Surface features on the disk and plumes at the limb related to the active volcanism can be observed...32 Figure 13 Simulated observations of Io in sunlit using the Keck NGAO 140 nm in va
A Next Generation AO System for the Keck Observatory
We propose a feasibility study for a Next Generation Adaptive Optics (AO) system at the Keck Observatory to extend its leadership in high-spatial-resolution laser guide star (LGS) AO The new system will deliver substantially higher Strehl ratios in the near-infrared and, for the first time, robust AO correction in the R, I, and z-bands It will enable unique extragalactic astrophysics capabilities through a multi-object AO system that feeds deployable integral field units (IFUs) The deployable IFUs will leverage MEMS deformable mirror (DM) development at the Center for Adaptive Optics and the demonstrated capabilities of AO integral field spectroscopy with the OSIRIS instrument, paving the way for a comparable capability on the Thirty Meter Telescope.
This proposal presents a compelling science case for the Next Generation AO system (NGAO), defining the scientific requirements, outlining a point design capable of meeting them, and describing instrument concepts that would fully exploit NGAO Over the coming year we will begin the design development with a feasibility study to deepen our understanding of the science requirements, perform trade studies among the AO system, instrument designs, and science case, and advance to a System Design Review In parallel, we will develop modular funding options for the new AO system and its instrumentation by identifying packages suitable for support from separate donors and agencies and outlining scenarios for phased funding.
The proposed new adaptive optics (AO) system will give Keck a genuinely unique role among the next-generation facilities under development worldwide While ESO, Gemini, and other 8–10 m telescopes are channeling substantial funding into extreme AO for planet hunting and wide-field ground-layer AO to improve seeing, none of these projects occupy the niche we find scientifically most compelling: precision AO that fully exploits Keck’s larger aperture and effectively multiplies that aperture for multi-object work through deployable integral-field units (IFUs).
Recent History and Planning
The precision AO approach we propose here has a strong heritage within the Keck Adaptive Optics Working Group (AOWG) strategic planning process.
In November 2002, the Keck Adaptive Optics Working Group (AOWG) completed a strategic plan for future adaptive optics (AO) systems at the Keck Observatory The plan was approved by the Science Steering Committee in 2003, and the AOWG reaffirmed it with an updated version released in September 2004 (KAON 271).
By 2006, the first three strategic plan areas are completed: the Keck II AO system is optimized, the laser guide star is in science operation, and OSIRIS has been commissioned, with the LGS and OSIRIS together leading the field The fourth component, the Next-Generation Wavefront Controller upgrade, is progressing well, with commissioning scheduled for late 2006 on Keck I and early 2007 on Keck II The upgrade will increase sensitivity to faint guide stars by at least one magnitude and replace obsolete components to ensure robust adaptive optics operations on both telescopes for the next five to ten years.
Originally, the fifth component of the 2002 strategic plan was to deliver an extreme adaptive optics (AO) planet finder for the Keck telescope However, that project has not materialized as planned Funding and development have shifted to the Gemini Observatory, and the instrument will be installed at the Gemini South Telescope.
Building on the 2002 AOWG strategic plan, the National Science Foundation funded the development of a solid-state laser guide star for Keck I The laser infrastructure is being designed, with delivery to Keck I scheduled for mid-2007 Upon laser commissioning, the OSIRIS instrument will be moved to Keck I to enable laser guide star adaptive optics (AO) at both Keck telescopes starting in 2008, while NIRC2 remains on Keck II.
Part six, and the final element, of Keck's 2002 strategic plan called for the development of a new adaptive optics facility—the Keck Precision Adaptive Optics System (KPAO) Although no hardware concept existed in 2002, KPAO was envisioned to provide substantially higher Strehl performance in the near-infrared and robust AO correction in the visible, potentially down to the Hα wavelength Since the start of fiscal year 2005, roughly one Keck full-time equivalent (FTE) per year, along with a portion of a postdoctoral researcher’s time, has been allocated to fleshing out the KPAO concept.
In fall 2005, the AOWG and the Science Steering Committee decided to intensify their assessment of potential future Keck adaptive optics (AO) systems To pursue this, the AOWG and the WMKO AO group jointly formed a science team and a technical working group, collaborating to develop the science case and a point design for Next Generation AO at Keck The current proposal reflects the outcomes of this six-month effort.
The Competitive Landscape
Background
Strategic planning starts with identifying the competitive landscape and using a global perspective to target opportunities for future projects The NGAO team—comprising science and technical working groups—carried out a broad survey of current and future adaptive optics systems worldwide Guided by our science goals, we aim to position Keck NGAO to assume a global leadership role in adaptive optics, rather than merely producing the second, third, or fourth version of a single next-generation AO system.
We found that the VLT and Gemini Observatories are planning on Ground Layer AO and Extreme
AO Gemini South and (eventually) the LBT plan to have MCAO systems By contrast precision
Adaptive Optics (AO), the AOWG’s four-year goal, has been neglected in the plans for other 8–10 meter telescopes, leaving an important and exciting competitive niche that Keck NGAO is well poised to exploit This strategic gap sets the stage for Keck NGAO to lead in high-resolution astronomy, and we shall report in Section 3 of this proposal on the precision capabilities and anticipated benefits of adopting NGAO.
AO enables a compelling science case for the Keck community.
Our survey of planned adaptive optics (AO) science instruments is fully described in Section 7 Table 1 presents an overview of what other observatories are planning for next-generation AO systems on 8–10 m telescopes By "next-generation AO," we mean systems that go beyond traditional single-conjugate AO with one laser guide star or are designed for specialized applications such as high-contrast imaging or interferometry We compiled the information from published papers, project websites, and the May 2006 SPIE meeting in Orlando, Florida.
