Near-Earth Objects, Part Four

Near-Earth Object Discovery Teams

There are several Near-Earth Object (NEO) discovery teams either in operation or in the planning stages. The early efforts to discover NEOs relied upon the comparison of photographic films of the same region of the sky taken several minutes apart. The vast majority of the objects recorded upon these films were stars and galaxies and their images were located in the same relative positions on both photographic films. Because a moving NEO would be in a slightly different position on each photograph and the background stars and galaxies were not, the NEOs appeared to “rise” above the background stars when viewed with special stereo viewing microscopes.

All of the NEO discovery teams currently use so-called charged couple devices (CCD) rather than photographs. These CCD cameras are similar to those used in camcorders and they record images digitally in many electronic picture elements (pixels). The length and width of CCD detector is usually given in terms of these pixels. A fairly common astronomical CCD detector might have dimensions of 2096 x 2096 pixels. While the CCD technology allows today’s detectors to be more sensitive and accurate than the older photographic methods, the modern discovery technique itself is rather similar. Separated by several minutes, three or more CCD images are taken of the same region of the sky. These images are then compared to see if any NEOs have systematically moved to different positions on each of the separate images. For a newly discovered NEO, the separation of the object’s location from one image to another, the direction it appears to be traveling, and its brightness are helpful in identifying how close the object was to the Earth, its size and its general orbital characteristics. For example, an object that appears to be moving very rapidly from one image to the next is almost certainly very close to the Earth. Sophisticated computer-aided analyses of the CCD images has replaced the older, manual stereo microscope techniques for all the current NEO search programs.

Not surprisingly, those discovery teams who search the largest amount of sky each month will have the most success in finding new NEOs. How much sky each telescope covers per month will depend upon a number of factors including the number of clear nights available for observing, the sensitivity and efficiency of the CCD detector, and the field of view of the telescope. Wide field of view telescopes can cover more sky per given time than telescopes with narrower fields of view. It is also important for search teams to extend their searches to greater and greater distances from the Earth or, in other words, go to fainter and fainter limiting magnitudes. A 6th magnitude star is roughly the limit of a naked eye object seen under ideal conditions by someone with very good eyesight. A 7th magnitude star would be 2.5 times fainter than a 6th magnitude star and an 8th magnitude star would be 6 times fainter than a 6th magnitude object (2.5 x 2.5 = 6.25). A difference of 5 magnitudes is a brightness difference of nearly 100 (2.5 x 2.5 x 2.5 x 2.5 x 2.5 is equal to about 100).

In terms of the discovery efforts for NEOs, NASA's current goal is to discover at least 90% of all NEOs whose diameters are larger than 1 kilometer within 10 years of project start. To meet the NASA goal, the rate with which new objects are discovered will necessarily be largest in the first few years. This is because during the latter years of the 10-year interval, more and more “discoveries” will actually be of objects that have been previously found. Currently, the best estimate of the total population of NEOs larger than one kilometer is about 1000. The progress toward discovering 90% of this population can be monitored under the "NEO Discovery Statistics" section on the NASA “Near-Earth Objects” Web site (http://neo.jpl.nasa.gov/).

Here is an example from the NEO Discovery Statistics section. As of April 24, 2012, here are the known numbers for the various categories.

Near-Earth Comet (NEC), 91
Atira Group, 11
Aten Group, 702
Apollo Group, 4808
Amor Group, 3326
Potentially Hazardous Asteroids (PHA), 1302
Near-Earth Asteroids (NEA), 8847
NEO, 8938


NEOWISE Space Infrared Survey

NEOWISE is the term used to describe the near-Earth object observing capability of the Wide-field Infrared Survey Explorer (WISE) telescope. WISE uses a 40 cm aperture telescope to observe the entire sky 1.5 times in its 9-month design lifetime. The cryogens used to cool the telescope are expected to run out in November 2010. The infrared wavelength regions are centered at 3.4, 4.6, 12, and 22 microns. The WISE spacecraft is in a nearly polar orbit with the telescope always directed 90 degrees from the sun's direction. NEOWISE is an enhancement to the WISE data pipeline that allows new moving objects to be followed up by ground-based telescopes operating in the visible region. These follow-up observations ensure that a NEO's orbit will be secure and the object will not become lost. Because dark asteroids re-radiate strongly in the infrared, NEOWISE observations can often provide better estimates for NEO diameters than can optical telescopes. That is, optical telescopes observe reflected sunlight so they cannot easily tell the difference between a small bright object and large dark object.

