Recent news reports suggested that a small asteroid might collide with the earth in 2030. The asteroid,if that's what it is, has been designated 2000 SG344. It was recorded as a 21st magnitude object on CCD images obtained with the 2.2-m University of Hawaii telescope by David Tholen and R. Whiteley from September 29 to October 3. Unfortunately their observations were not reported till October 24 by which time the object had faded to 24th magnitude, too faint for all but the biggest telescopes.

It was immediately recognised that the 2000 SG344 was in an orbit that frequently crossed earth's orbit. If earth was near the crossing point then the object would pass very close to the earth. Could it hit us?

Since 2000 SG344 circles the sun once in 354 days then it can come close to earth only at around 30 year intervals. First calculations from the Hawaii measurements indicated that SG344 would pass very close to earth on 21 September 2030. Several independent calculations performed for the IAU Technical Review Assessment Team gave it a 1 in 500 chance of hitting earth on that date.

No sooner had that report been aired than pre-discovery observations were located. Two observatories had on file CCD images obtained over three nights in 1999, between May 15 and 21. These increased the observation arc from three days to 17 months, greatly sharpening the orbital information. The improved orbit showed that 2000 SG344 will miss the earth by 4.2 million km -- 11 moon distances -- in 2030. However the same calculations showed that there was a 1 in 1000 chance that the object will hit the earth on 16 September 2071. (The change to a 40 year interval is due to the object's orbit being altered by earth's gravity in the 2030 encounter.)

Should we worry? Not really. The object's absolute magnitude, H, is 24.7. That translates to a size 30-70 metres, depending on its reflectivity, if it is a typical asteroid. The Technical Review noted that the earth is hit by an object of that size every year, on average. Most disintegrate in the atmosphere over uninhabited regions.

The Technical Review also noted the possibility that 2000 SG344 might not be an asteroid at all. Its orbit is very like that of five Saturn rocket stages launched during the Apollo programme in the 1970s. Though it is a bit bright for something that small, the possibility cannot be ruled out.

No doubt there will be intensive observations of the object when it comes within range next year.

For those who wish to do their own orbit computations the latest orbital elements by Gareth Williams in Minor Planet Electronic Circular V26 (2000 Nov. 8) are:

Epoch 2000 Sept. 13.0 TT = JDT 2451800.5
M 254.43448                    (2000.0)
n   1.01582959          Peri.  272.95216
a   0.9800663           Node   192.63931
e   0.0674466           Incl.    0.10906
P   0.97 years    H 24.7  G 0.15    U  4

If your software needs a perihelion date then

T = 2000 Dec. 25.9185 TT

On the afternoon of November 3, Carl Hergenrother of the Catalina Sky Survey (CSS) near Tucson, Arizona, obtained and made available additional observations of object 2000 SG344 from the CSS image archives. These pre-discovery observations significantly improved the certainty of the object's position in 2030 and effectively ruled out the chance of an Earth impact in that year. As explained in the earlier release from the International Astronomical Union (IAU), this was the most likely outcome of the continuing investigations. With the new data, we can say that the closest the object can approach the Earth in 2030 is 11 lunar distances on September 23. These results are in agreement with those of Andrea Milani at the University of Pisa, Italy.

The earlier announcement of a possible Earth impact in 2030 made by the Technical Review Team of the IAU followed the recently established process for reviewing Torino Scale 1 impact predictions. In the first use of this procedure, the Team formed a consensus opinion and, following the IAU mandate, made their results public within 72 hours.

While the new orbital calculations have ruled out the 2030 event, they have also increased the likelihood of encounters in years after 2030. For example, for the date September 16, 2071, current computations indicate roughly a 1 in 1000 chance of an Earth impact. However, these calculations assume that object 2000 SG344 is asteroidal in nature. There remains the significant possibility that this object is a spacecraft booster rocket from the Apollo era and studies are continuing to assess this possibility. Additional observations of the object will be possible in the coming months and these should further refine the calculations and conclusions.

