Shoemaker Levy 9's Anniversary Ends, but its Impact Continues

Composite image from the Hubble Space Telescope showing how the brightest region of Comet Shoemaker-Levy 9 changed over time. Credit: Dr. Hal Weaver and T. Ed Smith (STScI), and NASA Source: hubblesite.org

Composite image from the Hubble Space Telescope showing how the brightest region of Comet Shoemaker-Levy 9 changed over time. Credit: Dr. Hal Weaver and T. Ed Smith (STScI), and NASA Source: hubblesite.org

20 years ago amateur astronomers David H Levy and Carolyn and Gene Shoemaker discovered a comet - shattered into dozens of mountain-sized pieces - heading for Jupiter. The first piece of the comet slammed into Jupiter on July 16, 1994. Yesterday marked the anniversary of the final impact into the gas giant, but its impact here on Earth was more enduring.

Problem? What problem?

Until the last decade of the 20th Century amateur astronomers discovered comets and asteroids more often than professional astronomers. Studies of the “rubble” left behind by the Solar System’s birth did not rank high among the priorities for planetary science. Even when professionals wanted to search for asteroids, they struggled in a science economy. that forces scientists to compete for the limited observing time at professional telescopes. Searching for asteroids requires repeated observations over long periods. In the competition for grants and observatory time, professional asteroid hunters struggled against research proposals that promised higher-impact results in less time.

Amateur astronomers, on the other hand, devote their own time and money to observe targets that interest them. Without having to worry about applying for grants or publishing journal articles, amateurs can conduct the many observing sessions needed to spot an asteroid’s small point of light moving across the sky from night to night.

Mercury's cratered south pole region from the Mariner 10 mission in 1974. Credit: Nasa/JPL/Northwestern University  Source: Nasa Photojournal

Mercury's cratered south pole region from the Mariner 10 mission in 1974. Credit: Nasa/JPL/Northwestern University  Source: Nasa Photojournal

Beneath the surface, however, slow tectonic shifts in professional attitudes towards asteroid science were taking place. Viking, Voyager and other planetary space probes sent back images from worlds across the Solar System. Earth appeared to be the only world with an active geology. The others were covered with the 4-billion year accumulation of impact craters. Cratering appeared to be the dominant process shaping the surface of the terrestrial planets and moons. Understanding the role of asteroid impacts became a more important question for planetary scientists.

Chicxulub impact site from space. Top: Shuttle Radar Topography Mission relief map showing indentation from the impact. Bottom: Landsat image showing in white Merida, the capitol of Yucatan, for scale. The city dwarfed by the unseen crater below, now has a population of nearly 1 million. Credit: Nasa/JPL Source: Nasa Photojournal

Chicxulub impact site from space. Top: Shuttle Radar Topography Mission relief map showing indentation from the impact. Bottom: Landsat image showing in white Merida, the capitol of Yucatan, for scale. The city dwarfed by the unseen crater below, now has a population of nearly 1 million. Credit: Nasa/JPL Source: Nasa Photojournal

The images also raised questions about Earth itself. With so many craters on other worlds, where are the craters on Earth and what role did they play in our planet’s history?  Weathering, subduction, and other geological processes have buried or erased obvious signs of most impacts. But satellite imaging and new techniques in geology led scientists to find the faint traces of ancient impacts. In 1980 Nobel Prize-winning physicist Luis Alvarez and his son Walter identified a global layer of iridium in the geological strata separating the extinction of the dinosaurs - the smoking gun of an asteroid or comet impact. Ten years later scientists found the gun itself - a 70-kilometer wide crater buried under the Yucatan Peninsula.

Professional planetary scientists and space advocates convinced Congress in 1991 to have Nasa study a formal asteroid search program. The Space Guard Survey Report (available at Nasa’s Technical Report Server) pointed out that, at the time of the report:

the total worldwide effort to search for NEOs amounts to fewer than a dozen full-time-equivalent workers, a number of whom are volunteers!

Imagine! Amateurs doing useful things! For science! The report proposed a professional, international network of ground-based observatories dedicated to discovering asteroids. The small (by Nasa standards) project would scan the sky for asteroids 1 kilometer or larger that posed a risk of planet-wide destruction.

A cometary black eye for Jupiter changes professional asteroid hunting

The white flash of Fragment W hitting Jupiter's atmosphere as seen from Nasa's Galileo spacecraft. Credit: Nasa/JPL Source: Nasa Photojournal

The white flash of Fragment W hitting Jupiter's atmosphere as seen from Nasa's Galileo spacecraft. Credit: Nasa/JPL Source: Nasa Photojournal

By 1994 both planetary scientists and politicians were more receptive than ever to the idea that asteroids and comets posed a threat to Earth. And that’s when Levy and the Shoemakers made their discovery. Jupiter’s gravitational pull had already ripped SL9 into at least twenty pieces - the largest over 1 kilometer across. The many articles published to honor the 20th anniversary of SL9 do a better job than I can describing how scientists rushed to observe the comet’s collision with Jupiter:

Professionals around the world took the threat from asteroids more seriously in the years following SL9’s impact. In the United States, Congress required Nasa in 1998 to find all of the near-Earth asteroids larger than 1 kilometer. These are the dinosaur-killers whose impacts produce planet-wide effects. Congress expanded Nasa’s mandate in 2005 to include asteroids too small to cause mass extinctions, but at 140 meters across still alrge enough to cause massive regional damage on the scale of the largest hurricanes or earthquakes.

