9.      The Current Situation 

Although the possibility that comets or asteroids could impact with the Earth was discussed as far back as the late 17th century (by Edmund Halley), and, since then, by others such as Harvey Nininger in 1942 and Harold Urey in 1973, the scientific world's attention was really caught by the publication, in 1980, of a paper entitled "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction", by Louis and Walter Alvarez of the University of California, Berkeley. The paper linked the sudden extinction of about 70% of the recognised fossil species of maritime organisms, and possibly more than 90% of all species on Earth, including the dinosaurs, with a major impact of a ten-kilometre diameter body some sixty-five million years ago. Since then it has been determined that a 2-300 kilometre buried crater at Chicxulub in the Yucatan, Mexico, is the most likely "smoking gun". Although Frank Dachille and Allan Kelly had suggested this theory in 1953 and M.W. De Laubenfels in 1956, the Alvarez work received the greatest prominence. Nine years later the close passage of asteroid 1989 FC (now known as (4581) Asclepius) prompted a report from the American Institute of Aeronautics and Astronautics (AIAA) which recommended an urgent increase in the detection rate of potential impactors.

As a result of these events the United States House of Representatives charged the National Aeronautics and Space Administration (NASA), in the NASA Multi Year Authorization Act of 1990, with setting up two workshop studies, one "to define a program for dramatically increasing the detection rate for Earth-orbit-crossing asteroids", and the other to "define systems and technologies to alter the orbits of such asteroids or to destroy them if they should pose a danger to life on Earth".  The Detection Workshop (also known as the Morrison committee) report was finalised in January 1992, and recommended that a survey for near-Earth objects be initiated as soon as possible.  The Interception Workshop reported in February 1993.

 The plans for the Spaceguard Survey, which took its name from a similar project described by the celebrated science fiction author Arthur C. Clarke in his novel "Rendezvous with Rama", called for the establishment of a network of six 2.5 metre telescopes, positioned around the globe, and a central research establishment. The estimated cost of setting up the project was, in 1993, $50 million, with an additional $10 million per year running costs. The complete system of telescopes would take about five years to build and bring on line.

In 1993 NASA allocated an extra $0.5 million for asteroid research. The United States Department of Defense also allocated funds for a joint DoD - NASA project to send a small probe (Clementine) to survey the Lunar surface, and visit (1620) Geographos, an Earth crossing asteroid (sadly this part of the mission failed). Clementine 2 is under development, but the planned visits to two asteroids have not received funding due to the personal intervention of President Clinton.

During 1993/94, several significant events occurred that raised public and professional awareness of the threat.

The much publicised impact of Comet Shoemaker-Levy 9 on Jupiter in the summer of 1994.

The "near-miss" of Asteroid 1994 XM1 in December 1994.

The publication of a position paper, entitled "Response to the Potential Threat of a Near-Earth-Object Impact" by the AIAA in January 1995. 

The US Congress requested that NASA consider the threat with more urgency, and directed them to work with the US Air Force (who already operate a network of telescopes used to track man made satellites).   The NEO Survey Science Working Group was established, under the chairmanship of Eugene Shoemaker, and their report, published in June 1995, once again stressed the need for a comprehensive survey of NEOs, and proposed a refined proposal for a project costing $24 million in the first five years and $3.5 million annually for the following five years. These dramatically reduced costs reflected the huge advances in telescope, sensor and data processing technologies.  The report dealt only with the US component, but endorsed the recommendations of the earlier Morrison committee that international participation would be crucial. This is seen, not only as a way of spreading the financial burden, or as a means of providing "all round" surveillance, but as an effort to create a truly international forum for global defence. On 9 August 1995 NASA reported to Congress, but announced that, due to financial constraints, it was unable to fully fund the SPACEGUARD Survey. However, the agency stated that it remained committed to ongoing NEO survey activities, and would be providing over $1 million per year for these efforts. With this level of researching, the survey would take at least 100 years to catalogue 90% of potentially threatening objects.  In 1999 NASA funding was increased to $3.5 million per year.

