• Notice
• Committee
• Foreword
• Preface
• Contents
• Executive Summary

Printed in the United States of America

RONALD GREELEY, Arizona State University, Chair

JAMES ARNOLD,* University of California, San Diego

FRANCES BAGENAL, University of Colorado

JEFFREY R. BARNES, Oregon State University

RICHARD P. BINZEL, Massachusetts Institute of Technology

WENDY CALVIN, U.S. Geological Survey

PHILIP R. CHRISTENSEN,* Arizona State University

RUSSELL DOOLITTLE, University of California, San Diego

HEIDI B. HAMMEL, Massachusetts Institute of Technology

LARRY HASKIN, Washington University

BRUCE JAKOSKY, University of Colorado

GEORGE McGILL, University of Massachusetts

HARRY McSWEEN, JR., University of Tennessee

TED ROUSH, San Francisco State University

JOHN RUMMEL, Marine Biological Laboratory

GERALD SCHUBERT, University of California, Los Angeles

EVERETT SHOCK, Washington University

EUGENE SHOEMAKER, Lowell Observatory

DARRELL F. STROBEL,* Johns Hopkins University

ALAN T. TOKUNAGA,* University of Hawaii

ROGER YELLE,* Boston University

MARIA T. ZUBER,* Massachusetts Institute of Technology

Staff

DAVID H. SMITH, Study Director

ALTORIA B. ROSS, Senior Program Assistant

ELAINE E. HARRIS, Interim Program Assistant

STEPHANIE ROY, Research Assistant

CLAUDE R. CANIZARES, Massachusetts Institute of Technology, Chair

MARK R. ABBOTT, Oregon State University

JOHN A. ARMSTRONG,* IBM Corporation (retired)

JAMES P. BAGIAN,* Environmental Protection Agency

DANIEL N. BAKER, University of Colorado

LAWRENCE BOGORAD, Harvard University

DONALD E. BROWNLEE, University of Washington

JOHN J. DONEGAN,* John Donegan Associates, Inc.

GERALD ELVERUM, TRW Space and Technology Group

ANTHONY W. ENGLAND, University of Michigan

DANIEL J. FINK,* D.J. Fink Associates, Inc.

MARILYN L. FOGEL, Carnegie Institution of Washington

MARTIN E. GLICKSMAN,* Rensselaer Polytechnic Institute

RONALD GREELEY, Arizona State University

WILLIAM GREEN, former member, U.S. House of Representatives

NOEL W. HINNERS,* Lockheed Martin Astronautics Company

ANDREW KNOLL, Harvard University

JANET G. LUHMANN,* University of California, Berkeley

JOHN H. McELROY,* University of Texas, Arlington

ROBERTA BALSTAD MILLER, CIESIN

BERRIEN MOORE III, University of New Hampshire

MARY JANE OSBORN, University of Connecticut Health Center

SIMON OSTRACH, Case Western Reserve University

MORTON B. PANISH, AT&T Bell Laboratories (retired)

CARLÉ M. PIETERS, Brown University

THOMAS A. PRINCE, California Institute of Technology

MARCIA J. RIEKE,* University of Arizona

PEDRO L. RUSTAN, United States Air Force (retired)

JOHN A. SIMPSON, Enrico Fermi Institute

GEORGE L. SISCOE, Boston University

EDWARD STOLPER, California Institute of Technology

RAYMOND VISKANTA, Purdue University

ROBERT E. WILLIAMS, Space Telescope Science Institute

MARC S. ALLEN, Director (through December 12, 1997)

JOSEPH K. ALEXANDER, Director (from February 17, 1998)

ROBERT J. HERMANN, United Technologies Corporation, Co-chair

W. CARL LINEBERGER, University of Colorado, Co-chair

PETER M. BANKS, Environmental Research Institute of Michigan

WILLIAM BROWDER, Princeton University

LAWRENCE D. BROWN, University of Pennsylvania

RONALD G. DOUGLAS, Texas A&M University

JOHN E. ESTES, University of California at Santa Barbara

MARTHA P. HAYNES, Cornell University

L. LOUIS HEGEDUS, Elf Atochem North America, Inc.

JOHN E. HOPCROFT, Cornell University

CAROL M. JANTZEN, Westinghouse Savannah River Company

PAUL G. KAMINSKI, Technovation, Inc.

KENNETH H. KELLER, University of Minnesota

KENNETH I. KELLERMANN, National Radio Astronomy Observatory

MARGARET G. KIVELSON, University of California at Los Angeles

DANIEL KLEPPNER, Massachusetts Institute of Technology

JOHN KREICK, Sanders, a Lockheed Martin Company

MARSHA I. LESTER, University of Pennsylvania

NICHOLAS P. SAMIOS, Brookhaven National Laboratory

CHANG-LIN TIEN, University of California at Berkeley

NORMAN METZGER, Executive Director

Comets and asteroids are in some sense the fossils of the solar system. They have avoided most of the drastic physical processing that shaped the planets and thus represent more closely the properties of the primordial solar nebula. What processing has taken place is itself of interest in decoding the history of our solar neighborhood. Near-Earth objects are also of interest because one or more large ones have been blamed for the rare but devastating events that caused mass extinctions of species on our planet, as attested by recent excitement over the impending passage of asteroid 1997 XF₁₁.

