Astronomy - February 2014

ASTRONEWS

ASTROCONFIDENTIALBY KARRI FERRON

20 ASTRONOMY t FEBRUARY 2014

More than 200 asteroids are known to possess

a satellite, and astronomers have found them

in all populations of small solar system

objects. (87) Sylvia was the first triple asteroid

system ever discovered. When we announced

the existence of two moons, now called

Romulus and Remus (twin sons of the mytho-

logical figure Rhea Sylvia), we had only a

basic understanding of their orbits around

their large primary asteroid. But with 65

observations spanning 2001 to 2011, we were

able to better characterize the orbits of the

satellites and derive the shape and size of the

primary asteroid.

From this precious data set, we confirmed

the asteroid’s density to be slightly more than

water and less than carbonaceous chondrite

meteorite and revealed that the primary might

be differentiated, made of a dense core sur-

rounded by fluffy material. Our model was

robust enough to foretell the positions of the

moons at the time of an occultation across

Earth on January 6, 2013, when the triple

asteroid system was predicted to pass in front

of a bright star as seen from Europe. Fifty

amateur and professional observers were

mobilized to observe this event during a cold

winter night. Combining the exact details of

the occultation observations, we derived a

precise estimate of Romulus’ size — about 15

miles (24 kilometers) — and showed that its

shape is extremely elongated, most likely

stretched by the tides from the 170-mile-wide

(270km) primary. Our observations are

extremely useful in understanding the origin

and evolution of multiple asteroid systems,

and we are planning to expand this program

to 20 multiple asteroid systems for which we

have good satellite orbit models. We are very

thankful for the commitment of the large

community of amateur astronomers around

the world without whom this project would

not have been a success.

UNEXPECTED CHEMICAL. Gas around the active supermassive black hole at NGC 1097’s center shows a stronger

than expected hydrogen cyanide signal, the National Astronomical Observatory of Japan announced October 24.

Our galaxy rotates around the super-

massive black hole at its center. But, as

scientists describe online October 3 in

the Monthly Notices of the Royal Astro-

nomical Society, it also wobbles and

squishes in the north-south direction

— perpendicular to the galaxy’s main

disk, or its plane.

As part of the RAdial Velocity Experi-

ment (RAVE), astronomers looked at

the movements and speeds of “red

clump” stars — metal-rich giant suns

that all shine with the same brightness

— that are 6,500 light-years from the

Sun in any given direction. Because of

the giants’ predictable luminosity,

scientists can tell how distant each star

is by how much its light has dimmed

on its path to Earth. With that distance

in their pockets, they then can make a

map of how stars in different regions

are moving around.

This three-dimensional analysis

revealed that while the Milky Way is

rotating, its stars are sloshing in waves,

making a more complicated movie

than scientists expected. Now they just

have to figure out why. The galaxy’s

spiral arms could be leaving a wake in

which other stars bob, or the ripples

could be remnants of the galaxy’s colli-

sion with a smaller galaxy. — S. S.

The Milky Way wobbles

WHAT ARE WE LEARNING ABOUT TRIPLE ASTEROIDS?

COURTESY FRANCK MARCHIS

Although astronomers have discov-

ered a moon orbiting more than 200

asteroids, they have only uncovered

five with two satellites each.

FAST

FACT

Franck Marchis

Senior planetary astronomer at

the Carl Sagan Center of the SETI

Institute, Mountain View, California

W O N K Y W AY. The stars in the Milky Way do not simply orbit the galactic center in straightforward

ellipses but bob up and down in complicated patterns. The movement is perpendicular to the

galactic plane, seen here over the Cerro Paranal in Chile. BRUNO GILLI/ESO