Close Call

The short, intense saga of asteroid 1997 XF11

© 1998 by Robert C. Moler

Next month H.G. Smith will present a program on near earth asteroids. The events of this past month will make it very timely. I hope I'm not stealing his thunder, but I want to relate my adventures in finding information on the asteroid, how asteroid orbits are determined, and how, again, the media messed up the reporting.

March 11, 1998, Brian Williams ended his 9 p.m. newscast on MSNBC with a look at the next morning's newspapers. The New York Times was running a report on an asteroid that was to come within 30,000 miles of the earth in 30 years. October 26, 2028 at 1:30 p.m. EDT. was the appointed date and time. The asteroid had the provisional designation 1997 XF11. 1997 X means the asteroid was discovered in the first 15 days of December 1997. F11 is the consecutive number of the discovery in that period meaning the 281st object. The key to these designations can be found on the Minor Planet Circular web site. I'll give all the Internet addresses or URLs at the end of the article.

Anyway I checked the MSNBC web site for the text of the story. However I always make it a rule to get to the real sources of scientific information. The media can get things pretty scrambled. Next I checked out the Minor Planet Electronic Circulars and found one with orbital elements of the asteroid. It was MPEC 1998-E13. Unfortunately the orbital elements weren't in a form I could use with the Looking Up program I use for the planet and star charts for the Stellar Sentinel. I had to get out a book on celestial mechanics to review Kepler's third law of planetary motion and calculate a missing daily motion I needed. My other problem was that I hadn't a clue about the time parameter. I took a stab, at it, but was wrong. Anyway I could plot the orbit and see at least if the intersection point is correct. It was. The orbits cross twice, but the crossing point where the orbital plane of the asteroid also intersects the earth's orbital plane is where the earth is on October 26th. I played around with the time parameter until I got the asteroid to come close to the earth.

With some more poking around on the Internet, I was pleasantly surprised to find the International Astronomical Union Circulars (IAUCs) were being posted on the net. In the past they were available by subscription only. These are in the province of Brian Marsden and Dan Green of the Central Bureau for Astronomical Telegrams (CBAT) at the Smithsonian Astrophysical Observatory. I located the circular, IAUC 6837, which had the request for observations for 1997 XF11.

Asteroid 1997 XF11 will come close to the earth in 2028

But not as close as first predicted in International Astronomical Union Circular IAUC 6837 dated March 11, 1998. The next day a prediscovery observation from 1990 was added to the calculations and refined the orbit even more, showing the asteroid would not approach the earth as close as first thought, or as close as the moon. The new orbit was announced in IAUC 6839. Notice that the Circulars are devoid of all the hype that occurred in the popular media.

Since the asteroid's discovery there were 88 days of observations with which to calculate an orbit from. It takes at least three observations, with good positions, that is right ascension, declination of the object, time and location of the observer, in longitude, latitude, and altitude above sea level to get an idea of the orbital parameters of the object. The latter three are usually only relevant if the object is close. In reality, orbital determinations are more difficult. An object whose orbital inclination or tilt is close to the earth's orbital plane takes many more observations to get a good orbit. Then there are observational errors that are always present. So the more observations over a longer time give a more accurate orbit. Based on the 88 day orbital arc, several independent calculations suggested a close approach of the asteroid to the earth in 2028.

These calculations also take into account the gravitational effects of not only the asteroid and the sun, but all the major planets. The orbital elements of any body actually change slightly with time. The distance reported in the circular was 0.00031 AU. or about 30,000 miles center to center. A news release was issued also with an estimate of the asteroid's size based on its brightness and presumed albedo (reflectance) of a mile in diameter. This is when all hell broke loose. All the morning news programs on March 12th ran with the asteroid story, brushing aside the Presidential soap opera for a while. That day, using the orbital elements to plot the asteroid backward in time, Eleanor Helin, of the Jet Propulsion Laboratory (JPL) reported that a colleague had found an image of the asteroid on a photographic plates she took on March 22nd and 23rd, 1990. These data points, going back nearly 8 years, improved the accuracy of the subsequent orbital elements. The new closest approach distance in 2028 turns out to be 0.0064 AU or about 600,000 miles about two an a half times the moon's distance.

In many cases the news reports were that NASA's calculations by themselves gave a more correct answer. They ignored the real reason, the new data points. They didn't get it. I hope it doesn't take an asteroid hitting them on the head to get it right.

Here's some Internet URLs of places to get information directly from the source. All are prefixed with "http://".