A Rare Opportunity
By Laura Kinoshita, Al Conrad, and Bill Merline
During an interview about the discovery months later, Merline admitted that, at first, he was skeptical. “Most of the objects we see are moving extremely slowly, so they’ll orbit the asteroid once in about four days. Over several hours, it barely moves. You can only tell it has moved by actually measuring the pictures, but it’s really hard to see on the screen.”
Merline had looked at Daphne many times before, and had never seen a moon. Since 1996 he has discovered 15 other moons orbiting asteroids, most of them using AO systems.
“In this case, the thing we thought we saw was moving rather quickly. It would be in one place and then a couple hours later it was quite a ways away. And that almost made me think it wasn’t real, just because of that. I thought, “well it can’t be real, it’s moving too fast.” But then it kept moving in the same direction. A couple hours later we saw it moving yet further and still pretty fast. So by the end of the night I was pretty convinced it was real.”
Conrad immediately changed the observing program and spent the rest of the night looking at the new object until it was lost near the horizon at the telescope limit. The next morning, Merline plotted the position points. It was real. They had found a new world. “I was pretty worn out,” he says, “but it was exciting.”
It was the first time Conrad had discovered a moon. Once he realized what he had discovered, he compared his feelings to what it must have been like to be Lewis and Clark, explorers of the Pacific Northwest. “I always sort of think they would turn around at some bend in their trail and realize they’re the first people seeing a particular thing. There’s the initial excitement of just being the first person to see it. Then there is the interpretation; just enjoying the natural world and being fascinated by it. These are places that could someday be visited by our descendants.”
Fifteen years ago, asteroids only appeared as faint points of light in telescope images, a single pixel element. Today, the Keck Observatory can see as many as 300 different “resolution elements” on the surface of an asteroid. Conrad doesn’t like to use the term “pixels” because it can be misunderstood. He explains that the way the AO system works with light is different than the way computers render images on the screen. It’s an important distinction, because part of the credit for the discovery is due to a new, high-speed wavefront controller in the AO system, a technology upgrade funded by the W. M. Keck Foundation.
Conrad explains. “The enhanced AO system has become more effective at shorter wavelengths. At these wavelengths we can resolve detail as small as 35 to 45 kilometers (25 miles) on the surface of an asteroid. In addition, for fainter sources, the enhanced system outperforms its predecessor. Both of these improvements played a key role in the discovery of the tiny moon near Daphne.”
“This body has been searched for satellites many times before,” Conrad says. “But this was the first time it had been observed with the new wavefront controller on the Keck 10-meter telescope. We used this new wavefront controller together with the NIRC2, which is one of the best infrared cameras on the planet.”
Daphne, an asteroid about 200 kilometers in size (125 miles), was also in a favorable viewing position. Its orbit was lined up with the Earth and the Sun such that Earth was in the middle and the asteroid was opposite the Sun in the sky, or at “opposition.” In this position, Daphne was fully lit by the Sun. In addition, Daphne, Earth and the Sun had lined up while Daphne was also nearest the Sun in its rather elliptical orbit, something that only takes place once every 20 to 40 years. The fact that this event took place in 2008 was the reason Daphne was on the observing list that night.
“A moon that is very close to the primary is going to be hard to see unless we’re pretty close to it, because otherwise it gets lost in the glare of the primary,” says Merline. “But because the asteroid was so close to Earth, we were able to resolve the secondary.”
Daphne was closer to Earth than it has been since 1962, and it won’t be this close again until the year 2031.
The newly discovered moon is exceptionally small, just 2 kilometers wide (1 mile). It is 10,000 times fainter than Daphne, the most extreme size difference ever found in an asteroid moon. The moon also orbits extremely close to the asteroid, about half the distance of any other moon of the same class found orbiting an asteroid. It is separated from Daphne by just 460 kilometers (290 miles). The fact that it orbits so close to Daphne tells scientists it probably formed after a collision between two asteroids, much as the way our moon formed after a proto-planet collided with Earth.
Merline continues, “There are several different mechanisms that we think are operating to make these satellites. Most of them involve collisions, but not all of them. We’re pretty sure this system fits into the same mold as many of the other ones we’re seeing.”
The collision, he explains, probably didn’t break the asteroid apart, but instead caused a giant cratering event that threw up a lot of material, some of which became the tiny moon. Collision events, like those thought to have occurred on Daphne and on Earth, would have thrown most of the debris away from the system and a small amount would have fallen back to the surface. Any material that remained in orbit eventually formed into a moon.
There could also be more moons around Daphne, though scientists will have to wait to find out. Daphne is already becoming harder to observe as it makes its way back to the middle of the asteroid belt. But examples of second and third moons have been found around small bodies, most famously around Pluto, which is now known to have three moons.
“We always try to look for additional ones, but if they haven’t been seen yet, they’re obviously going to be hard to find,” Merline says.
The presence of a moon near Daphne was the key to determining Daphne�s density. The asteroid seems to be more compact and heftier than expected, perhaps more than any other asteroid of its type or class (C-type). Jack Drummond, another collaborator on the team, gives early estimates for a density of about 2 grams per cubic centimeter, more than the density of water (1 g/cc), but less than rock (3 g/cc).
“Maybe it’s more like the density from Daphne’s original formation, with some impact that broke up the surface layers a bit,” Merline says. “The density is low compared to rock, so it still has to be broken up to some extent, but probably just not as much as the other C-types that we’ve seen, most of which are thought to have been completely broken apart and reassembled into a so-called “rubble pile”.” The estimated density is so unusual that the research team is having a hard time believing their figure is accurate.
“Some of our solutions do have a density that goes lower, it’s just that they are not a very good fit to the data,” explains Merline. Since the discovery, Daphne and its satellite have been observed several more times, both at Keck and by colleagues working at the European Southern Observatory. “I’m certain with the new data we’re going to find the absolute answer. It would be interesting if it turns out to have a high density, but I’d say it’s a 50/50 chance that it’s lower than that.”
The compact nature is surprising because the numbers are about 40 percent higher than what they should be. No other asteroid in Daphne’s class has ever been found to have such a high density figure.
“This might just be a more unusual one, but that’s okay. If all of these are exactly the same, then something probably happened to all of them. But if there’s a range, then we can see that the process maybe isn’t complete in some places, so we could learn even more.”
Of all the large main-belt asteroids studied, only two percent have been found to have companions. It is a low number compared with the percentage having moons among other types of small bodies in the Solar System. The rate for Kuiper Belt objects, near the edge of the Solar System, is closer to 10 to 30 percent. What could account for the difference?
“[Daphne] is an example, wherein, if the asteroid hadn’t come this close, then we would have never found the moon—- not with today’s technology,” says Merline. “It’s possible that a lot of these asteroids have satellites that we can’t see yet.”
Understanding relics like Daphne not only tell us about the history of the early Solar System, but also about the processes affecting young planets around other solar systems throughout the universe. Extrasolar planets is a field of research that did not exist when the Keck telescopes were designed, yet now it has become a regular experiment in the Observatory’s research program.
Merline and Conrad presented their results July 14-18, 2008 at the 10th meeting of the Asteroids, Comets, Meteors conference in Baltimore. It was an opportunity to share their findings with international experts and colleagues representing a wide variety of backgrounds and expertise. “Our satellite discovery and density measurement were well received at the conference,” reported Conrad. “We are really homing in on what these tiny worlds are made of.”