The comets and asteroids whose orbits bring them close to Earth are clearly the most accessible to detailed investigation, both from the ground and from spacecraft. When nature kindly delivers the occasional asteroid to the surface of Earth as a meteorite, we can scrutinize it closely in the laboratory; a great deal of information about primordial chemical composition and primitive processes has been gleaned from such objects.

This report reviews the current state of research on near-Earth objects and considers future directions. Attention is paid to the important interplay between ground-based investigations and spaceborne observation or sample collection and return. This is particularly timely since one U.S. spacecraft is already on its way to rendezvous with a near-Earth object, and two others plus a Japanese mission are being readied for launch. In addition to scientific issues, the report considers technologies that would enable further advances in capability and points out the possibilities for including near-Earth objects in any future expansion of human exploration beyond low Earth orbit.

Claude R. Canizares, Chair
Space Studies Board

Asteroids and comets continually pass by Earth, sometimes at uncomfortably close distances. Impacting objects in the geologic past have created large craters and may have caused the extinction of many living organisms. Over the past decade, scientific and popular interest has grown in assessing the likelihood that Earth may be struck in the future by large meteoroids, commonly known as near-Earth objects (NEOs). In 1990, Congress asked the National Aeronautics and Space Administration (NASA) to study the danger. NASA responded with two reports in 1992, calling for increased efforts to locate NEOs and to address issues of hazard mitigation.^1,2 A second request from Congress to NASA in 1994 sought a plan for discovering all NEOs larger than 1 km in diameter within a time period of 10 years, as a cooperative effort among NASA, the U.S. Air Force, and international partners. A responding report was released in 1995.³ Several telescope facilities and new instruments now coming into operation will dramatically increase the rate of discovery of NEOs and determine their orbits. This program of intensified discovery efforts offers a unique opportunity to broaden scientific understanding of the distribution, composition, and origin of the population of small bodies in interplanetary space.

Previous reports of the National Research Council have stated that asteroids and comets offer important constraints on the early history of our planetary system, and comets, in particular, have highest priority for scientific study.⁴ Moreover, the Space Studies Board and its committees have stressed the appropriateness of initiating a program of asteroid and comet study that includes both reconnaissance and exploration phases.⁵ The most accessible of these bodies, both for observation by ground-based telescopes and for study by spacecraft, are to be found among the NEOs.

Against this background of renewed interest in the study of near-Earth objects, the Space Studies Board charged the Committee on Planetary and Lunar Exploration (COMPLEX) to review current knowledge of NEOs derived from ground- and space-based studies and to answer the following questions:

• What is the present understanding of the origin, composition, and physical characteristics of near-Earth objects?

• What is the expected level of understanding of NEOs in the next decade?

• What levels of ground-based telescopic observation are needed to increase our understanding of targets of high scientific interest?

• What are the likely opportunities for low-cost flyby, rendezvous, landing, and sample return missions to these bodies, and to what degree will these missions address fundamental scientific issues?

This project was formally initiated in May 1996, and the bulk of the material was written in early and mid- 1997. This material was extensively revised and updated in the late summer of 1997. Although many COMPLEX members past and present worked on this report, the bulk of the task of assembling their many individual contributions was performed by Harry Y. McSween and Eugene Shoemaker with the assistance of James Arnold, Richard Binzel, and Alan Tokunaga. The work of the writing team was made easier thanks to the invaluable assistance rendered by Alan Harris (Jet Propulsion Laboratory).

This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's (NRC's) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. COMPLEX thanks reviewers Donald Hunten (University of Arizona), Clark Chapman (Southwestern Research Institute), Margaret Kivelson (University of California, Los Angeles), George Wetherill (Carnegie Institution of Washington), and John Wood (Harvard-Smithsonian Center for Astrophysics) for many constructive comments and suggestions. Responsibility for the final content of this report rests solely with the authoring committee and the NRC.

Executive Summary

1 Introduction to Near-Earth Objects

Scientific Goals for the Study of Near-Earth Objects
References

2 Status of Current Research Programs

Measuring the Physical Characteristics of Near-Earth Objects
Understanding the Mineralogical and Chemical Compositions of Asteroids
Deciphering the Relationships Among Asteroids, Comets, and Meteorites
Understanding the Formation and Geologic Histories of Near-Earth Objects
References

3 Future Research Activities

Detecting Near-Earth Objects
Issues Related to Increased Discoveries of Near-Earth Objects
Observations Needed to Identify Objects of High Scientific Interest
Opportunities for Low-Cost Missions
References

4 Technological Aspects of Studies of Near-Earth Objects

Support and Development Required for Ground-based Observations
Technology Status and Development for Robotic Missions to Near-Earth Objects
Human Exploration of Near-Earth Objects
References