Table 1 Next-generation AO systems under development for 8-10 meter telescopes
Next-Generation AO Systems Under Development for 8 - 10 meter Telescopes
Type Telescope GS Next-Generation AO
Systems for 8 to 10 m telescopes Capabilities Operations
High-contrast Gemini-S NGS Near-IR Coronagraphic
Imager (NICI) Good Strehl, 85-act curvature, dual-channel imager 2006
High-contrast Subaru N/LGS Coronagraphic Imager
(CIAO) Good Strehl, 188-act curvature, 4W laser 2007
High-contrast VLT NGS Sphere (VLT-Planet
Finder) High Strehl; not as ambitious as GPI 2010
High-contrast Gemini-S NGS Gemini Planet Imager
Wide-field Gemini-S 5 LGS MCAO 2’ FOV 2007
Wide-field Gemini 4 LGS GLAO Feasibility Study Completed ?
Wide-field VLT 4 LGS HAWK-I (near IR imager)
+ GRAAL GLAO 7.5' FOV, AO seeing reducer,
Wide-field VLT 4 LGS MUSE (24 vis IFUs) +
GALACSI GLAO 1' FOV; 2 x EE in 0.2" at
Narrow-field VLT 4 LGS MUSE (24 vis IFUs) +
Interferomete r LBT NGS AO for LINC-NIRVANA
Phase 1: Single conj., 2 tel’s Phase 2: MCAO 1 telescope Phase 3: MCAO both telescopes
Gemini Observatory
Gemini has three ambitious new AO systems and two new AO-dedicated instruments under development and/or study:
Near-IR Coronagraphic Imager (NICI) is being commissioned at Gemini as a high-contrast instrument for Fall 2006, featuring an 85-element curvature adaptive optics (AO) system and a dual-channel imager tuned to detecting ultracool substellar objects Gemini plans an ambitious two- to three-year dedicated NICI observing campaign with the goal of direct imaging and characterization of giant planets around the nearest young stars.
Funded by the Gemini Observatory, the Gemini Planet Imager (GPI) is an ambitious extreme adaptive optics project led by Bruce Macintosh of LLNL and designed for direct imaging of exoplanets This $24 million instrument combines an adaptive optics system with about 1800 active degrees of freedom, a coronagraph for high-contrast imaging, and a low spectral resolution integral field unit (IFU) led by James Larkin of UCLA GPI is specifically aimed at detecting giant planets around young stars, enabling new insights into planetary formation and early evolution.
Gemini has funded and is close to installing its multi-conjugate adaptive optics (MCAO) system on Gemini South, which will use five laser guide stars to provide a 2-arcminute field of view Its back-end instrument is GSAOI, the Gemini South AO Imager—a dedicated near-infrared imager built by the Australian National University.
• GLAO: Gemini has completed a feasibility study for a Ground Layer AO system (HerzbergInstitute of Astrophysics, Durham University, and University of Arizona) The intended completion date is not yet clear.
European Southern Observatory
The VLT has embarked on an impressive long-term plan for adaptive optics that includes three new AO systems, a new laser facility, and five new AO-fed instruments:
• SPHERE, the VLT planet-finder This is a high-order AO system with three different back- end instruments (a differential imager, an integral field spectrograph, and a visible-red coronagraph
• The “AO Facility,” a four-laser-guide-star facility feeding two different AO systems, and using a new 1170-actuator adaptive secondary (description of AO systems follows)
• GRAAL, a ground-layer AO system that sends light to the new wide-field HAWK-I infrared imager (7.5 arc min field of view)
• GALACSI, a ground-layer AO system that sends light to the new MUSE instrument (this remarkable instrument consists of 24 visible-light IFUs, each with a 1 arc min field of view)
Subaru
Subaru is upgrading its adaptive optics by replacing its previous AO system and dye laser with a higher-order configuration optimized for high-contrast imaging The new setup is a 188-degree-of-freedom curvature-based AO system—the largest of its kind ever built—paired with a 4-watt solid-state sum-frequency laser This advanced LGS AO system will drive Hi-CIAO, Subaru’s near-infrared coronagraphic imager, enabling improved high-contrast observations.
LBT
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Summary
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Figure 1 Expenditures and future plans for adaptive optics for ESO and for the US.
Science with the Existing Keck AO Systems
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2005 (23% solar system, 46% galactic and 31% extragalactic)
Figure 2 Keck AO science papers by year and type of science
Figure 3 TAC-Allocated NGS and LGS AO science nights in semesters 06A and 06B
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AO mode is very high although a significant number of NGS nights are still requested.
Educational Impact
The educational impact of a more capable Keck adaptive optics (AO) system is evident in how the existing AO facilities support the research of students and postdocs A rough count of author participation in Keck AO science papers published through 2005 shows involvement by 26 graduate students and 20 postdocs.
Introduction
Ground-based astronomy's long-standing dream of high angular resolution has driven the development of adaptive optics, which over nearly the last two decades has been conceived and built to realize this potential Natural guide star (NGS) AO has produced numerous high-impact results thanks to greatly improved angular resolution and sensitivity However, NGS AO has largely been restricted to solar system and galactic science due to its very small sky coverage The current generation of single LGS systems is opening the door to high angular resolution extragalactic astronomy, but subject to modest Strehl ratios (typically < 0.5 in K band), relatively poor performance at J band and below, and a small field-of-view.
At WMKO, adaptive optics (AO) observations using natural guide star (NGS) AO and laser guide star (LGS) AO are increasingly demanded across solar system science, galactic science, and extragalactic science AO has made significant strides since its first implementation nearly two decades ago, delivering sharper and higher-contrast views of celestial targets Yet, current AO capabilities remain relatively limited when weighed against the expansive ambitions and imagination of today’s science community.