Principal Investigators: Ned Wright (UCLA) for WISE & Amy Mainzer (JPL) for NEOWISE

Look here for more information on WISE and
NEOWISE: http://www.jpl.nasa.gov/news/news.cfm?release=2010-174 and http://wise.ssl.berkeley.edu/

Look here for a tally of NEOWISE discoveries of comets and near-Earth objects: http://neo.jpl.nasa.gov/stats/wise/


Lincoln Near-Earth Asteroid Research (LINEAR)

In cooperation with the Air Force, MIT's Lincoln Laboratory has been operating a near-Earth object discovery facility using a one-meter aperture GEODSS telescope. GEODSS stands for Ground-based Electro-Optical Deep Space Surveillance and these wide field Air Force telescopes were designed to optically observe Earth orbital spacecraft. The GEODSS instruments used by the LINEAR program are located at the Lincoln Laboratory's experimental test site in Socorro, New Mexico. Tests in early 1996 indicated that the search system, now known as LINEAR, had considerable promise. In the period between March and July 1997, a 1024 x 1024 CCD pixel detector was used in field tests and, while this CCD detector filled only about one fifth of the telescope's field of view, four NEOs were discovered. In October 1997, a large format CCD (1960 x 2560 pixels) that covered the telescope's 2 square degree field of view was employed successfully to discover a total of 9 new NEOs. Five more NEOs were added in the November 1997 through January 1998 interval when both the small and large format CCD detectors were employed. Beginning in October 1999, a second one-meter telescope was added to the search effort.

Currently, LINEAR telescopes observe each patch of sky 5 times in one evening with most of the efforts going into searching along the ecliptic plane where most NEOs would be expected. The sensitivity of their CCDs, and particularly their relatively rapid read out rates, allows LINEAR to cover large areas of sky each night

Principal Investigator: Grant Stokes

Co-Investigators: Scott Stuart and Eric Pearce

Look here for additional information on the LINEAR program: http://www.ll.mit.edu/LINEAR/


Near-Earth Asteroid Tracking (NEAT)

The NEAT discovery team at the NASA/Jet Propulsion Laboratory has a cooperative agreement with the U.S. Air Force to use a GEODSS telescope to discovery near-Earth Objects. The NEAT team designed a CCD camera and computer system for the GEODSS telescope located on Haleakala, Maui, Hawaii. The CCD camera format is 4096 x 4096 pixels and the field of view is 1.2 x 1.6 degrees. When used for NEO discovery efforts, Air Force contractor personnel operate the telescope and the data are routed directly to the Jet Propulsion Laboratory for analyses. The NEAT system began observations in December 1995 and observed for 12 nights each month centered on the new moon through December 1996. Beginning in January 1997, the number of observing nights was reduced to the six nights each month preceding the new moon because of increased Air Force operational requirements upon the facility. In February 2000, NEAT operations were transferred from the one-meter GEODSS telescope to the nearby AMOS 1.2-meter telescope. While the field of view of the AMOS 1.2-meter telescope is about that of the 1-meter GEODSS telescope, the AMOS telescope is available for more nights per month than was the GEODSS telescope.

Beginning in April 2001, a 1.2 meter aperture Schmidt telescope at Palomar mountain (southern California) was also put into service to discover and track near-Earth objects. This telescope is equipped with three cameras, each of which has its own 4096x4096 CCD array.