The most important requirement for any discovery program is a telescope with a large field of view (FOV); however, it is equally important to have adequate focal plane instruments. Until the late 1980’s most of the telescopes used for asteroid detection and follow-up used photography. This was the case with the two pioneering programs conducted at the Palomar Observatory by the teams led by Eleanor F. Helin and Eugene M. Shoemaker. Both teams used the Palomar 0.46m Schmidt telescope, with photographic films producing a (circular) FOV of 56 square degrees. The advantage of the large FOV was, however, negated by the amount of time needed to visually scan the films using a stereomicroscope. It is not surprising that the discovery of new objects was rare when compared with current rates.  During the early stages using such techniques 1-2 asteroids and an occasional comet were discovered for 13,000 square degrees of sky surveyed each year, rising to about 20 NEOs for 80-100,000 square degrees at the end of the two programs. The strategy adopted by the teams at Palomar was to repeatedly image a portion of the sky near opposition. This is the best place to search for asteroids because the small phase angle increases their apparent magnitude. This strategy was adequate when very few objects were known  (136 in May 1993, as opposed to the more than 700 in May 1999) but was clearly biased towards low-inclination and large objects.

The first group to adopt electronic devices at the focal plane of their telescope was the Spacewatch team.  The Spacewatch telescope (see figure 1) was established during the 1980’s, and since 1989 has been fully operational.  The telescope, a 0.91m Newtonian with a 2048 x 2048 CCD at the focus, produces a FOV of roughly 0.5 x 0.5 degrees. The use of a new imaging technique compensates for the small FOV by maintaining the telescope in a fixed position and reading out the CCD, line by line, at sidereal rate. In this way a specific point in the sky "moves" from line to line until it is read by the system; the effective integration time (i.e., the time needed to "cross" the CCD) is about 2.5 min. This technique, called "staring mode" produces a strip on the sky of width 0.5 degrees and length corresponding to the time of operation (usually half an hour of right ascension). At the end of the strip the telescope is moved back to the original position and another strip, covering the same region of the sky, is taken. This procedure is then repeated a third time. The team has written semi-automatic detection software that compares the three strips while they are being taken.  In principle, any object that has moved in different strips could be an asteroid. However, in order to eliminate a number of false detections (artificial satellites, cosmic rays, and defects in the read-out), the continuous presence of an operator is needed. The Spacewatch telescope produces a large amount of data (about 2 GB every night), containing large numbers of asteroids of all kinds (about 500 for each scan). Currently the Spacewatch team is installing and testing a new 1.8-m telescope that has been largely funded by private donations. The difference between the early discovery programs (Palomar and Spacewatch) was not limited to the different imaging technique, but concerned also the "philosophy" of the research. Due to the limited magnitude attainable by the Schmidt (about 17.5), the Palomar teams were unable to detect faint objects. On the contrary, the Spacewatch telescope was able to reach magnitude 20.5 thanks to the use of a CCD array. This meant that Spacewatch was imaging a "deeper" portion of space. Moreover, the Spacewatch team was also able to detect very fast moving (therefore close) objects with very small diameters. It is still controversial as to whether it is better to scan a larger portion of the sky at a lower magnitude or to reach higher magnitudes in a more limited region, but the recent outstanding results of the LINEAR project (see below) seem to favour the former strategy.

The two Palomar surveys were terminated in the early 1990’s, mainly because it soon became evident that the future for NEO discovery lay in the use of powerful CCD arrays and dedicated (even if not large) telescopes. It is important to note, however, that another project started at the beginning of the decade and terminated in 1995.  The Anglo-Australian Near-Earth Asteroid Survey (AANEAS) led by Duncan I. Steel made use of the large UK Schmidt Telescope (UKST) located at Siding Spring (Australia) for search and follow-up, covering about 40,000 square degrees per annum.  Another major, and very important, activity of the team was the inspection of all of the photographic plates that were taken at the UK Schmidt site.  They demonstrated that archival searches of past positions of asteroids (the so-called "pre-discoveries", or "precoveries") is as very method of following-up discoveries as a single precovery may lengthen the observational arc of an asteroid by decades. The recent example of Asteroid 1997 XF11 showed how efficient such a technique can be.  Notwithstanding the excellent results obtained during its operation, the AANEAS project was terminated by the Australian government in 1995.