Extending the benefits of adaptive optics (AO) to a wider range of science hinges on three key characteristics for next‑generation AO systems: (1) achieving very high Strehl performance in the near-infrared to produce a stable, high-contrast PSF; (2) enabling correction at optical wavelengths toward the red to attain the highest angular resolution and access critical physical diagnostics; and (3) expanding the corrected field of view to support statistical studies of large samples By enabling these capabilities, AO will transition from a specialized tool to a fundamental instrument for advancing astronomical observations.
Observatory facility, capable of meeting the demands of many quite different science programs.
We quantitatively developed a focused set of science cases drawn from topics of high interest to the Keck scientific community, spanning the range of modern observational astronomy and intentionally including a diverse mix of scenarios to rigorously test the parameter space of a new NGAO (next-generation adaptive optics) system While not all potential future science areas are pursued, the results demonstrate the breadth and outstanding promise of new opportunities within reach of NGAO Importantly, the evidence suggests the right NGAO system will have broad appeal across a very large community of users, driving transformative capabilities for astronomy.
Solar System Science
Introduction
Planetary science has become an interdisciplinary field that has expanded dramatically over the last four decades, driven by advances in space exploration Ground-based telescopes, once marginal in the study of solar-system bodies, now play a central role thanks to Adaptive Optics (AO) Improvements in angular resolution provided by AO enable the remote study of surface and atmospheric features on planets, their satellites, and other minor bodies.
Continuous monitoring of solar system bodies is essential to understand and constrain variable phenomena on their surfaces and in their atmospheres, including volcanism, geysers, resurfacing, erosion, clouds, hazes, vortices, and rain Some phenomena are tied to seasonal cycles or other long-term changes, while dramatic events can unfold over short timescales, such as comets breaking up in giant planet atmospheres or Io’s volcanic outbursts Unpredictable events like these require studying them on timescales that do not align with spacecraft preparation and launch timelines.
The Keck telescope—the first 8–10 m-class facility equipped with an adaptive optics (AO) system—has already produced numerous results with a significant impact on planetary science Despite the magnitude threshold of the natural-guide-star AO system (m_v ≈ 5), which limits the number of observable targets, and the relatively small size of the planetary-science community compared with other subfields, about one third of all refereed articles and roughly 40% of the Keck Observatory’s science press releases since 2000 are based on solar-system studies.
A next-generation adaptive optics (AO) system on a 10-meter telescope, providing high-resolution visible and near-infrared imaging and spectroscopy, is poised to surpass the quality of the Hubble Space Telescope (HST) We discuss a representative set of science cases envisioned for this state-of-the-art instrument and illustrate its capabilities with simulations wherever possible This concise list is by no means exhaustive and reflects current research activity across the astronomical community.
Multiplicity in the Asteroid Populations
Contributors: F Marchis (UC-Berkeley), Josh Emery (NASA-Ames), A Bouchez (Caltech)
Thousands of small bodies orbit the Sun and are classified as asteroids, Trojans, Centaurs, or Trans-Neptunian Objects (TNOs) based on their orbits, while their surface reflectivity—tied to chemical composition—helps categorize them further These objects are believed to be remnants from the Solar System’s formation, preserving valuable information about the composition and conditions of the proto-planetary environment, making their study a forefront area of scientific research.
Until recently, little was known about the internal composition and structure of small bodies in the solar system For decades, researchers have pursued evidence of satellites around these minor bodies, because the orbits of these companions provide unique information about the primaries’ intrinsic properties—such as mass and, when size is known, density and porosity—and shed light on their formation, history, and evolution Moreover, analyzing satellite orbits helps constrain dynamical models of formation and long-term stability, improving our understanding of how small bodies form and evolve.
Discovery: After the Galileo spacecraft discovered Dactyl, the first asteroid companion in, 1993
Belton et al (1995) showed that satellites may be common around main-belt asteroids Merline et al (1999) reported the first direct detection of a satellite, Petit-Prince, orbiting asteroid (45) Eugenia, using adaptive optics on the Canada-France-Hawaii Telescope (CFHT).
Since then, 20 visible binary systems have been discovered using 8-meter-class ground-based telescopes equipped with adaptive optics and the Hubble Space Telescope Most systems consist of a moonlet companion (a few kilometers in diameter) orbiting a larger primary (~100 km across) We now know the orbital elements as well as the relative sizes and shapes for twelve systems (Marchis et al., 2003, 2004a, 2005abc), revealing a surprising range of orbital diversity and pointing to multiple formation scenarios For example, the first triple system, 87 Sylvia, comprises two moonlets orbiting an irregular rubble-pile primary, supporting a collisional origin for this system (Marchis et al., 2005c) Four other systems host satellites in notably elliptical orbits (e > 0.10) and/or high inclinations, and these systems also show small primary-to-satellite size ratios, consistent with formation by capture or by non-disruptive impacts followed by gravitational capture of ejecta Finally, one system features two equally sized components (each with a radius of about 45 km) orbiting their center of mass; while it has been suggested that this doublet formed by a splitting event after a close encounter with a larger body, such events are extremely rare, leaving the formation mechanism of this pair unresolved (Descamps et al., 2005).
Taking into account the detection limits of the current adaptive optics (AO) system installed at Keck Observatory—approximately 1/50 the size of the primary—a survey of 33 main-belt asteroids shows that fewer than 4% of large asteroids (diameter greater than 50 km) have a companion More recently, two independent groups led by P Pravec and F Colas report the discovery of several binary systems in surveys based on detecting mutual events and/or multi-component periods in their light curves These findings reveal a non-negligible fraction of close binary systems, with separations in the 1–20 km range, among observed asteroids.
