NASA Science News
In 2071 a relic of NASA’s earliest space exploration efforts might return to Earth, if current estimates are confirmed.
November 7, 2000 — Last Saturday a syndicated sportswriter compared LA Lakers center Shaquille O’Neal to 2000 SG344, a newly-discovered near-Earth object (NEO). During Shaq’s game the night before, O’Neal had barreled into the opposing center "like 2000 SG344 — that object hurtling toward Earth." Fortunately for basketball fans, O’Neal is far more likely to score a free throw than 2000 SG344 is to crash land on our planet.
Although SG344 is nearby now, scientists say there is no appreciable chance of a collision for at least 70 years. (Shaq, on the other hand, should make plenty of baskets between now and then.)
2000 SG344 was discovered by asteroid-hunters on Sept. 29th as it was gliding by Earth approximately 20 times farther away than the Moon. Astronomers quickly realized that the faint object was unusual. Its 354 day orbit is very much like Earth’s, so much so that 2000 SG344 might not be an asteroid at all, but rather a piece of manmade rocket debris.
Our planet and 2000 SG344 move through space like two runners racing along a track at nearly the same speed; it takes a long time for one to lap the other. The NEO, which is moving a bit faster than Earth, is slowly drifting away and won’t return for 30 years.
As recently as last Friday astronomers were concerned that the next encounter might be too close for comfort. A panel convened by the International Astronomical Union (IAU) announced on Nov. 3rd that 2000 SG344 might hit Earth in the year 2030. The chances of an impact were slim, they said, and new data to refine the object’s orbit would likely rule out a collision altogether.
That’s exactly what happened. Shortly after the IAU announcement, astronomer Carl Hergenrother found "pre-discovery" images of 2000 SG344 from May 1999 in archives from the Catalina Sky Survey.
"The pre-discovery images let us calculate a better orbit that absolutely rules out a collision in 2030," says Donald Yeomans, manager of NASA’s Near Earth Object program at the Jet Propulsion Laboratory. "It won’t come any closer to Earth than 11 lunar distances. However," he added, "the new orbit increases the chances of encounters in years after that. For example, there is a 1-in-1000 chance of a collision on Sept. 16, 2071."
Just as the possible 2030 encounter was excluded by better data, Yeomans anticipates the same will happen to the 2071 date. "Additional observations this year and in 2030 when SG344 comes back again will certainly alter our conclusions as we learn even more about its orbit."
If Earth and 2000 SG344 do cross paths in the future, what happens will depend on the nature of the near-Earth object. When the object was first discovered it appeared to be a small asteroid, but another possibility is gaining favor among researchers. "The orbit of SG344 is so Earth-like, it makes you wonder if came from our own planet," mused Yeomans.
In 1971, the last time 2000 SG344 was in the vicinity of Earth, NASA’s Apollo program was in full swing. 2000 SG344 may well be debris from an Apollo-era rocket masquerading as a space rock.
"Initially we thought it was too bright (and thus too large) to be a rocket fragment, but it’s possible that this is the S-IVB stage from a big Saturn V," continued Yeomans. "S-IVBs" were booster rockets that propelled Apollo Command and Service Modules toward the Moon from their parking orbits around Earth. "Many of those boosters were targeted to hit the Moon, but the S-IVBs from Apollo 8 through 12 went into orbit."
If SG344 is a derelict rocket booster, it’s probably no larger than 15 meters and wouldn’t pose much of a threat even if it did strike Earth. An incoming S-IVB would burn up in the atmosphere as a dazzling but mostly-harmless fireball. Spectators in Texas and Oklahoma witnessed just such a display last month when the casing from a Russian Proton rocket disintegrated over North America.
On the other hand, if 2000 SG344 is a bona fide space rock, it’s likely to be bigger and more dangerous. Typical near-Earth asteroids reflect about 3% to 20% of the sunlight that falls on them. The apparent brightness of 2000 SG344 corresponds to such an asteroid 30 to 70 meters across.
"Whatever it is, 2000 SG344 is certainly no dinosaur killer," Yeomans added, referring to a 10 km space rock that may have triggered mass extinctions when it hit Earth 65 million years ago. A 70-meter asteroid (the worst-case scenario for 2000 SG344) could obliterate a city if it landed on one, but it would not trigger a global catastrophe.
At the lower end of the scale, around 30 meters, 2000 SG344 might not even reach the ground. "It depends on what it’s made of," says Yeomans. "A 30-meter slab of iron could make it through the atmosphere and do some local damage." On the other hand, a fragile carbonaceous chondrite like the widely-publicized Yukon fireball of January 2000 would mostly disintegrate high overhead.
The September discovery of 2000 SG344 by astronomers David Tholen (University of Hawaii) and Robert Whiteley (now with the Catalina Sky Survey) was no accident. Using the Canada-France-Hawaii telescope atop Mauna Kea, the pair were looking for unusual near-Earth asteroids in orbits whose greatest distance from the Sun coincides with the Earth’s orbit. Such objects could hit our planet, but they would rarely be found by search programs that hunt for space rocks coming from the general direction of the main asteroid belt.
"We’re not doing a full-fledged survey like LINEAR, Spacewatch, or LONEOS," says Tholen. "We observe only a few nights each year." Nonetheless, the pair have discovered ten near-Earth objects since they began their part-time hunt in 1996. Their most celebrated find, 2000 SG344, is technically classified as an "Aten": an Earth-crossing object with an orbital semi-major axis smaller than one astronomical unit. Atens spend most of their time inside Earth’s orbit, crossing over for brief intervals only.
But is 2000 SG344 an Aten asteroid or an Aten rocket shell? That’s the 10-megaton question.
Tholen thinks it may be an asteroid. "The only Apollo launches in 1971 (the last time 2000 SG344 was close to Earth) were Apollo 14 and 15. But the S-IVB stages from those missions crashed into the Moon."
"The wild card is Apollo 12," he continued. "Its S-IVB stage apparently wound up in an Earth-circling orbit. There is a possibility that the Moon perturbed Apollo 12’s S-IVB into an orbit around the Sun." In that case, the important time is not when Apollo 12 launched in 1969, but instead when the Moon might have nudged the booster into its new orbit."
"Another problem [in assessing the rocket hypothesis] is that we don’t know what happens to white paint after 30 years of micrometeoroid bombardment," says Tholen. Does it remain white, or would the paint erode, revealing a less reflective surface beneath? "We have to guess at the reflectivity to convert the measured brightness into a diameter estimate and those diameter estimates seem too big to be an S-IVB. We do have evidence in the images of a rapidly rotating (about 10 minute period) elongated (maybe 2 to 1) object. That sure sounds like the shape of a Saturn booster, but we’ve also found asteroids that match that description!"
Whatever it is, astronomers plan to track SG344 as it slowly recedes from Earth — more data will reduce the uncertainties about SG344’s orbit and naturally answer many of the questions about this mysterious NEO.
But there’s not much time. The orbit of 2000 SG344 is carrying it towa
rd the blinding glare of the Sun. By mid-2001, the object will be dimmer than 25th magnitude as its solar elongation dips below 90 degrees. "That’s a tough combination for observers," noted Yeomans. "We might find more pre-discovery images," he added hopefully. "Pictures from 1971 would really lock down the orbit and probably tell us whether SG344 is a rocket or an asteroid. We just have to keep looking."