In December 1995 one of the teams working at Palomar (Helin) started a new program at JPL, the Near-Earth Asteroid Tracking (NEAT) project.  The team has been allocated observing time of a GEODSS telescope of the US Air Force, located in Maui, Hawaii (see figure 2). The GEODSS telescopes, used for tracking of artificial satellites and space debris, have 1m apertures. The NEAT team has access to the telescope for six nights per month and, using its own camera consisting of a 4096 x 4096 CCD, produces a FOV of 2.5 square degrees.

At the beginning of an observing run the controlling computer is loaded with a "script" that contains all the information about the fields (that are concentrated near the ecliptic and near opposition) to be observed.  The telescope is then driven by an automatic control program that optimises the collection of images. At the end of the run an area of about 45 degrees on each side of the opposition is sampled. Other automated procedures are used to analyse the data.  Each field is imaged three times and the software detects all moving objects, measuring their positions. The processed data, consisting of small portions of the images containing the objects, is in the region of 15 MB per night) and are transmitted to JPL over a commercial telephone line. There the NEAT team checks the reliability of the detections and files them to the Minor Planet Center (MPC).

Another major advance in NEO discovery was achieved in 1998 when the LINEAR project began its operations. The Lincoln Near-Earth Asteroid Research is the brainchild of the Lincoln Laboratory of MIT, which maintains an Experimental Test Site at White Sands, near Socorro (New Mexico), collocated with another GEODSS telescope. The instrument used by the LINEAR project is not an operational part of the GEODSS system but is used, together with other instruments at the site, for testing and developing new devices to be used by the system.  LINEAR uses a new, 2560 x 1960, frame transfer CCD, whose major advantage is the ability to read it while the new image is being integrated. After an initial test period, LINEAR started regular operations in March 1998, and it quickly demonstrated its capabilities by sending 151,000 asteroid observations to the MPC in the first month.  Amongst these were the detections of 13 new NEOs.  At the beginning of the programme LINEAR searched close to the opposition point, like NEAT, but subsequently it has extended the search out of the ecliptic. Four more discovery programs are worthy of mention.

LONEOS is a project led by Ted Bowell at the Lowell Observatory. It uses a 44-cm Schmidt located at the Lowell site on Anderson Mesa.  The camera has a 4096 x 4096 CCD chip, with a FOV of about 10 square degrees, that reaches magnitude 19.6 with 68 seconds of integration time. The project is still not fully operational, however, due to problems encountered during set-up. The planned activity consists in continuous, automated scanning of the entire sky, so LONEOS is likely to become one of the major discovery programs.

ODAS is a joint French-German project. The telescope is a large Schmidt (150/90 cm) located at the Caussols Station of the Observatoire de la Côte d'Azur, near Nice. The camera has a 2040 x 2048 Loral chip, with a FOV of about 0.3 square degrees. An upgrade to a 4096 x 4096 chip is anticipated. This is the only discovery program active in Europe, but is likely be terminated soon due to the decommissioning of the Schmidt telescope.

The SCAP program of the Beijing Astronomical Observatory (China) uses a 90/60 Schmidt telescope coupled to a 2048 x 2048 Ford CCD. The FOV is of about one square degree. This is the only discovery program active in Asia.

The Catalina Sky Survey uses a 46 cm Schmidt telescope with a 4096 x 4096 chip, resulting in a FOV of 8.4 square degrees and capable to reach magnitude 20 with 240 seconds of integration time. The system is currently being adjusted and calibrated and the team plans to start routine operations soon.

All the major discovery programs active at this time have been mentioned in the previous paragraph.  

These projects find about 15-20 new NEOs every month (statistics as at June 1999). Although not co-ordinated among themselves (nor with centres devoted to follow-up) the efficiency is increasing, mainly due to the large amount of sky covered. 

It must be noted, however, that the situation is evolving rapidly and that detailed here is likely to change within in a few months.