2 and 10 km is therefore significantly larger (~10-15%) It should emphasize that the mechanism of formation for this population is still unexplained
As of now, the catalog of known or suspected binary asteroid systems has grown to 85 and continues to expand rapidly Their discovery has spurred imaginative and unconventional thinking in planetary science For example, a three-body interaction could explain Triton's highly eccentric, retrograde orbit It is possible that the object we observe as a satellite is actually part of a binary pair that was captured following a close encounter in Neptune's gravitational field (Agnor and Hamilton, 2006).
Ground-based telescopes equipped with adaptive optics have become essential for studying multiple asteroid systems, a field that is still relatively young but increasingly important The discovery and characterization of these systems rely on high angular resolution to accurately measure orbital parameters and determine size and mass ratios, thereby quantifying angular momentum and enabling meaningful comparisons with formation scenarios Achieving this level of detail requires extensive, long-term observations of many asteroids, a task well suited to the substantial telescope time available with AO-enabled ground facilities The Hubble Space Telescope has also made remarkable contributions to this field, including the discovery of Pluto’s small moonlets (Weaver et al., 2006) and the first binary Centaur (Noll et al.).
As of 2006, the telescope is clearly oversubscribed and its lifetime is limited There is no plan for a mission toward a binary asteroidal system yet Consequently, adaptive optics (AO) will play a major role in the future, especially if new instruments deliver better sensitivity and more stable wavefront correction.
Binary and multiple Trans-Neptunian Objects first became identifiable in seeing-limited ground-based observations, but adaptive optics now offers a major sensitivity boost for detecting these systems and precisely determining their orbits Today, adaptive optics enables efficient discovery and orbital analysis of binary Kuiper Belt Objects, though only the brightest KBOs are reachable with laser guide star (LGS) AO systems when the objects themselves are used as their own natural guide stars for tip/tilt and focus corrections In practice, only about eight KBOs are known with R-band magnitudes brighter than 19.0.
Among these objects, Pluto and 2003 EL61 (Haumea) each host multiple satellites, while at least one other body, 2003 UB313 (Eris), has a single known moon Appulses with moderately bright stars provide an opportunity to extend satellite searches and improve orbit determinations for smaller and more distant Kuiper Belt Objects (KBOs).
The next-generation Keck adaptive optics (AO) system could deliver two key benefits for discovering and characterizing Kuiper Belt Object (KBO) moons First, improved Strehl performance would enable the detection of closer and fainter companions Second, expanded sky coverage would allow surveys to reach more distant and diverse objects.
Figure 4 highlights the first known triple asteroidal system, formed by asteroid 87 Sylvia and its two moonlets, Romulus and Remus, discovered with the VLT/NACO adaptive optics system in August 2004 The moonlets' orbits are observed nearly edge-on, a geometry that complicates the detection and characterization of the satellites.
Table 2 Number of asteroids observable using the NGAO system per asteroid populations and considering various limit of magnitude for the tip-tilt reference (assuming on-axis observations)
Populations by brightness (numbered and unnumbered asteroids)
Current adaptive optics (AO) studies of main-belt multiple asteroid systems are limited by the small number of asteroids observable within the wavefront sensor magnitude limit The Keck Near-Infrared Guide Star AO (NGS AO) system reaches about magnitude 13.5, enabling observations of roughly 1,000 main-belt asteroids with perihelion greater than 2.15 AU and aphelion below 3.3 AU In contrast, populations further from the Sun, such as Trojan asteroids and trans-Neptunian objects (TNOs), remain out of reach with this setup Table 2 summarizes the total number of observable asteroids per population under different wavefront sensor magnitude limits (see Appendix: Number of Observable Asteroids) For this study, we focus on an on-axis reference scenario, treating the asteroid itself as the reference.
With NGAO, we can achieve excellent wavefront correction down to magnitude 17, enabling the survey of roughly 10% of the main-belt population and the potential discovery of about 1,000–4,000 multiple asteroid systems Because NGAO provides a more stable correction than the Keck LGS AO system, the halo from uncorrected phase is greatly reduced, allowing us to detect closer and fainter satellites and thus more multiple asteroids The superior angular resolution in the visible range (FWHM ≈ 14 mas in the R band) further enhances our ability to characterize close binaries While about 12 visual binary systems are currently known, we propose focusing on 100 new main-belt binaries discovered by HST light-curve or snapshot programs and by previous AO work An order-of-magnitude increase in known orbits will shed light on their formation, considering collisional family membership, heliocentric distance, size, shape, and other parameters.
To reach a peak SNR~1000-3000 on an AO image, the typical total integration times for a 13, or
For this program, 17-magnitude targets require 5-minute and 15-minute integrations, respectively Accounting for a typical 25-minute overhead to slew the telescope, point at the target, and close the adaptive optics loop (Marchis et al 2004b), the total telescope time per observation is about 30 minutes The asteroid orbit, described by orbital elements (P, a, e, i), can be constrained after eight consecutive observations taken over 1–2 months to limit parallax effects, corresponding to roughly 0.3 nights per object In total, thirty observation nights are requested for this program over a three-year period to achieve the required astrometric and orbital determinations.
Size and Shape of Asteroids
Contributor: Joshua Emery (NASA-Ames), F Marchis (UC-Berkeley)
Asteroids are the debris left over from the formation of the Solar System Because of their small to moderate sizes, they have generally not undergone late-stage endogenic alteration, so their surfaces preserve the scars of early and late-stage collisional evolution and early geologic processes, along with ongoing exogenic surface processes like space weathering Adaptive optics observations of asteroids can play a key role in revealing what this debris has to tell us about the formation and evolution of the Solar System.