As part of the NEAT effort, a SkyMorph system was developed whereby searches can be made for pre-discovery images of newly discovered objects. These pre-discovery images can then immediately improve the initial orbits of newly discovered NEOs and ensure that these objects will not be lost. Searches within the SkyMorph system can be made upon the archive of approximately 40,000 CCD images made by the NEAT system or within either the original or second generation Digitized Sky Surveys (DSS and DSS2).

Raymond Bambery: Principal Investigator

Steven H. Pravdo: Co-Investigator and Project Manager

David L. Rabinowitz, Ken Lawrence and Michael Hicks: Co-Investigators

Look here for additional information on the NEAT program: http://neat.jpl.nasa.gov/


Spacewatch

Beginning in 1984, the 0.9-meter, Newtonian f/5 Steward Observatory Spacewatch telescope has been used full time for surveying comets and asteroids under the leadership of Tom Gehrels. First installed on the University of Arizona campus in 1923, this telescope was moved to Kitt Peak, Arizona in 1963. In 1983, this instrument was donated to the Spacewatch team and in 1984 it then became the first telescope to detect and discover asteroids and comets with electronic detectors (CCDs, as opposed to photograph= ic plates or film).

The initial 320 x 512 RCA CCD detector used from 1984 to 1988 was replaced with a large format 2048x2048 CCD detector used during the interval 1989-1992. This system had a field width of 38 arc minutes and limiting magnitude of 20.5. The sensitivity of the CCD (quantum efficiency) was doubled to 70% in 1992 when a thinned 2048 x 2048 CCD was installed and extended the limiting magnitude down to 21.0. The 0.9-meter telescope is used about 23 nights per month to search for near-Earth objects. By locking the right ascension axis in place and allowing the star fields to drift through its field of view ("drift-scan") while the CCD detector was constantly read out, this telescope scanned at a rate that covers about 200 square degrees of sky each month down to magnitude 21. Each region of sky is scanned three times, about thirty minutes apart, to examine which objects have moved relative to the background stars.

This system was the first to discover NEOs with CCDs, the first to discovera comet with a CCD, and the first to discover an NEO with automated image processing software. In 2000, large minor planet (20000) Varuna in the outer solar system was discovered with this system. Spacewatch has discovered many of the smaller near-Earth objects that pass close to the Earth, including a ten-meter sized asteroid (1994 XM1) that had a then record close Earth passage (105,000 km on 1994 Dec 9). Discoveries of Potentially Hazardous Asteroids (PHAs) with the 0.9-m drift scan system total 45, NEOs total 274, and follow-up observations of NEOs many thousands.

In 2001, the Spacewatch group began observing with a newly built 1.8-meter aperture telescope designed for follow-up of asteroids that get fainter after discovery.

In late 2002, a large-mosaic CCD camera (four 4608 x 2048 CCDs) was added to the 0.9 meter, and the optical system has been modified to allow a wider field-of-view (2.9 square degrees). The 0.9 meter design now operates in the "stare" mode rather than in the previous "drift-scan" mode, whereas the 1.8-meter telescope operates in the "drift-scan" mode.

From 2005 through 2008 the Spacewatch group gradually shifted their emphasis to follow-up observations that are critical for securing accurate orbits. In addition to these activities, the Spacewatch team has been involved with studies of the Centaur and Trans-Neptunian minor planet populations and the sizes of short period comet nuclei.

Robert S. McMillan: Principal Investigator

Robert Jedicke and Jeff Larsen: Collaborators

Joe Montani and Jim Scotti: Senior Research Specialists

Look here for additional information on the Spacewatch program: http://pirlwww.lpl.arizona.edu/spacewatch/


Lowell Observatory Near-Earth Object Search (LONEOS)

Begun in 1993, the LONEOS system utilizes a 0.6-meter f/1.8 Schmidt telescope in Flagstaff Arizona to discover near-Earth comets and asteroids. The telescope was acquired from Ohio Weslayan University in 1990. Using a 4K x 4K CCD detector to cover a field of view of 2.9 x 2.9 degrees, the telescope is designed to make four scans per region over the entire visible sky each month down to a limiting magnitude of about 19. In 1999-2000, the efficiency of the LONEOS program in discovering NEOs increased significantly due to upgrades to the CCD camera and data reduction software.

