August 27, 1997

Asteroid 3753 -- Earth's Newly Discovered Companion

Orbits of Asteroid 3753 and the Sun's inner planets. The sizes of the Sun and planets are not drawn to scale.

Image Credit: Paul Wiegert, York University, Canada.

The Earth and Moon share their orbit around the Sun with another planetary body -- Asteroid 3753!

This discovery was made earlier this year, when scientists at York University in Canada and Turku University in Finland calculated the orbit of a small rocky object first observed in 1986. As the above illustration shows, the orbit of this object, known as Asteroid 3753 or 1986TO, is fairly elongated. It crosses the Earth's orbit and extends inward to the orbit of Venus and outward to that of Mars. The orbit is inclined by about twenty degrees relative to the ecliptic plane (the plane in which the planets orbit).

Asteroid 3753's orbit is much more interesting and complex than the above image would suggest. The complexity becomes apparent when the orbit is projected onto the ecliptic plane and viewed as if one is traveling with the Earth around the Sun. In this co-rotating frame of reference, the Earth appears to stay in one place, as illustrated in the next diagram. (The other planets do not stay in place since their orbital periods differ from that of the Earth.)

In this co-rotating reference frame, the projected orbit of Asteroid 3753 traces out a kidney bean-shaped path each year, as illustrated in image 2. Over the course of 385 years, this yearly path drifts around the Sun in the clockwise direction. Images 3, 4, and 5 show the drift after 125 years, 250 years, and 385 years. After 385 years, the drift reverses direction and becomes counterclockwise.

After another 385 years, the paths reach again the position shown in image 2. In reality, the kidney bean-shaped paths are much more closely spaced than shown in the illustration.

Why does Asteroid 3753 trace out such an unusual orbit? This is difficult to explain. In fact, the orbit cannot be determined with pencil and paper. Lengthy computer calculations are required. Ultimately, the orbit's behavior is due to the interplay between the attractive gravitational forces of the Sun and Earth, the centrifugal force that arises from the asteroid's orbital motion, and the orbit's elongated shape and inclination to the ecliptic.

Asteroid 3753 is not big. Its diameter has been estimated to be about three miles.

There is no danger that the asteroid will crash into Earth any time soon, if ever. At its closest approach to the Earth, which happens every 385 years as we already noted, it never gets closer to us than about 9.5 million miles. That's comparable to forty times the Earth-Moon distance. The last time this happened was 100 years ago.

More Cool Stuff

We obtained the above images and information from Web pages by Paul Wiegert and Kimmo Innanen of York University in Ontario, Canada, and Seppo Mikkola of the Tuorla Observatory at the University of Turku in Finland. At the end of their discussion, the researchers have links to sites that offer additional information about asteroids and near-Earth objects:

http://www.asteroid.yorku.ca/companion or

http://www.asteroid.yorku.ca/units/phas/people/wiegert/3753/

For additional information on Asteroid 3753, go to the "Frequently Asked Questions" pages by Dr. Wiegert:

http://www.asteroid.yorku.ca/units/phas/people/wiegert/3753/faq.html

There are several NASA-supported educational sites on the Web that feature the Sun's planetary system:

"Eyes on the Sky, Feet on the Ground," a set of pages of the Web site "Using Science and the Internet as Everyday Classroom Tools" from the Smithsonian Astrophysical Observatory. The solar system is discussed in chapter 5:

http://hea-www.harvard.edu/ECT/the_book/

The "Public Connection" Web site, an outreach project of Rice University and the Houston Museum of Natural Science:

http://space.rice.edu/hmns/connect.html

The "Science Information Infrastructure" project of the University of California at Berkeley:

http://www.cea.berkeley.edu/~edsci/sii/sii_sii.html

http://www.cea.berkeley.edu/Education/sii/SEGway/

"Windows to the Universe," an innovative and engaging Web site about the Earth and space sciences:

http://www.windows.umich.edu/