Astro Bob blog: Venus' little buddyPulled this way and that by an icy moon plus Venus pairs up with a double star tonight
By: Bob King, Duluth News Tribune
Venus' little buddy
A photo of Saturn's moon Enceladus just prior to Cassini's close encounter last week. Enceladus is 314 miles in diameter and its icy surface makes it one of the most reflective objects in the solar system. Faint water plumes can be seen to the moon's right side. Credit: NASA/JPL/Space Science Institute
Cassini flew only 60 miles away from Saturn's icy cue ball moon Enceladus last week and for 26 hours took precise measurements of the moon's gravitational field in hopes of probing its deep interior. Enceladus is famous for its plumes of water vapor and organic particles that shoot from long fissures near its south pole called "tiger stripes". What exactly is going on inside the moon to get water to the surface is what's making scientists scratch their chins.
During the flyby, the gravitational pull of Enceladus changed the speed of the spacecraft and the frequency of the radio signals its sends back to Earth. Depending on what's doing the tugging, the frequency or "pitch" of the signal can drop or rise. The radio science instrument onboard the craft is so sensitive it can measure a change in Cassini's speed of one-millionth of a meter (micron) per second. Scientists use the data to wring out information about the composition and movement of the moon's interior.
No one knows yet what the "wiggles" in the radio signal will indicate. Does Enceladus have an ocean in its core like some humongous liquid-center candy or might the plumes' source be a single "lake" closer to the surface? Results from the experiment will also tell scientists if bubbles of warmer ice in the interior rise toward the surface of the south polar region like an underground lava lamp. A natural lava lamp in orbit about Saturn? That would be too good to be true.
The sky was peaceful last night. Off in the northeast Vega and the Northern Cross were on the rise. Seeing them gave me that good feeling that summer is coming. Venus had fallen away in the west, but when it returns tonight, bring out your binoculars and see if you can spot the planet's temporary companion Tau Tauri (taw TAUGH-rye). Venus, being closer to the sun than the outer planets like Mars and Jupiter, moves more quickly through the zodiac during the year. As it does, the planet "visits" numerous stars, star clusters and planets along its busy way. Tonight is Tau Tauri's turn, a star too faint to see in twilight but which should pose no problem in binoculars. Venus will be less than one full moon diameter to its right. Although Tau Tauri appears faint that's only because it's 400 light years away. In reality it's 200 times brighter than the sun.
This map shows the sky as you look northwest about an hour after sunset. Point your binoculars at Venus to see Tau Tauri. Created with Chris Marriott's SkyMap software
Tau is a double star with a dimmer companion immediately below and to its left. It will prove a worthy challenge to see in binoculars. If by good favor you have clear skies both tonight and tomorrow night, you'll be able to see for yourself how quickly Venus moves as it leaves Tau behind. Helping push things along is the Earth's own motion around the sun which causes the stars in the western sky to set four minutes earlier each night while those in the east rise four minutes earlier. These small increments of time add up until pretty soon the constellations in the west disappear and are replaced by the new summer season groups in the east. Summer always comes in the stars. It's that inevitability that gives sky watchers hope on cold winter nights.
(Lava lamp photo by Saltmiser)