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Gaia Asteroid Census

August 8, 2022

One method that governments have to monitor demographic trends in their countries is by census. In the United States, a census is legally mandated by the U.S. Constitution, and a census has taken place every decade since 1790. To encourage honest reporting, answers to census questions are confidential. No one is allowed to associate any person, household, or business with identifiable information.

One controversy of the US Census process is its extended data collection beyond that required to apportion the House of Representatives of the United States Congress. One year, I was asked to give the number of toilets in my house! The answer is only two, which pales in comparison to the supposed number in Buckingham Palace. That residence has 78 bathrooms and likely as many toilets.

A census tabulator from 1960

A census tabulator from 1960s. Note the wall of vacuum tubes on the right.

Shown is the FOSDIC (Film Optical Sensing Device for Input to Computers), with possible reference to Fearless Fosdick, a parody of the Dick Tracy comic strip of Chester Gould (1900-1985) that appeared in the Li'l Abner comic strip by Al Capp (1909-1979).

The name most associated with census tabulating machines is Herman Hollerith (1860-1929), who developed an electromechanical tabulating machine for punched cards used in the 1890 United States census.

FORTRAN programmers will remember Hollerith constants, a way to insert character data into their programs. The code 7HTIKALON is the way that the seven character string, TIKALON, was printed (early impact printers could print just upper-case characters).

(Wikimedia Commons image from U.S. Census Bureau. Click for larger image.)

Our Solar System has eight planets, along with their many moons, and probably nine dwarf planets, the largest of which is the demoted planet, Pluto. These are listed below.

• Pluto      • Haumea • Quaoar
• Ceres • Makemake • Gonggong
• Eris • Orcus • Sedna

The total number of these large bodies pales in comparison with the number of known asteroids and the estimated much greater number of unknown asteroids. It's estimated that there are millions of asteroids, most of the known asteroids being found in the main asteroid belt between the orbits of Mars and Jupiter. There are more than 200 asteroids larger in diameter than 100 kilometers (60 miles) in the main asteroid belt, but there might be 1-2 million asteroids larger than 1 kilometer (0.6 miles), and millions of smaller ones.

There are asteroid populations besides the main asteroid belt. The Trojan asteroids orbit the Sun in the same orbits as a planet at its Lagrange points. These are locations about 60 degrees ahead or behind the planet at which the gravitational pull of the planet balances that of the Sun. Jupiter, the largest planet, has a retinue of more than 10,000 Trojan asteroids, while Neptune has 30, and Mars has 9. Uranus and Earth each have just a single discovered Trojan.

A mosaic image of the asteroid, Bennu

A mosaic image of the asteroid, 101955 Bennu, composed from images collected on December 2, 2018, by the OSIRIS-REx spacecraft from a range of 24 kilometers (15 miles).

Bennu, an Apollo asteroid, is a carbonaceous asteroid of mean diameter of 490 meters, and it has a cumulative 1-in-1,800 chance of impacting Earth between 2178 and 2290 with the greatest risk being on September 24, 2182.

The OSIRIS-REx spacecraft successfully collected a sample of Bennu for return to Earth.

(Wikimedia Commons image by NASA/Goddard Space Flight Center/University of Arizona. Click for larger image.)

The Apollo asteroids, named after the first discovered of this class, 1862 Apollo, are asteroids whose orbits cross Earth's orbit. There are presently more than 10,000 known Apollo asteroids, and about the same number of other near-Earth asteroids that aren't Apollo asteroids. More than 2,000 near-Earth asteroids, about three-quarters of which are Apollo asteroids, are sufficiently large enough to be hazardous if they impact Earth. As an example, the Chelyabinsk meteor that exploded over Chelyabinsk, Russia, on February 15, 2013, was an Apollo asteroid. NASA's Center for Near-Earth Object Studies monitors such objects.

There is reason enough beyond scientific curiosity to do a careful census of Solar System asteroids, and the Gaia space observatory of the European Space Agency is conducting such a census. Gaia was launched in 2013 and it is expected to operate until 2025. The Gaia spacecraft was designed primarily for astrometry, the measurement of the the positions, distances and motions of stars, but it tracks all points of light visible to its sensors, and these include asteroids and comets.

Gaia sky coverage visualization

Looking very much like modern art this is a visualization of Gaia's sky coverage. (A still frame from a Wikimedia Commons image by B. Holl of the University of Geneva, Switzerland, and A. Moitinho and M. Barros of CENTRA, University of Lisbon.)

Gaia's first data release to the Gaia Archive, done after its first 14 months of operation, cataloged the positions and magnitudes for 1.1 billion stars.[2] The second data release cataloged the positions, parallaxes and proper motions for about 1.3 billion stars, the positions of an additional 300 million fainter stars, and some asteroid data.[1-2] The most recent data release of June 13, 2022, includes data on Solar System objects, including a ten-fold increase in the number of cataloged asteroids.[1-2]

Gaia resides about 1,5 million kilometers from the Earth near the Sun-Earth Lagrange L2-point.[1] Gaia has two optical telescopes, and it scans the sky by rotating about its axis in a period of about six hours, and its instruments allow for accurate determination of object positions, velocities, and spectra. All this data allows determination of the chemical compositions, temperatures, colors, masses, brightness, ages, and radial velocities of stars and the physical properties of asteroids.[1] About fifty scientific articles are being published coincident with the latest data release.[1]

Asteroid Lutetia

The photocenter of asteroid Lutetia (brown bullet) and center of mass (blue bullet) as viewed by Gaia in three different viewing geometries. The illumination by the Sun is in the direction of the red bar, and the angle between Gaia and the Sun is, from the left to the right, 16.7, 24.2 and 20.9 degrees and the photocenter-barycenter distance is 7.42, 5.96 and 10.91 kilometer, respectively. Image from P. Tanga, K. Muinonen, A. Penttilã, et al., 2022, Astronomy & Astrophysics, in press.

The large number of cataloged asteroids means that there is a significant increase in the number of detected asteroids in close proximity to each other. Such close encounters allow estimates of asteroid mass.[1] Gaia's astrometry is so accurate that a correction is required for the angular offset between the center of mass of an asteroid and the center of the area illuminated by the Sun.[1]

The latest data release contains about 60,000 spectra of Solar System asteroids (see figure). An asteroid's spectrum allows an estimate of its composition and offers clues to its origin and the evolution of the Solar System.[1] Gaia has increased the number of available asteroid spectra by more than an order of magnitude.[1]

Gaia asteroid spectra histogram

Histogram of the asteroids with spectra in the latest Gaia Data Release, no. 3.

The number of asteroids is in the vertical axis, and the horizontal axis shows the distance of the asteroid to the Sun in astronomical units.

The asteroid belt lies between 2.2 and 3.2 astronomical units, and this is reflected in the histogram data.

(Gaia Collaboration image from L. Galluccio, K. Muinonen, G. Fedorets, M. Granvik, A. Penttilã, L. Siltala, et al., 2022, Astronomy & Astrophysics, in press. Click for larger image.)

Apart from the asteroid observations, a significant result of Gaia's measurements is its data for about 1,6 million quasars. Such data can be used to create a more accurate visible light Celestial Reference Frame that will enhance the accuracy of satellite positioning and measurements by Earth observation satellites.[1] The present International Celestial Reference Frame (ICRF3) is based on the radio positions of a few thousand quasars obtained using very long baseline interferometry (VLBI).[1]


  1. Gaia space telescope rocks the science of asteroids, University of Helsinki Press Release, June 17, 2022. This press release also appears here.
  2. Gaia Archive at ESA.

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