Twenty years since Voyager's last view
12-02-2010 13:17 CST
On Sunday comes the twentieth anniversary of an iconic image from the Voyager mission: the "Pale Blue Dot" photo of Earth caught in a sunbeam, which was captured by Voyager 1 as part of a Solar System Family Portrait. This panoramic view of our planetary cradle wouldn't have happened without years of advocacy by Planetary Society founder Carl Sagan, whose vision still inspires our organization.
Nothing like it has been done since. There hasn't been the opportunity -- Voyager 1 is the last spacecraft to have departed the solar system with a functional camera. The next one to be in a similar position will be New Horizons. I asked Alan Stern whether the Pluto-bound craft will take a family portrait from its distant perspective. He told me "Absolutely! But because of the bright Sun, we have to be careful for now, so we can't do it until after Pluto. We can however (and soon will-- this summer!) look back to Jupiter and see it and the Galilean satellites from 2+ billion km away near the orbit of Uranus. It should be quite evocative." Indeed it will.
There are several retrospectives on Voyager's solar system family portrait being posted across the Internet today, including this one from JPL and this really neat story from National Public Radio, featuring an interview of Candy Hansen, who was apparently the first person to spot Earth fixed in that sunbeam. Here's one written by the Society's Charlene Anderson a few years ago, on the occasion of the 25th anniversary of the Voyagers' launches.
by Charlene Anderson
This article is reprinted from the September/October 2002 issue of The Planetary Report.
Home. Family. This will be Voyager's enduring legacy: It has changed forever the feelings raised by those words. Through its robotic eyes we have learned to see the solar system as our home. Through its portraits of the planets we know that they are part of our family.
Apollo astronauts showed us a tiny Earth alone in the blackness of space. Now, with these images, Voyager has shown us that Earth is not really alone. Around our parent Sun orbit sibling worlds, companions as we travel through the Galaxy.
These family portraits of the Sun and planets were Voyager's final photographic assignment. Planetary Society President and Voyager Imaging Team member Carl Sagan worked for a decade to get these pictures taken. Between the two Voyager spacecraft, they returned some 67,000 images of the four outer planets and their 56 known moons. Voyager 1 had the slightly easier assignment: It encountered Jupiter in March 1979 and swung by Saturn in November 1980. Then it headed out in search of the heliopause, the edge of our Sun's sphere of magnetic influence, and where the solar wind gives way to the wind from the stars. In August 1989 Voyager 2 flew by Neptune, completing its reconnaissance mission, having visited Jupiter in 1979, Saturn in 1981 and Uranus in 1986. After passing Neptune, Voyager 2 joined its twin on the way to interstellar space.
The Voyagers had been launched in 1977 to take advantage of a planetary alignment that occurs only once every 176 years. The outer planets were lined up so that a spacecraft could swing from one to another, threading its way past the 4 gas giants in only 12 years. Mission planners at the Jet Propulsion Laboratory could select their paths from among many possible trajectories and targets.
For Voyager 1, they chose to send the spacecraft close by Titan's south pole to obtain close-up data on Saturn's largest moon. Titan's thick nitrogen atmosphere proved to be heavy with complex, carbon-rich organic molecules, and its surface is possibly dotted with lakes of liquid hydrocarbons. For carbon-based lifeforms living in a primarily nitrogen atmosphere - such as ourselves - a world like Titan is well worth a close look.
But to fly close to Titan, the project team had to sacrifice Voyager 1's encounters with Uranus and Neptune or a close-up look at Pluto. Its path around Saturn swung the spacecraft up and out of the ecliptic, the plane defined by Earth's orbit about the Sun. Looking from its Pasadena home on Earth's northern hemisphere, the spacecraft now appears to be coasting above our solar system.
Voyager 1 was chosen to take the family portrait because fewer instruments might be damaged by looking back toward the Sun. And, to Voyager 2, now beyond Neptune and traveling much closer to the ecliptic, Jupiter was too close to the Sun to be picked up by the spacecraft's cameras.
So on February 14 - Valentine's Day 1990 - Voyager 1 aimed its cameras at a string of small colored dots clustered just to the right of the constellation Orion - the Hunter. The spacecraft was then 32 degrees above the ecliptic and nearly 6 billion kilometers (3.7 billion miles) from the Sun. It took 39 wide-angle views and 21 narrow-angle images. The narrow-angle camera, with a lens resembling a telephoto, took three consecutive images through colored filters of seven of the nine planets. This enabled image processors at the Jet Propulsion Laboratory to construct the colored portraits of the planets seen on pages 16 and 17. The Multi-Mission Image Processing Laboratory then pasted together the wide-angle images into the mosaic on the next page.
Voyager had produced the first portrait ever of our Sun and planets together.
But like shy family members at a holiday gathering, the smallest planets avoided having their pictures taken. Mars and Mercury were lost in the glare of the Sun. The outermost planet, Pluto, was too tiny and far away. So this family portrait is incomplete. The next generation of spacecraft will be unable to take another family portrait. Magellan and Galileo, and the planned missions, such as the Soviets' Mars '94 and NASA's Mars Observer and Comet Rendezvous/Asteroid Flyby, plus the joint NASA/European Space Agency Cassini mission, will be locked in orbit about their target planets. None of these will ever gain a perspective from which they could see the solar system as Voyager did.
Voyager alone could look homeward and capture our family of planets as they looked on February 15, 1990. Voyager alone could so graphically show us how Earth and the planets are inextricably linked to our parent Sun.
Home is now a corner of space brightened by a small yellow star. Family is now a company of planets circling that star together. Our home and family now encompass an entire solar system.
Thank you, Voyager.