This section identifies three specific areas of asteroid research that can be advanced through disk-resolved observations Although this list is not exhaustive, it illustrates how improved adaptive optics (AO) enable new asteroid science and suggests that many additional applications will emerge as more researchers explore the possibilities The section concludes with an overview of NGAO's potential to increase the number of asteroids that can be resolved, expanding the observable asteroid population.
3.3.3.1.1 Collisional Evolution of the Asteroid Belt
High-resolution imaging of asteroids can significantly advance our understanding of the accretional and collisional evolution of the Solar System The observed properties of the Main Belt arise from a complex mix of initial conditions—such as total belt mass, the compositional distribution of that mass, and the timing of Jupiter’s formation—and evolutionary processes, including collisional fragmentation, giant-planet migration, and the degree of mixing These processes are being modeled with increasing sophistication, yet they require robust observational constraints Properly observed asteroids offer essential clues that help disentangle these factors As Bottke et al (2005b) state, “Like archaeologists working to translate stone carvings left behind by ancient civilizations, the collisional and dynamical clues left behind in the Main Belt, once properly interpreted, can be used to read the history of the inner Solar System.”
One key constraint in understanding the asteroid cratering record is the occurrence of large craters on large asteroids For example, imaging with the Hubble Space Telescope (HST) at a spatial resolution of about 36 km per pixel has revealed a large impact basin, roughly 460 km in diameter, at the south pole of a basaltic asteroid.
The differentiated asteroid 4 Vesta, with a diameter of about 560 km, has been used as a primary constraint in several collisional evolution models (e.g., Bottke et al 2005a; O’Brien and Greenberg 2005) The reasoning is that large collisions must be frequent enough to make Vesta’s impact history not too unlikely, yet not so frequent that many large impacts should have occurred across the asteroid belt However, statistical conclusions drawn from a sample size of one warrant caution: Vesta could be a statistical outlier, and extending its properties to the entire asteroid belt would be an astronomical red herring.
Spatially resolved imaging of other large asteroids is critical in order to place the results for Vesta into context and to derive truly reliable statistical constraints on large collisions throughout the Main Belt Observations of the 15 or 20 largest asteroids would provide the statistics necessary to put much stronger constraints on the frequency of these large collisions We estimate that 20 Main Belt asteroids will be resolved with sufficient resolution with NGAO in R-band (33 in V-band) for mapping comparable to that done previously for 4 Vesta This compares with only one (Ceres) that is available from the current Keck AO (K-band) The criterion for these results is that the fractional resolution (spatial resolution divided by diameter) be equal to or smaller than for the HST observations of Vesta (36km/560km = 0.065) The NGAO resolution in R-band on Vesta is sufficient to meet this criterion.
With an ~11 km resolution, these observations achieve more than a threefold improvement over HST data The majority of this gain comes from extending high Strehl, diffraction-limited performance to shorter wavelengths Imaging large asteroids of different taxonomic types—and thus varying compositions—will reveal how surface structure and strength vary among asteroids, consistent with findings such as O’Brien et al (2006).
Any model aiming to describe the asteroid population must explain the size distribution of the Main Belt as a whole and of its sub-populations The initial Main Belt size distribution was set by accretion, governing how many objects of each size formed during that era Since then, collisional and dynamical erosion have modified this distribution, leaving measurable signatures of their influence The size distributions of other populations likewise depend on their formation history and evolutionary environments In asteroid families, the initial size distribution is set by fragmentation laws, which remain uncertain and can vary with composition For near-Earth objects, the size distribution is ultimately determined by the delivery mechanism from the Main Belt, a process that is very likely size-dependent.
Without accurate knowledge of asteroid sizes, it is impossible to decode the information contained in their size distributions; disk-integrated visible photometry cannot determine size, since size and albedo cannot be disentangled without additional information Direct imaging is the most straightforward method to measure asteroid sizes, whereas radiometry—measuring thermal emission alongside visible reflected flux—depends on many poorly known parameters such as thermal inertia, thermal-IR phase functions, and surface beaming due to roughness The radiometric approach has been used to size many Main Belt asteroids, but its calibration for large objects does not transfer to smaller bodies and especially to near-Earth objects observed at high phase angles The ideal solution—a large direct-imaging campaign of thousands of asteroids—is likely infeasible on Keck, but NGAO will enable direct size measurements for a substantial sub-sample spanning sizes, compositions, shapes, orbital classes, dynamical families, and viewing geometries These observations can anchor the size distributions of each subgroup and recalibrate other methods to improve reliability With NGAO in the R-band, about 1193 objects would be observable, and an estimated ~300 directly imaged asteroids, carefully chosen, would be adequate to anchor the distributions Marchis et al (2006) began such a survey with Keck NGS AO, imaging 30 asteroids over a few half-nights, and, accounting for roughly 20 minutes overhead per object and 5–15 minutes of integration each, a program of this scale could be completed in about 12 nights.
Well-calibrated size distributions of asteroid families will enable the investigation of the physics of disruption and fragmentation, a key uncertainty in evolutionary models, and the same is true for properly anchored size distributions of near-Earth objects There are currently very few NEOs with known sizes, which presents a problem for hazard mitigation since the number of objects in near-Earth space that could cause regional catastrophes is unknown, complicating efforts to detect and stop potentially devastating impactors.
