Univeristy of Arizona planetary scientists on Sept. 25 will celebrate the 20th anniversary of the first automatic discovery of a near-Earth asteroid using computer software.
David Rabinowitz, then a UA astronomer and now a research scientist at the Yale Center for Astronomy and Astrophysics, designed the software, and Spacewatch, the UA asteroid-tracking team, found the asteroid. The software used to make the discovery in 1990 was called the Moving Object Detection Program, or MODP, and was modeled after the original program designed by UA astronomer Jim Scotti.
Since then, Spacewatch has found hundreds of asteroids using this technology, building knowledge about the early solar system and objects that could in the future come close enough to hit the Earth.
How does MODP work? Like planets, asteroids reflect sunlight, making them appear as small bright objects. The computer compares the positions of objects in a sequence of images and flags any object that moves relative to the other objects. Since stars don't change position relative to each other, movement could indicate an asteroid. Longer image exposure times allow the motion of some fast-moving objects to leave trails on the image, which also indicate that the object is moving relative to the position of surrounding stars.
"We take three images of the same place in the sky at intervals of minutes to tens of minutes," said Robert McMillan, the principal investigator of Spacewatch. "The computer examines the images first for stars, whose positions don't change from one look to the next. Images that look like stars or slightly trailed stars that move during the observation interval are flagged by the software as candidates for solar system objects and presented to the observer for review."
The astronomers compare the computer-select objects with those already catalogued to see if the trajectory, size and brightness match up with a known object. If no match is found, the new discovery is listed by the International Astronomical Union's Minor Planet Center, or MPC, in Cambridge, Mass. The community of observers tracks the asteroid until the MPC considers its orbit to be definitive. At that time it is assigned a permanent number in a sequence of nearly 247,000 objects catalogued since 1801.
No. 11,885 of that sequence was the landmark discovery for UA astronomers. On Sept. 25, 1990, Scotti was observing with the 0.9-meter telescope on Kitt Peak when MODP flagged an object for him to analyze. Because main-belt asteroids move relatively slowly, speed is one indication of a near-Earth asteroid, or NEA, which helped to recognize the discovery.
Scotti still had to wait until daybreak to rewind the tape and reread the images to be certain of what he found. The discovery made a big impression on the team. "I think we were pretty excited," said Scotti. "It was our first automatically-detected NEA using MODP. It became 1990 SS."
NEAs are objects whose orbits approach the orbit of the Earth. Most NEAs originate in the asteroid belt between the orbits of Mars and Jupiter. Perturbations in the asteroids' orbits caused by heat from the sun that is unevenly re-radiated off the asteroids' surfaces can cause them to drift into resonances with the orbital period of Jupiter. This in turn causes their orbits to change enough to cross into the region of the inner planets. The process takes millions of years, which means that if the team were to find all of the asteroids, there wouldn't be new ones to discover anytime soon.
"The success of MODP allowed Spacewatch to start discovering NEAs on a more regular basis," McMillan said. "It also encouraged, directly or indirectly, the development of other asteroid-search programs using CCDs and computers rather than photographic film. This resulted in a rapid increase in the rate of detection of NEAs."
CCDs, or charged coupled devices, are components of digital cameras used for astrophotography that allow astronomers to set the camera to take images at pre-determined time intervals.
"Before electronic detectors and software, NEAs were found by visual inspection of photographic plates or film," said McMillan. "Photography held sway for almost a century because it could cover a lot of sky and the time exposures ...could detect stars, galaxies, nebulae and asteroids much dimmer than could be seen by eye looking through a telescope eyepiece."
Astronomers would visually compare sequences of photographs to see if any object left a streak indicative of motion, or changed position relative to the stars from one image to the next.
Spacewatch led the way into the digital age. Founded in 1980 by Tom Gehrels and McMillan, the goals of the project are "to explore the various populations of small objects in the solar system, and study the statistics of asteroids and comets in order to investigate the dynamical evolution of the solar system."
The UA group was the first to pioneer discovering asteroids with CCD cameras at a time when everyone else was shooting film, Scotti said. Technological advances through the years have resulted in bigger telescopes, bigger CCDs and more storage space on computer hard drives and discs, all of which contributed to the group's ability to detect more asteroids.
In astrophotography, pixels equal sky coordinates, so cameras with more pixels mean more sky can be imaged per photograph. "We can cover a lot more sky than we could back then," said Scotti.
Use of automatic detection programs doesn't mean there's no one at the telescope. "We always have someone observing," Scotti said. The team rotates shifts at Kitt Peak, scanning the skies more than three weeks each month (the gap is around the full moon, when the moon's light interferes with imaging and observation).
Why track asteroids? McMillan believes there are three reasons to survey and follow NEAs: "They are scientifically interesting as primitive remnants of the processes that occurred in the early solar system, they can be material resources for advanced space flight and colonization, and they can pose a hazard of impact on Earth."
"By surveying the objects, we're trying to find every kind of object out there," said Scotti. "We've probably found all the objects down to 5 kilometers." Astronomers believe the impact on Earth of a 1-kilometer asteroid could cause drastic climate change because of dust and debris ejected into the upper atmosphere.
"The probability of impact decreases as you consider larger-sized objects because the bigger ones are fewer in number. An impact like the one that wiped out the big dinosaurs probably doesn't happen more often than once in 100 million years, on average." McMillan said. "As you consider smaller and smaller sizes, the uncertainty of probability increases. Surveys are trying to count more of the smaller asteroids for this reason."
If Earth were endangered by an asteroid, all it would require to protect the Earth would be either to hasten or to delay the asteroid's approach so that it passed through the point of intersection with Earth's orbit either before or after Earth.
One of the many achievements of Spacewatch was the rediscovery in 2000 of an asteroid that was originally found in 1911 – found, and then lost. The long-lost asteroid known as Albert (No. 719 in the sequence of asteroids that began in 1801) was discovered by the Vienna Observatory in Austria and named after a benefactor of the observatory. Until its rediscovery, Albert was the last catalogued object whose position still was unknown.
Spacewatch is funded by grants from NASA and the The Brinson Foundation, and by contributions from corporations and private individuals.