Edward Bowell: Principal Investigator

Bruce W. Koehn: Computer Programming

Karri Muinonen: Asteroid detection modeling

Look here for additional information on the LONEOS program: http://asteroid.lowell.edu/asteroid/loneos/loneos.html


Catalina Sky Surveys

The Catalina Sky Surveys (CSS) is currently the most efficient NEO survey program for finding new near-Earth objects. CSS utilizes three refurbished telescopes all using identical thinned, multichannel cryogenically cooled 4K x4K CCD cameras; 1) The original Catalina Sky Survey (CSS, MPC COD 703) using a 0.7-meter f/1.8 Schmidt telescope with a 2.9 x 2.9 degree field at the Steward Observatory Catalina Station (2510m elevation, 20 km northeast of Tucson, Arizona), 2) The Siding Spring Survey (SSS, MPC COD E12) using the Uppsala 0.5-m f/3.5 Schmidt telescope with a 2.0 x 2.0 degree field operated jointly with the Australian National University Research School for Astronomy and Astrophysics at Siding Spring Observatory, Australia (1150m elevation), and 3) the Mt. Lemmon Survey (MLS, MPC COD G96) using the 1.5-meter f/2.0 prime focus telescope with a 1.0 x 1.0 degree field at the Steward Observatory Mt. Lemmon station (2790-m elevation, 18 km north of Tucson). The 1.5-m Mt. Lemmon and 1.0-m Siding Spring telescopes are also used for astrometric follow up and physical observations of interesting NEOs.

Edward Beshore: Principal Investigator

Steve Larson and Rob McNaught (SSS): Co-investigators:

Andrea Boattini, Gordon Garradd, Alex Gibbs, Al Grauer, Rik Hill and Richard Kowalski: Observers.

Look here for additional information on the Catalina Sky Surveys: http://www.lpl.arizona.edu/css/ and http://msowww.anu.edu.au/~rmn/


Japanese Spaceguard Association (JSGA)

Japan's National Space Development Agency (NASDA), the National Aeronautic Laboratory, and the Space and Technology Agency have allocated the necessary funds to bring this facility on-line. This observatory is located near Bisei town, Japan. In addition to the search for NEOs, this facility will be used to track debris in Earth orbit. The 1-meter Cassegrain telescope has a field of view of 3 degrees and there are plans to use a mosaic of ten CCD detectors each one of which will have dimensions of 2096 x 4096 pixels. A 0.5-meter telescope with a field of view of 2 x 2 degrees began operations in February 2000. Once the 1-meter NEO search telescope begins operations, the 0.5-meter telescope will be used to provide follow-up astrometric observations.

Syuzo Isobe: Principal Investigator

Look here for additional information on the JSGA program: http://www.spaceguard.or.jp/ja/index.html


Asiago DLR Asteroid Survey (ADAS)

ADAS is a dedicated asteroid search and follow-up program located at Asiago-CimaEkar, Italy. It is a joint venture between the Department of Astronomy of the University of Asiago and the Astronomical Observatory of Padua in Italy and the DLR Institute of Space Sensor Technology and Planetary Exploration, Berlin-Adlershof, Germany. The current system uses a 2K x 2K CCD detector and a 0.6 m aperture Schmidt telescope. Observations began in February 2001 and this effort will concentrate their searching at small solar elongation angles in an effort to discover near-Earth objects in the inner solar system such as Atens and the putative Inner Earth Objects whose orbits are completely inside that of the Earth.

Cesare Barbieri (Padua) and Gerhard Hahn (DLR-Berlin): Principal Investigators

Look here for additional information on the ADAS program: http://planet.pd.astro.it/planets/adas/index.html


To be continued...

NASA's Near-Earth Object (NEO) Program coordinates NASA-sponsored efforts to detect, track and characterize potentially hazardous asteroids and comets that could approach the Earth. To learn more, visit the home page of NASA's Near-Earth Object Program: http://neo.jpl.nasa.gov/

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