3.3.3.1.3 Geologic Properties and Surface Heterogeneity
Among the largest asteroids, geological activity may have occurred in their own right, and some could still be active today Evidence indicates that a number of these bodies differentiated, with Vesta’s basaltic crust and M-type asteroids considered to be remnant cores of disrupted, differentiated asteroids, though many others did not differentiate The reasons for this divergent evolution remain unexplained, with hypotheses ranging from volatile content inhibiting differentiation, to shifts in silicate mineralogy with heliocentric distance, to heat sources such as radioisotopes or induction heating that may not have been uniformly distributed Direct observation of large asteroids, both differentiated and undifferentiated, is the best approach to resolving this current conundrum.
Imaging reveals surface albedo variations that map to distinct geologic units, such as lava flows on Vesta or carbonate/organic/water/clay deposits on Ceres, while detailed shape analysis provides clues to internal composition and structure For example, HST imaging shows Ceres as nearly spherical, supporting a differentiated icy object with a water mantle surrounding a rocky core, whereas Vesta’s non-homogeneous shape reflects diverse rheologies The accretion and collisional history of the inner Solar System was not uniform, but varied with distance from the Sun due to differing materials, and NGAO imaging will enable investigations of these differences through combined shape and albedo mapping.
Disk-resolved spectroscopy is a powerful tool for mapping asteroid geology Extending NGAO to shorter wavelengths will enable complete characterization of the important 1 μm silicate band, permitting detailed mineralogical mapping of silicates on individual surfaces A water-of-hydration band near 0.7 μm can also be mapped to understand how water has affected particular asteroids, whether localized by impacts or influenced by broader events across groups There is also recent spectral evidence for silicates on the surfaces of some M-type (presumably metallic) asteroids, which raises questions about whether these bodies are not purely metallic or simply possess a silicate mantle covering, perhaps as remnant material If such a mantle provides only partial coverage, NGAO’s disk-resolved spectroscopy could map this distribution and reveal the extent of silicate material on these surfaces.
3.3.3.1.4 Improvements in Number of Resolvable Asteroids by NGAO
Table 4Table 4Error: Reference source not found summarizes the number of asteroids resolvable from visible to near-IR domain and per population (see Appendix Number of Observable
Thanks to the high angular resolution in the V and R bands, roughly 800 main-belt asteroids could be resolved and have their shapes estimated with a precision better than 7% With the current adaptive optics system, about 100 main-belt asteroids are resolvable Determining the size and shape of Trojan asteroids, even if limited to a small sample, is useful for estimating their albedo For near-Earth asteroids (NEAs), the large number of resolvable objects arises from very close approaches to Earth, though many of these are unnumbered, and refined orbits may show that their closest approaches are not as near as initially thought.
Table 4 Number of asteroids resolvable with Keck NGAO in various wavelength ranges and per population
Unnumbered asteroids (most of the NEAs) have poorly known orbits
Resolvable asteroids in each band (numbered and unnumbered)
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Marchis, F Kaasalainen, M., Hom, E.F.Y., et al 2006 Size, Shape, and multiplicity of main-belt asteroids I Keck Adaptive Optics Survey, submitted to Icarus.
O’Brien, D.P., and R Greenberg 2005 The collisional and dynamical evolution of the main belt and NEA size distributions Icarus 178, 434-449.
O’Brien, D.P., R Greenberg, J.E Richardson 2006 Craters on asteroids: Reconciling diverse impact records with a common impacting population Icarus in press (available online).
Thomas, P.C., R P Binzel, M.J Gaffey, et al 1997 Impact excavation on asteroid 4 Vesta: Hubble Space Telescope results Science 277, 1492-1495.
Thomas, P.C., J.Wm Parker, L.A McFadden, et al 2005 Differentiation of the asteroid Ceres as revealed by its shape Nature 437, 224-226.
Walker, R.G 2003 IRAS diameters and albedos revisited DPS 35, abstract #34.19.
Wolters, S.D., Green, S.F., McBride, N., Davies, J.K 2005 Optical and thermal infrared observations of six near-Earth asteroids in 2002 Icarus 175, 92-110.
Moonlet Spectroscopy
Contributor: Franck Marchis (UC-Berkeley), Joshua Emery (NASA-Ames)
3.3.4.1 Scientific Background: Satellites around minor planets
Section 3.3.2.1 examines the existence of multiple asteroid systems, the detection of moons in these systems, and the orbital dynamics studied with adaptive optics (AO) As of writing, about 85 binary asteroid systems are known or suspected, underscoring the diversity and richness of these populations A key goal in the subfield of multiple asteroid studies is to reveal the nature and formation of these systems, shedding light on their architectures and evolution.
Multiple asteroid systems may form through several pathways, including capture of a fragment after an oblique impact, tidal splitting during a close encounter, fission, disruption and reaccretion of large fragments followed by capture of smaller ones, and capture after a close encounter, among others Reflectance spectroscopy of the primary and its moonlet across the visible to near-infrared range (approximately 0.65 to 2.5 μm) can help constrain the origin of each system Broadly differing spectra, with variations in the number, depth, width, and positions of absorption features, indicate distinct surface mineralogies.
Different surface mineralogies between a moonlet and its asteroid primary are expected under several formation scenarios If the multiple system formed from disruption and reaccretion of large fragments, interior compositions of both the impactor and target would be exposed and mixed with exterior material, producing heterogeneous fragments with signatures of core, mantle, and crust for differentiated bodies, or unweathered primordial material for undifferentiated ones; visible and near-infrared (NIR) spectra can identify these compositions In contrast, tidal splitting and fission should yield components with identical compositions, so systems with very similar spectral characteristics would support those scenarios If the system formed by capture after an oblique impact or after a close encounter, the moonlet could have a different composition due to the original objects' differing makeup, reflecting the diversity of asteroid compositions that could undergo close encounters or collisions.