The reasons for the fast evolution of this field can be summarised as follows:

·         The increase in funding from NASA to the American discovery programs

·         The increase in public awareness of the threat posed by asteroids and comets

·         The experience acquired in the past decade

NASA has recently increased the budget devoted to NEO research, and the majority of this money will be spent on upgrading existing facilities. NASA has been instructed by the US Congress to "discover 90% of the NEOs larger than 1 km in ten years". The prospects for the future are very good. Spacewatch will have the second, 1.8m telescope available soon and will increase its discovery rate by at least a factor of two (probably much more). NEAT, that has suffered in the last months from a refurbishment at the Maui telescope, will be moved to a second GEODSS in Hawaii, and its camera will probably be upgraded; moreover, the number of nights available at the site will increase from 6 to 18 per month. LINEAR will soon have a second telescope available in New Mexico and will count on 18 nights per month. LONEOS should start regular operations soon.

This increased support is also, at least partially, due to the publicity that the NEO threat has received world-wide through the media and following the spectacular collision of P/Shoemaker-Levy 9 with Jupiter in 1994.  It is not limited to the USA, however.  The Japanese government has funded the set-up of a Japanese discovery program that will start operations within the year and a new project is being evaluated at the Klet Observatory (Czech Republic).

It is possible to evaluate the current degree of completeness. Table 7 (prepared by D. Morrison and A.W. Harris) shows the statistics for 96 NEOs larger than 1 km (actually, brighter than mag 18) discovered in the 24 months between July 1997 and June 1999 (1999 values are a projection based on the discoveries from January to April). 

The average rate of discovery of large NEOs for 1998 was 54 per year, or almost 5 per month. The discovery rate in the second half of 1998 was actually a factor of 5 greater than the same six-month period in 1997. The value for 1999 is of the order of 70 objects per year.

In a ten-year Spaceguard survey expected to detect 90% of the NEO population with D > 1 km, we must discover just over 20% in the first year, with the rate declining exponentially thereafter as greater completion is reached and more of the objects found are rediscoveries. Therefore, to achieve the stated Spaceguard goal of finding 90% of the 1500-2000 NEOs in a decade, we need to increase detection rates by a factor of about 7 over the 1999 anticipated values.

However, almost all the major discovery programs are sampling the sky close to the opposition point, with the noticeable exception of LINEAR and LONEOS. This strategy optimises the searches for objects with semi-major axes larger than 1 AU, because these NEOs spend most of their time outside the Earth's orbit, and may be observed in the best possible conditions near opposition. However, these strategy introduces a strong bias in favour of low-inclination objects of the Apollo type (a > 1, q < 1.017, a being the semi-major axis and q the perihelion distance, in AU) and Amors (1.3 > q > 1.017). Atens, on the contrary, have a < 1 and Q > 0.987 (Q is the aphelion distance); if their orbits are slightly crossing the Earth's, it may be that their positions in the sky are high on the ecliptic plane, for perspective reasons, and their periods outside the Earth's orbit can be very short. For these reasons it is very probable that the Aten population is under sampled in our databases. In addition, it is very probable that another population of objects is completely missing in our sample.  Theoretical investigations, coupled to numerical integration of NEO orbits over extended periods of time, indicate that Aten objects may evolve into orbits completely inside that of the Earth. These objects, sometimes called also Arjunas or IEOs (Inner Earth Objects), have never been detected simply because they are never located at opposition.

A strategy optimised for the search of Atens (which, account for the largest number of close encounters with Earth) would require the scanning of the sky at high declinations - something that LONEOS and LINEAR are going to provide - and at small solar elongations. This last requirement is currently unaddressed, because observations close to the sun are difficult and inefficient. 

The current status of the discovery programs can be summarised as follows:

Seven programs are active (but one may terminate soon), five of which located in the USA

The programs are not well co-ordinated: each of them has its own survey strategy and data processing techniques

The original data are permanently stored only in a few cases

The sky coverage in not uniform, especially at high declinations and low solar elongations

There is great unbalance between the northern and southern hemispheres: no search program is active (nor planned) in the south

Some of the NEO populations (Atens and IEOs) are definitely under sampled.