This week the Red Planet welcomes the latest robotic emissary dispatched from Earth. You can find Curiosity's destination, along with Saturn and the bright star Spica, clustered together in the western sky after sunset.
Hey, have you heard that Mars is coming so close to Earth that it'll look as big as the full Moon? Fortunately, that long-lived Mars myth seems finally to have gone away after dragging on nearly a decade.
Mars was especially close and bright in August 2003. But that's not the case at all right now, as NASA's Curiosity finally reaches the Red Planet after cruising across 350 million miles interplanetary space over 8½ months. Mars is practically on the far side of the Sun from Earth, 154 million miles (1.7 astronomical units) away.
Even so, you can step outside this week to spot Curiosity's destination low in the west after sunset (and maybe cheer the spacecraft on while you're at it). As the chart here shows, the Red Planet is joined by Saturn and the bright star Spica. All three are nearly equal in brightness, and they make a striking celestial triangle about 5° on a side. Can you detect any difference in color among them? Spica's color is icy white, Saturn is slightly creamy, and ruddy Mars is tinged with orange.
It'll be fascinating to watch the shifting geometry of this trio in the days ahead. Saturn's relative position above Spica doesn't change much, but Mars is moving rapidly eastward. Watch as Mars slides between the other two, forming a nearly straight line with them on the evenings of August 13th and 14th. After that, the Red Planet continues its migration eastward and forms a second pretty triangle about a week later. A thin crescent Moon joins the scene on the 21st and adds to the spectacle.
The nearly-equal brightness of these three beacons belies their true distance. Spica is about 260 light-years away — when the light you see left its surface, Benjamin Franklin was flying his first kites in lightning storms. Saturn, though much closer, is still 10 a.u. (930 million miles) away. Its reflected sunlight takes 83 minutes to reach Earth, as do the radio transmissions from NASA's Cassini spacecraft, which is orbiting the planet.
Mars is the closest of the three, but it still takes Curiosity's radio signal 13.8 minutes to reach Earth. This means that by the time ground controllers confirm the start of its dramatic plunge through the Martian atmosphere, the spacecraft itself will have already been sitting inside Gale crater for 6 minutes (preferably in one piece).
Plans call for Curiosity to operate for a minimum of one Martian year, or 98 weeks here on Earth. As the months flip by, Mars and the Sun will draw nearer in the sky. By mid-April next year, they'll appear only 8 arcminutes apart, and for a few days the Sun's interference will disrupt the ability of ground controllers to send the spacecraft any commands. During that period the rover will be executing of commands that don't include any driving.
A year later, in April 2014, the Red Planet will be at its biggest (15 arcseconds across) and brightest (magnitude –1.2) as it reaches opposition from the Sun in the sky. However, then as now, it still won't look as big as the full Moon!