Bottke et al (2005) argue that the Main Belt exhibits spatial mixing of asteroid taxonomic classes, creating a compositional gradient with heliocentric distance: S-types ("stony") dominate the inner belt, C-types ("carbonaceous") are most common in the middle belt, and D-types—likely C-type bodies enriched in organics and ices—predominate in the outer belt, with metallic M-types also present The transitions between these groups are not abrupt, and substantial overlap exists (for example, some C-types appear in the inner belt) This moderate mixing provides a constraint on the dynamical state of the early Solar System By studying the compositions of multiple asteroid systems, researchers can gauge how much collisional interaction has occurred between different asteroid types, thereby helping to constrain the early Solar System's dynamical and collisional environment.
Dominant mafic minerals on terrestrial bodies—pyroxene, olivine, and spinel—exhibit markedly different spectral morphologies in the near-infrared region (see Figure 7) To date, C-type asteroids have largely featureless NIR spectra, although modern instrumentation can reveal weak absorption features For example, Hardersen et al (2005) reported weak features (~1–3%) attributed to orthopyroxenes on M-type asteroid surfaces A low-contrast hydration band centered near 0.7 μm has been studied by Vilas and Gaffey (1989) and Vilas and Sykes (1996) (see Fig X) Consequently, previously “featureless” asteroid spectra warrant re-observation with higher sensitivity and better angular resolution (Rayner et al., 2004) In addition, C-type and D-type asteroids show a wide range of continuum slopes, likely reflecting various physical processes such as space weathering.
Figure 7 shows typical spectra of a mafic asteroid, where the depth, width, and central position of two broad absorption bands constrain the surface ratios of pyroxene, olivine, and spinel The observed spectra of asteroid 105 Artemis, a C-type object, collected at various rotation phases, reveal an extended, poorly contrasted absorption feature centered near 0.7 microns.
New integral-field imagers with wavelength coverage from 0.65 to 2.5 μm and sufficient signal-to-noise ratio enable the acquisition of high-quality spectra Moderate SNR spectra (~30) are adequate to reveal modest spectral differences between the primary and its satellite arising from variations in composition (abundances of major mineral phases) or differences in the degree of space weathering High SNR spectra (>100) allow these compositional differences to be quantified and the potentially confounding effects of space weathering to be eliminated High-S/N data permit detailed characterizations of surface assemblages and the detection of subtle differences between the primary and its satellite, providing the means to test models of their origin. -**Sponsor**Need help making your research article shine and comply with SEO rules? [Soku AI](https://pollinations.ai/redirect-nexad/oJemTcRt?user_id=983577), the AI trained by ad experts, can assist by identifying those key sentences that capture the essence of each paragraph for maximum impact Think of it as having a team of experts ensuring your work is not only scientifically sound but also discoverable Let Soku AI handle the complexities of SEO while you focus on the science!
Using the published sensitivities of OSIRIS (R ~ 3800) and NIRC2 (R ~ 2500), we assess the feasibility of spectroscopy of Sylvia’s moonlets (see Section B) and the Strehl ratios for single LGS adaptive optics in J, H, and K (0.26, 0.35, 0.46) and NGAO at 140 nm (0.71, 0.83, 0.90) Table 5 lists the S/N estimates for each object with NIRC2 and OSIRIS for a 1-hour integration The NGAO system is expected to raise the J-band spectral S/N by a factor of 2–4 In this calculation we neglect scattered light from uncorrected AO phase, which would predominantly affect the closest moon, New2 The estimated residual background intensity on the image implies about a sixfold S/N reduction for the S/New1 moon. -**Sponsor**Looking to refine your astronomy article and boost its SEO? Let's focus on the core meaning for each paragraph To delve deeper into optimizing your research, consider a [1 Year Subscription: Nature Astronomy](https://pollinations.ai/redirect-nexad/2SBtx2MU?user_id=983577) It provides unparalleled access to pivotal advancements and comprehensive reviews in the field Specifically, for your work, note that NGAO significantly enhances spectral S/N in the J band, though scattered light impacts the closest moon The subscription can help you keep current with advances in mitigating such effects for even better data!
Table 5 presents the S/N estimates for the spectra of the Pseudo Sylvia moons with a 1-hour exposure The NGAO system is expected to increase the S/N in the J-band by at least a factor of 2 to 4 For the closest moon, S/New1, the S/N gain could be even higher, in the range of 12 to 24, because the NIRC2 image is limited by the halo around the primary asteroid (this limitation is not included in the calculation).
Observing at wavelengths shorter than 1 μm enables access to the crucial 1 μm silicate band, a key diagnostic for asteroid silicate mineralogy, and the standard analysis relies on the positions and areas of both the 1 μm and 2 μm bands If the spectrum cuts off at 1 μm, it is not possible to reliably determine the band center or band area, hampering mineralogical interpretation Additionally, a water-of-hydration band near ~0.7 μm, whose exact position depends on the specific mineral, has been used—with the 3 μm band—to map hydration features in the main belt Without extending adaptive optics capability at least to the R-band, we cannot reliably assess the hydration states of moonlets.
The gain in sensitivity offered by NGAO compared with the current Keck AO is crucial for this study, as is the capability for AO-assisted spectroscopy at wavelengths shorter than 1.0 micrometer With a tip-tilt magnitude limit near 18, the sample could include roughly 50 binary systems, making the number of observable targets substantial Acquiring two observations at opposite rotational phases will enhance the characterization of each system We plan eight observing nights, assuming 1-hour integrations per spectrum in the z, J, H, and K bands, and covering about a quarter of the sample Our preferred instrument is a visible and NIR camera with slit spectroscopy, with an integral field spectrograph as a second choice.
