4.1 Introduction

The first Earth-crossing asteroid, Apollo, was discovered photographically in 1932 at Heidelberg and then lost until 1973. In the following decades only a handful of additional ECAs were discovered, and many of these were temporarily lost also. Not until the 1970s were regular searches initiated, using wide-field Schmidt telescopes of modest aperture. Some of these photographic survey programs continue today with steadily increasing discovery rates. In the early 1980s these photographic approaches were supplemented by a new technique of electronic CCD scanning implemented at the University of Arizona, and by the late 1980s this more automated approach was also yielding many new discoveries. Even today, however, the total worldwide effort to search for NEOs amounts to fewer than a dozen full-time-equivalent workers! In this chapter we briefly review the history and current status of both the photographic and CCD searches.

4.2 Photographic Search Programs

Photographic techniques

The overwhelming majority of discoveries of near-Earth asteroids (and increasingly of comets) has been obtained from photographic searches carried out with wide-field Schmidt telescopes. The bulk of discoveries has been made in the last decade, and the rate of discovery is rapidly increasing. This increase is due in part to improved technology but principally to increased interest within the astronomical community.

To date the two most productive photographic teams in this field have been those directed by E. F. Helin and E.M. Shoemaker. Most of their work has been done using the 0.46-m Schmidt telescope at Palomar Observatory, California. Observing programs on three large Schmidt telescopes located in France, Chile,and Australia have also contributed but rather sporadically, as has work carried out with a narrower-field astrograph in Ukraine. A new successful program has recently been started on the UK Schmidt in Australia. The three main photographic programs now in operation are described briefly below.

Various techniques are used to detect and measure NEOs, but the search process must be carried out very soon after the exposure in order to permit rapid followup. In some programs the films are exposed in pairs with a gap in time between the first and subsequent exposure, then scanned with a specially built stereo comparator. Images which move noticeably between the first and second exposure may be detected in this way. Alternatively, a visual search can be carried out using a binocular microscope, and trailed images (produced by the motion of the NEO during the time exposure) are noted. The angular velocity may be inferred from the motion between exposures or in the case of a single exposure, from the trail length. Selection of potential NEOs is carried out on the basis of this angular velocity, and only those objects with anomalous motions are followed up to determine precise orbits.

A variety of photographic emulsions have been used in NEO searches, but the most effective have been the IIIa-type emulsions coated on glass from Kodak, introduced twenty years ago, and a panchromatic emulsion coated on a film base released in 1982, again from Kodak. The new film (4415) has been particularly useful and is now the emulsion of choice for this work. CHECK

Planet-Crossing Asteroid Survey (PCAS)

The PCAS survey for Earth-crossing and other planet-crossing asteroids was initiated by E.F. Helin and E.M. Shoemaker in 1973 and is now directed by Helin. It is the longest running dedicated search program for the discovery of near-Earth asteroids and is carried out with the 0.46-m Schmidt telescope at Palomar Observatory in California. Early in the survey, about 1000 square degrees of sky were photographed each month. In the last ten years, the use of fast film has allowed shorter exposures leading to greater sky coverage. This fact, in combination with a custom-made stereo-microscope, has resulted in a five-fold increase in the discovery rate over the early years of the program. Using the stereo pair method, up to 4000 independent square degrees of sky can be photographed per month. This program has been particularly successful in getting out early alerts on new discoveries so physical observations can be obtained during the discovery apparition. There has also been an organized international aspect to this program, called the International Near-Earth Asteroid Survey (INAS), which attempts to expand the sky coverage and the discovery and recovery of NEAs around the world.

Palomar Asteroid and Comet Survey (PACS)

A second survey with the Palomar 0.46-m Schmidt was begun by E.M. and C.S. Shoemaker in 1982 and has continued with the collaboration of H.E. Holt and D.H. Levy. About 3000 square degrees of sky are photographed each month. Both the PACS and PCAS programs center their sky coverage at opposition and along the ecliptic and attempt to cover as much sky as possible in every 7-night observing run at the telescope. The two programs combined produce about 6000 independent square degrees of sky coverage per month.

