With the AP's Alicia Chang in the lead, a few media outlets in the last week or so are relaying news from NASA that its ancient Voyager 1 spacecraft is reaching a milestone, or AU-stone, long after its August 20, 1972 launch. It already swivel-hipped through a series of gravitational slingshots in the late 1970s and 80s that sent it close past Jupiter and Saturn. Its path heaved it onward toward interstellar space. Its sister craft Voyager 2 added Uranus and Neptune to the itinerary but is well behind Voyager 1. The two are the longest-operating spacecraft. That's amazing. Any hook that brings attention to these mechanical Methuselahs seems appropriate.
The news, set off by an August 20 NASA press release (Grist below), is that Voyager I is now wafting its way into the edge of the heliopause.That's the region where the sun's spray of gases, or solar wind, exhausts itself and blends into the interstellar medium of similarly hyperthin stuff, a collective emanation of the Milky Way's stars along with leftovers from the galaxies formation. It has enough instruments still working and enough radio oomph to still be delivering data treasures to scientists here on Earth.
That's a good story. It's also a reminder of the audacious missions NASA could muster in its early, post-Apollo era despite the budget cuts that accompanied the manned Moon mission's close.
Chang gets the history of this saga down well. She puts the present situation dramatically:
Sooner or later, the workhorse spacecraft will bid adieu to the solar system and enter a new realm of space – the first time a manmade object will have escaped to the other side.
The press release plants a large seed for that "other side" line. It phrases it this way in its hed: "Voyager at 35: Break on Through to the Other Side." If your mind does not turn to Jim Morrison and the Door's first big hit well, you're probably one of those Who are not part of my generation.
I have a small comment, then a question about the definition of solar system. The comment is merely that this news invites some recognition of Pioneer 10, an even older spacecraft that also reconnoitered Jupiter, is also out there far beyond Neptune although not quite as far as the speedier Voyager I, and on the opposite side of the sun. It's out of radio range now, but is the one carrying the iconic plaque that Carl Sagan helped to compose with the engraved images of a man and woman and a chart to the Sun's location. Both Pioneer 10 and a fictional Voyager 6 have even appeared in Star Trek episodes and a movie.
The meatier question: Does entry into interstellar space, as defined by the character of the vanishingly-slight gruel of matter to be found, equate with leaving the solar system? The NASA press release says so. Well… Voyager 1 is more than 9 billion miles, or 120 or so astronomical units (Earth-Sun distance) away. But if the solar system were defined as the family of objects orbiting the sun, does that not add extreme ambiguity to its dynamical extent? The Kuiper Belt of frozen things, some of them approaching planet size, extends to at least 50 AU, astronomers say. But some of those experts say it could go a long way farther, maybe more than 100 AU. And while hypothetical the vast Oort Cloud of a trillion or so sun-orbiting comets is speculated to extend a thousand times farther out, or a fair fraction of the way to the nearest star. If that cloud is not there it's hard to explain the origin of long-period comets that occasionally enter the inner solar system on trajectories asymptotic to an infinite distance of origin. But their composition seems to place them in the Sun's family. Certainly any story stoutly declaring that the heliopause is the edge ought to recognize that there are different ways to define said edge.
One more thing. A paper this week in Nature, from a team led by a Johns Hopkins space scientist, complicates NASA's effort to highlight the 35th anniversary by trumpeting that Voyager I is entering the border zone separating us from interstellar space. Researchers say recent data don't look like what one would expect from that twilight region. Could be this whole news burst is a solar system shaggy dog story.
Other Stories, mostly prompted by this week's Nature:
- Nature.com – Ron Cowen: Voyager's long goodbye ; Rather than a tumultuous region of shifting breezes, the probe seems to be in a relatively calm spot, a sort of dead zone, it says here. The researchers suspect the spacecraft, communicating over the vast gulf (Cowen writes) with a transmitter "about as powerful as a refrigerator light bulb," is not even close to the heliopause.
- New Scientist – Victoria Jaggard: Voyager 1: reports of my exit are greatly exaggerated ; Jaggard gets just right the party-pooper effect of the Nature paper on NASA's celebration. But she buys the idea that the heliopause is the edge of the solar system. Poor little comets orbiting beyond it – nobody to call their Mama?
- Ars Technica – Matthew Francis: Missing: Voyager 1 yet to find the boundary line of the Solar System ;
- Bloomberg – Meg Tirrell: Voyager 1 Reaches Pocket of Calm at Solar System's Edge ;
- Space.com – Charles Q. Choi: Voyager 1 Spacecraft Farther From Solar System's Edge Than Thought;
- Register (UK) Brid-Aine Parnell: Voyager's 35th birthday gift: One-way INTERSTELLAR ticket / Veteran space probe bores outward into the deep void ; Fancy writing here, what with a "surrounding expanse of obsidion nothingness" hyperbole right in the lede. This also is a fine summary of all the signs to NASA – prior to this week's Nature paper – of entry into the heliopause.
Grist for the Mill: NASA Press Release ;
– Charlie Petit