5 Conclusions and Recommendations

Ground-based Telescopic Observations and Instrumentation
Laboratory Studies and Instrumentation
Spacecraft Technology, Instrumentation, and Missions
References

Glossary

Near-Earth objects (NEOs) are asteroids and comets with orbits that intersect or pass near that of our planet. About 400 NEOs are currently known, but the entire population contains perhaps 3000 objects with diameters larger than 1 km. These objects, thought to be similar in many ways to the ancient planetesimal swarms that accreted to form the planets, are interesting and highly accessible targets for scientific research. They carry records of the solar system's birth and the geologic evolution of small bodies in the interplanetary region. Because collisions of NEOs with Earth pose a finite hazard to life, the exploration of these objects is particularly urgent. Devising appropriate risk-avoidance strategies requires quantitative characterization of NEOs. They may also serve as resources for use by future human exploration missions. The scientific goals of a focused NEO exploration program are to determine their orbital distribution, physical characteristics, composition, and origin.

Physical characteristics, such as size, shape, and spin properties, have been measured for approximately 80 NEOs using observations at infrared, radar, and visible wavelengths. Mineralogical compositions of a comparable number of NEOs have been inferred from visible and near-infrared spectroscopy. The formation and geologic histories of NEOs and related main-belt asteroids are currently inferred from studies of meteorites and from Galileo and Near-Earth Asteroid Rendezvous spacecraft flybys of three main-belt asteroids. Some progress has also been made in associating specific types of meteorites with main-belt asteroids, which probably are the parent bodies of most NEOs. The levels of discovery of NEOs in the future will certainly increase because of the application of new detection systems. The rate of discovery may increase by an order of magnitude, allowing the majority of Earth-crossing asteroids and comets with diameters greater than 1 km to be discovered in the next decade.

A small fraction of NEOs are particularly accessible for exploration by spacecraft. To identify the exploration targets of highest scientific interest, the orbits and classification of a large number of NEOs should be determined by telescopic observations. Desired characterization would also include measurements of size, mass, shape, surface composition and heterogeneity, gas and dust emission, and rotation. Laboratory studies of meteorites can focus NEO exploration objectives and quantify the information obtained from telescopes. Once high-priority targets have been identified, various kinds of spacecraft missions (flyby, rendezvous, and sample return) can be designed. Some currently operational (Near-Earth Asteroid Rendezvous [NEAR]) or planned (Deep Space 1) U.S. missions are of the first two types, and other planned U.S. (Stardust) and Japanese (Muses-C) spacecraft missions will return samples. Rendezvous missions with sample return are particularly desirable from a scientific perspective because of the very great differences in the analytical capabilities that can be brought to bear in orbit and in the laboratory setting.

Although it would be difficult to justify human exploration of NEOs on the basis of cost-benefit analysis of scientific results alone, a strong case can be made for starting with NEOs if the decision to carry out human exploration beyond low Earth orbit is made for other reasons. Some NEOs are especially attractive targets for astronaut missions because of their orbital accessibility and short flight duration. Because they represent deep-space exploration at an intermediate level of technical challenge, these missions would also serve as stepping stones for human missions to Mars. Human exploration of NEOs would provide significant advances in observational and sampling capabilities.

The Committee on Planetary and Lunar Exploration (COMPLEX) has considered appropriate baseline research efforts, as well as a number of augmentations to existing programs for the discovery and characterization of NEOs. With respect to ground-based telescopic studies, the recommended baseline is that NASA and other appropriate agencies support research programs for interpreting the spectra of near-Earth objects (NEOs), continue and coordinate currently supported surveys to discover and determine the orbits of NEOs, and develop policies for the public disclosure of results relating to potential hazards. Augmentations to this baseline program include, in priority order, that relevant organizations do the following:

1. Provide routine or priority access to existing ground-based optical and infrared telescopes and radar facilities for characterization of NEOs during favorable encounters, or

2. Provide expanded, dedicated telescope access for characterization of NEOs.

The baseline recommendation with respect to laboratory studies and instrumentation is that NASA and other appropriate agencies should support continued research on extraterrestrial materials to understand the controls on spectra of NEOs and the physical processes that alter asteroid and comet surface materials. An appropriate augmentation to this baseline is to support the acquisition and development of new analytical instruments needed for further studies of extraterrestrial materials and for characterization of returned NEO samples.

Spacecraft missions and the development of the associated technology and instrumentation are essential components of any program for the study of NEOs. The baseline recommendation in this area is to support NEO flyby and rendezvous missions. Appropriate augmentations include, in priority order, the following:

1. Develop technological advances in spacecraft capabilities, including nonchemical propulsion and autonomous navigation systems, low-power and low-mass analytical instrumentation for remote and in situ studies, and multiple penetrators and other sampling and sample-handling systems to allow low-cost rendezvous and sample-return missions.

2. Study technical requirements for human expeditions to NEOs.

Although studies evaluating the risk of asteroid collisions with Earth and the means of averting them are desirable, they are beyond the scope of this report.