Anglo-Australian Near-Earth Asteroid Survey (AANEAS)

The AANEAS program began in 1990 under the direction of D.I. Steel with the collaboration of R.H.McNaught and K.S.Russell using a visual search of essentially all plates taken with the 1.2-m U.K. Schmidt Telescope as part of the regular sky survey. Up to 2500 square degrees are covered each month to a limiting stellar magnitude near 22.

4.3 The Spacewatch CCD Scanning Program

An alternative to photographic search programs was developed at the University of Arizona under the name "Spacewatch" by T. Gehrels in collaboration with R. MacMillan, D. Rabinovich, and J. Scotti. This system makes use of a CCD detector instead of photographic plates. It differs from the wide-field Schmidt searches in scanning smaller areas of sky but doing so to greater depth. In 1981, the Director of the University of Arizona Observatories made the Steward 0.9-m Newtonian reflector on Kitt Peak available, and initial funding for instrument development was obtained from NASA. By 1983 Spacewatch had a 320 x 512 pixel CCD in operation, which was too small for discovery of near-Earth asteroids on that telescope, but was exercised in order to get experience with CCD modes of operation. Later this was upgraded to a 1048x1048 pixel CCD.

The basic construction and operation of the CCD are ideal for scanning. It is referred to as the "scanning mode"; in older literature it is called Time Delay Integration (TDI). The scanning is done by exactly matching the rate of transfer of the charges, from row to row of the CCD chip, with the rate of scanning by the telescope on the sky. A basic advantage of scanning is the smooth continuous operation, reading the CCD out during observing, compared to stop-and-go resetting the telescope for each exposure and waiting for the CCD to be read out before the next exposure can be started. Another advantage of scanning is that the differences in pixel sensitivity are averaged out, and two-dimensional "flat fielding" calibration is therefore not needed.

As each line of the CCD image is clocked into the serial shift register, it is read out by the microcomputer and passed on to the workstation. There the data are displayed, searched for moving objects, and recorded on magnetic tape. As each moving object is discovered, from the three repeated scan regions of about 30 minute length, its image is copied to a separate "gallery" window for verification by the observer. Some five years of computer programming went into this system.

Currently this Spacewatch system is discovering approximately as many NEOs as the photographic surveys. As a consequence of its more sensitive detector, it also tends to discover more smaller objects, including three objects found in 1991 that are only about 10 m in diameter. Substantial increases in capability are proposed with a new telescope of larger aperture (1.8 m) to replace the current Spacewatch telescope in the same dome.

4.4 Potential of Current Programs

The following Chapters of this Report describe a survey program based on a new generation of scanning telescopes. However, there is still excellent work to be done with current instruments during the transition to the new survey. The near-term potential of photographic techniques may be considered in the following context. With the provision of about $1 million capital costs and $1 million per year operating expenses it would be possible to boost the current worldwide photographic discovery rate from about 20 per year to 100 per year. Similarly, an upgrade of the Spacewatch CCD scanning system to 1.8-m aperture would more than double the output of this system, and still greater gains are possible utilizing advanced, large-format CCDs. This instrument can also be used as a test-bed for new NEO survey techniques such as use of CCD arrays, optimizing of scanning strategies, and refinement of automated search software.

By the time large search telescopes with CCD detectors become available later in this decade it would be possible to have a sample of at least 1000 NEO's with well determined orbits. From this sample, which should include about 10 percent of the larger bodies, we will gain a much better idea of the physical properties and dynamical distribution of the total population. Such information will be invaluable in optimizing the search strategy of the large new telescopes. In addition, the operation of the large CCD search facilities will require trained personnel and a complex organization to utilize them to the fullest extent, and expansion of current programs can provide the experienced staff that will be required if and when the full survey begins operation.

We assume here that wide-field photography will continue in a substantially productive manner for a number of years. CCD work is expected at the Spacewatch telescope on Kitt Peak in Arizona (with proposed upgrade to 1.8-m aperture) and with the French OCA Schmidt and the Palomar 0.46-m Schmidt, both of which are proposed for conversion to CCD operation.