NASA Ames Space Science Division
The Spaceguard Survey
Chapter 4: History and Current Programs
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.