Bottke, W.F., D.D Durda, D Nesvorny et al 2005b Linking the collisional history of the main asteroid belt to its dynamical excitation and depletion Icarus 179, 63-94.
Hardersen, P.S., M.J Gaffey, P.A Abell 2005 Near-IR spectral evidence for the presence of iron- poor orthopyroxenes on the surfaces of six M-type asteroids Icarus 175, 141-158.
Rayner, J.T., P.M Onaka, M.C Cushing, W.D Vacca 2004 Four years of good SpeX In Ground- based Instrumentation for Astronomy: Proceedings of the SPIE (A Moorwood and I
Vilas, F and M.J Gaffey 1989 Phyllosilicate absorption features in Main-Belt and Outer-Belt asteroid reflectance spectra Science 246, 790-792.
Vilas, F and M.V Sykes 1996 Are low-albedo asteroids thermally metamorphosed? Icarus 124, 483-489.
Titan – The coupled surface-atmosphere system with NGAO
Contributors; Máté Ádámkovics (UC Berkeley), Franck Marchis (UC Berkeley), Antonin Bouchez (Caltech)
Titan, Saturn's largest moon, possesses a dense atmosphere dominated by nitrogen at about 1.5 bar with roughly 5% methane, accompanied by complex haze layers and a variety of cloud types, posing questions about how such a small body retains an atmosphere while other satellites do not Methane is short-lived in Titan's atmosphere due to photolysis and must be constantly replenished; surface reservoirs of liquid hydrocarbons were once thought to supply methane, but they do not presently exist, though Titan may have harbored hydrocarbon oceans in the past, and the surface measured by the Huygens probe was found to be moist The source of methane could lie in deep interior processes or near the surface Methane plays the role of Earth's water, linked by a methane-based meteorological cycle to the surface, because Titan is near its methane triple point, creating a surface-atmosphere coupling where temporal variations in one domain drive changes across the planet, though this coupling has so far been inferred rather than directly observed Studying seasonal differences in cloud properties and surface albedo will help determine Titan's evolutionary path to its current state.
Titan’s year spans roughly 30 Earth years, so studying its seasonal responses requires combining Voyager-era data with a growing suite of ground-based and spacecraft observations As ground-based instruments gain sensitivity and resolution, they reveal more dynamical variations on shorter timescales because small-scale processes—such as cloud formation and haze density changes—occur more rapidly than large-scale ones For example, the Cassini spacecraft and the Huygens probe have provided exceptionally high spatial resolution measurements that show small-scale (0.7 µm) and near-infrared will enable robust surface composition characterization by detecting broad pyroxene bands at 0.9 and 2.0 µm and crystal SO2 bands at 1.98 and 2.12 µm.
Figure 14 R-band observation simulation of Io (angular diameter of 0.9”) with KNGAO and HST/ACS
To take advantage of the high angular resolution provided at visible wavelengths by Keck NGAO, the top priority is a visible imager with low spectral resolution (R ≲ 1000) A near-IR camera with thermal capabilities up to 5 μm (or an additional thermal imager) is the second instrument priority to detect thermal emission from low-temperature hot spots As discussed, Io observations in eclipse are an interesting science driver that benefits from a moderate AO correction over a large field of view (MCAO).
3.3.6.5 References de Pater, I., Marchis, F., Macintosh, B et al., 2004 Keck AO observations of Io in and out of eclipse, Icarus 169, 1, 250-263
Geissler, P., McEwen, A., Porco, C et al 2004 Cassini observations of Io’s visible aurorae, Icarus
Keszthelyi, L Jaeger, W.L., Turtle, E.P et al 2004 A post-Galileo view of Io’s interior, Icarus 169,
Marchis F., de Pater, I.; Davies, A G et al 2002.H igh-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io, Icarus 160, 1, 124-131.
Marchis F., Le Mignant, D., Chaffee, F.H et al 2005 Keck AO survey of Io global volcanic activity between 2 and 5 mm, Icarus 176, 96-122.
Conclusion
The study clearly demonstrates the NGAO system's promise for selected science cases, and while we highlight a few representative examples for conciseness, many more studies will become accessible for additional solar-system bodies In Appendix: Satellites of Giant Planets Observable with NGAO, we list several satellites that could be observed with an on-axis approach (using the planet as a reference), limited by satellite brightness and distance from the planet to avoid glare that would hinder wavefront analysis NGAO will be an excellent instrument to map the surface composition of the Galilean satellites with spatial resolution close to, and potentially better than, the Galileo NIMS/SSI observations in the visible Resolution of 6–25 pixels will be attainable on images of mid-sized Saturnian satellites (Enceladus, Mimas, Rhea, Dione), enabling continuation of Cassini-era studies A study of giant-planet atmospheres is another important topic that benefits from moderate adaptive optics correction over a large field of view (>45 arcsec) to characterize atmospheric activity, such as the recent discovery of a second red spot on Jupiter These measurements will constrain the internal structure, heat flow, and composition of giant-planet atmospheres and thus inform our understanding of the evolution of giant planets and exoplanets.
To enable NGAO to observe moving targets, the design must include differential guiding when the tip-tilt source is not the object itself or moves relative to the target The scientific return of the Keck telescope and the NGAO system would be greatly enhanced if service observing (queue scheduling) is offered With an error budget of 140 nm, NGAO is expected to achieve a Strehl ratio of about 20% in the R band under moderate seeing Bright targets such as the Galilean satellites (mv ≈ 6) can be observed even under seeing conditions worse than average in the near-infrared (>1.2 arcseconds) Very challenging observations, such as studying multiple trans-Neptunian objects (mv > 17), could be scheduled when seeing is excellent (