(IAU Technical Review Team Assessment on Asteroid 2000 SG344)

Recognizing the public interest and concern over possible impacts from small Near-Earth Asteroids (NEAs), the International Astronomical Union has established a process to provide international expert review of any discoveries or calculations that predict a close encounter with a non-neglibible chance of future impact. This review process has been exercised to confirm the calculations, based on current observations, of a close encounter with a low probability of impact on 21 September 2030 by a very small asteroid-like object, designated 2000 SG344.

Computations made earlier this week by a group of international experts suggest that the object 2000 SG344, has a remote 1 in 500 chance of impacting the Earth in 2030. These results have been verified over the course of the past 72 hours by a Technical Review Team of the International Astronomical Union. The greatest likelihood is that future observations of the object will yield higher precision orbit computations that will show with certainty that it will miss the Earth entirely. The unusual nature of the orbit of 2000 SG344 suggests the possibility that it might simply be a man-made rocket booster from the Apollo era.

Object 2000 SG344 was discovered on September 29, 2000 by David J. Tholen and Robert J. Whiteley using the Canada-France-Hawaii 3.6-meter aperture telescope on the island of Hawaii. Shortly thereafter, re-discovery observations taken in May 1999 by MIT's LINEAR observatory team were also identified. Given the observed brightness of the object and its assumed reflectivity, an estimate can be made for its diameter. While the reflectivity of this object is not known, values typical for near-Earth asteroids imply this object's extent is about 30 - 70 meters.

Orbital calculations in late October by Andrea Milani (University of Pisa, Italy) first indicated the possibility of a future impact. Paul Chodas of the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory estimates a one in 500 chance of the object hitting the Earth on September 21, 2030. The possibility of an Earth impacting orbit was confirmed by Steven Chesley (NASA/JPL), Giovanni Valsecchi (Italian National Research Center in Rome, Italy) and Karri Muinonen (University of Helsinki). If the object is near the large end of the estimated size range for an asteroid, it would be classified as category 1 within the 10 point Torino Scale, meaning the object is one that merits careful monitoring. If the object's size is closer to the lower limit of 30 meters, it would be classified as Torino Scale 0 and hence not of immediate concern.

Because the orbital period of this object about the sun is 354 days, it moves a bit faster than the Earth about the Sun so it is drifting slowly away and will not return to the Earth's neighborhood until nearly three decades. It was last in the Earth's neighborhood in 1971. As yet undiscovered pre-discovery observations made in 1971 and additional observations made in the coming months would provide the data for further refining this object's orbit and the circumstances of its close Earth approach in 2030. During the 2030 close approach, the perturbative effects of the Earth upon the object could change its orbital period so that numerous encounters might be possible after 2030. The likelihood of this situation is also under study.

Because of its Earth-like orbit, this object is an obvious candidate for being a left-over space probe or rocket stage. For example, the S-IVB stages of the five Apollo rockets (Apollo 8-12) entered into heliocentric orbits that are similar to the orbit of object 2000 SG344. If this object is a man-made rocket booster, it would have a higher reflectivity than a natural asteroid and hence it would have to be smaller (about 15 meters) to reflect as much light as a much darker asteroid. While object 2000 SG344 seems too bright to be an Apollo rocket booster, the possibility of its being man-made has not been completely ruled out.

While object 2000 SG344 will likely pass close to the Earth in 2030, it should be made clear that the probability of the object missing the Earth is at least 500 to 1. If the ongoing studies determine that this object is likely to be a relatively small man-made booster then such a lightweight object would pose no hazard. It is interesting to note the chance of object 2000 SG344 striking the Earth in 2030 is actually somewhat less than the chance of an undiscovered object of the same size striking the Earth in any given year. Thus object 2000 SG344 is more interesting than threatening but the international efforts to characterize the nature and future motion of this object will continue.

In line with its policy decisions, the IAU does not intend to make any further statements on the eventuality of an impact by 2000 SG344.