A new age of amateur asteroid hunting

Amateur and professional astronomers had discovered 305 near-Earth objects by 1995. By the end of 2013, that number reached 10,481. Data Source: Nasa/JPL

Amateur and professional astronomers had discovered 305 near-Earth objects by 1995. By the end of 2013, that number reached 10,481. Data Source: Nasa/JPL

Nasa’s Near Earth Object Program, based at JPL, now leads the space agency’s search for asteroids and works with other search programs around the world. Statistics it publishes show that professional programs now account for almost all of today’s asteroid discoveries. Larger telescopes at mountaintop observatories, more sophisticated instruments, and automated software gives professional asteroid searches abilities that amateurs can’t match - especially since the remaining undiscovered asteroids are too faint for most amateur telescopes to detect.

Yet amateurs still have one advantage over the professionals: time. The professional observatories are great at making the handful of observations needed to discover an asteroid. But figuring out orbits, sizes, masses, and other properties of these newly-discovered asteroids requires thousands of hours of follow-up observations. Amateur asteroid observers devote as much of their free time (and money) as they want to their observations. Amateurs can also react faster to new discoveries since they don't need permission from review committees to make their observations.

British amateur astronomer Jonathon Tate is an example of the lengths amateurs are willing to go. The UK government failed to create its own near-Earth asteroid program so Tate founded Spaceguard UK in 1996. From his observatory - the Spaceguard Center - in the Welsh countryside, Tate and other volunteers use 4 telescopes to conduct asteroid follow-up observations. Admissions fees to the visitors center and contributions from donors cover the Spaceguard Center’s operating costs, but aren't enough to pay the volunteers.

The Minor Planet Center is professional astronomy’s clearing house for asteroid observations. As the data flows in, the MPC’s scientists calculate the orbits of new asteroids and determines whether any of the near-Earth objects might become a threat to Earth. Tim Spahr, Director of the MPC, discussed the role of amateurs - who he calls “unfunded experts” - at the Nasa Grand Challenge seminar earlier this year:

While they don’t discover that many near-Earth objects anymore, under 2% annually, they make valuable contributions. We get follow-up observations for objects on the Near Earth Object observation page. These are critically important.

He gave a more detailed explanation to the Planetary Society's Mat Kaplan last year on the Planetary Radio podcast:

I want to make clear that amateurs play an important role in this, although I also want to be honest in that that role for discovery and follow-up for the orbits is diminishing….  It’s very hard then for amateurs to compete [with professional observatories] in the discovery process. Now where amateurs do contribute is in orbital refinement and in particular now the physical observation area - light curves especially. Amateurs are doing a fantastic job in that area now and they are likely to do more.

The Planetary Society itself is a key supporter of amateur asteroid hunters. Its Shoemaker Near-Earth Object Grant Program provides amateur astronomers - and professionals in developing countries - money to upgrade their observatories. Moore’s Law and similar trends gives amateurs ever-better telescopes and instruments to tease data out of the faint light reflected by asteroids across the Solar System. The Planetary Society has donated $270,000 to astronomers in 16 countries, supporting some of the most prolific amateur asteroid observers in the world.

Other professionals consider amateurs essential resources for their own research projects. The amateurs collect a large number of observations that supplement the small number of observations professional make with the large, sophisticated instruments at the big observatories. Some example of pro-am mashups include:

These collaborations depend on amateurs whose skills and equipment place them at one extreme in the amateur spectrum. So how do novices develop those skills? The best way is to contact a local astronomy club which will have several members who conduct asteroid observations.

But the professionals also have ways for you to begin observing asteroids. The University of Maryland created the Amateur Observers’ Program over a decade ago as the outreach program for NASA’s Deep Impact cometary mission. It now supports Nasa’s Dawn, and Epoxi asteroid exploration missions. The AOP’s online resources teach new amateurs how to find and observe asteroids and guides them through beginner, intermediate, and advanced observing projects. More recently the Slooh robotic telescope service and Nasa announced an agreement to recruit more amateurs into the study of asteroids. Slooh will add more web-connected telescopes to its network and, with Nasa’s help, will recruit and train amateurs to conduct the follow-up observations for newly-discovered asteroids.

Shoemaker-Levy 9’s Enduring Impact

The scars SL9 left in Jupiter’s cloud bands have long since disappeared, but the comet’s impact on Earth will last much longer. It galvanized politicians and professional scientists to take the threat of asteroid impacts more seriously. And it highlighted the way amateurs produce real science that complement the work of professionals. Now as dashcams and smartphone cameras capture more evidence of the small asteroids bombarding Earth every day, the public’s role in asteroid science has never been greater. Their individual contributions may be small, but step-by-step amateur asteroid observers make our world a little safer.