is MBAs, NEOs, Trans-Neptunian Objects, and comets, belies the true complexity of the Solar System. The results of observations and modeling in the last decade make it clear that characteristics once thought to indicate very distinct populations overlap in any number of ways. For example, asteroids can display cometary activity and comets may constitute as many as 5%-10% of NEOs. Specific regions in the Main Belt affected by resonances furnish MBAs to the NEO population. Centaurs, orbitally unstable objects similar to Scattered Disk Objects (members of the Trans-Neptunian population), appear to originate from both the Scattered Disk and the Oort Cloud. Studies of the orbits and interactions of these populations will refine characterization of the dynamics and evolution.
LSST will not only explain the objects, populations, and history of the Solar System but undoubtedly discover new and mysterious aspects that will continue to challenge theories of formation and evolution.
Reducing Potential Hazards to Earth
The Earth’s atmosphere protects the planet—and its life forms—from most incoming objects: small Near-Earth asteroids or comets (in combination, called NEOs). But large NEOs might not disintegrate and could potentially impact Earth causing damage to the biosphere’s health.
The damage that objects cause upon impact with Earth varies primarily due to size. As an object reaches approximately 40 m in diameter, it has potential to do tremendous albeit localized damage. From 40 m to 1 km damage moves from localized to regional extent and above a diameter of 2 km, the object delivers energy of a million megatons and will produce devastating environmental damage on a global scale. Larger impacts can cause mass extinctions—whether of dinosaurs or humans.
In 2005 Congress mandated the detection and tracking by 2020 of 90% of all NEOs larger than 140 m in diameter whose orbits pass within 0.5 AU of Earth (140 m diameter approximately marks the transition from a projectile, which causes localized damage to one that causes regional damage). NASA estimates there are around 100,000 NEOs greater than 140 m in diameter. Identifying 90% of the NEOs larger than 140 m diameter would also identify virtually all of the potential impactors greater than 1 km diameter, as well as 60%-90% of objects that could produce potentially dangerous air blasts.
Current simulations indicate that the LSST baseline cadence will provide orbits for around 82% of PHAs larger than 140 m after ten years of operation. If 15% of time is spent optimizing observations to reach fainter limiting magnitudes for NEOs near the ecliptic, the 90% completeness level is reached in twelve years without significant effects on other science goals.
The Solar System becomes stranger, far more complex and ever more interesting as our instruments permit more detailed study. LSST will take this study to whole new levels, providing the potential to thoroughly understand our Solar System’s origin and evolution in all its complexity. And LSST’s survey has the potential to identify hazards coming from the Solar System so that we can protect the planet.
This article written by Anna H. Spitz and R. Lynne Jones
LSST is a public-private partnership. Funding for design and development activity comes from the National Science Foundation, private donations, grants to universities, and in-kind support at Department of Energy laboratories and other LSSTC Institutional Members:
Brookhaven National Laboratory; California Institute of Technology; Carnegie Mellon University; Chile; Cornell University; Drexel University; Google Inc.; Harvard-Smithsonian Center for Astrophysics; Institut de Physique Nucléaire et de Physique des Particules (IN2P3); Johns Hopkins University; Kavli Institute for Particle Astrophysics and Cosmology at Stanford University; Las Cumbres Observatory Global Telescope Network, Inc.; Lawrence Livermore National Laboratory; Los Alamos National Laboratory; National Optical Astronomy Observatory; Princeton University; Purdue University; Research Corporation for Science Advancement; Rutgers University; SLAC National Accelerator Laboratory; Space Telescope Science Institute; The Pennsylvania State University; The University of Arizona; University of California, Davis; University of California, Irvine; University of Illinois at Urbana-Champaign; University of Pennsylvania; University of Pittsburgh; University of Washington; Vanderbilt University
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