[New to the Journey? Read this for a brief introduction!]
by Gideon Marcus
With the Apollo missions taking so much of our attention (there were four flights this year), it is understandable that unmanned missions and science have gotten short shrift. I'm going to try to address this oversight now.
Far out!
Do you remember Pioneer 6 (launched Dec. 16, 1965) and Pioneer 7 (launched Aug. 17, 1966)? They are deep space probes designed to observe the Sun from widely different vantage points. In fact, we've been a bit remiss: since '66, two more identical Pioneers have gone up: Pioneer 8 (December 13, 1967) and Pioneer 9 (November 8, 1968). A fifth and final Pioneer was launched August 27, 1969, but its carrier rocket exploded. The loss of that one is pretty bad; whereas the others are all spread out fairly equidistantly around the Sun, more or less as far away from it as the Earth, Pioneer "E" was going to be put in an orbit that kept it close to Earth, where it would be used to give as much as a two-week warning of dangerous flare activity.
Nevertheless, NASA is blazing along with four satellites. Indeed, thanks to the longevity and spread-out positions of Pioneers 6 and 7, they were able to perform an unique experiment. On Nov. 6, the two satellites were 175 million miles apart on a common line with the Sun, and scientists observed the difference in behavior of solar wind particles due to their passage through space in opposite directions. In a similar vein, on Dec. 2, when the spacecraft reached points on a common spiral line leading out from the Sun (the star rotates, so it flings out particles in a spiral rather than linear fashion), scientists measured different kinds of solar particles coming from the same events on the Sun.
We'll have to wait for the journals to publish any papers, but this is the kind of large-scale, long-term science made possible by the Pioneer probes!
Another cool example of Pioneer science
Far in!
While the Pioneers study the Sun far from Earth, there are a host of spacecraft monitoring our home star from Earth orbit. For instance, we haven't talked about the Orbiting Solar Observatories (OSOs) for a while, but there have been six so far. They were the first heavy satellite series to be launched by NASA, providing nearly continuous coverage of the Sun since 1962, in wavelengths we can't observe from Earth because they are blocked by the Earth's atmosphere: ultraviolet, X-Ray, and gamma ray.
Why was the Sun such an early focus? Three major reasons: 1) understanding the dangers posed by flares and their relation to the high energy particles trapped by Earth's magnetic field is critical to ensuring astronaut safety, 2) surveying the Sun and comparing changes on the solar surface with fluctuations of space weather near Earth tell us both about the interactions of the two as well as the nature of both, and 3) the Sun is the closest star at hand, and what we learn about the Sun as a star can be applied to the millions of other stars we can observe.
The revelations OSO have given us are not easily conveyed. It's not like Explorer 1, which discovered the Van Allen Belts—a hitherto unexpected phenomenon—or the TIROS weather satellite, which discovered storms we hadn't even known about. Rather, they give us a huge body of data with which we can refine our understanding of how the Sun works, and also so that we can better predict space weather. What's called "basic research."
OSO 1 operated continuously from March-May 1962, and intermittently on to August 1963, returning data on 75 solar flares—most importantly, what events preceded, succeeded, and coincided with them in many different wavelengths, a fingerprint of an eruption, so to speak.
(ground-taken picture of the Sun flaring)
OSO 2 expanded its coverage to the corona, that bright bit of the Sun you can only see during a lunar eclipse. Its launch was delayed until February 3, 1965 because the original OSO B was damaged in a launch explosion, April 14, 1964, that killed three technicians! Though OSO 2 returned data for nine months, I can't find a single article on the Sun that stemmed from it. There's one on about 20 other stars observed by the satellite, though, and the difficulties of seeing through the Sun's glare to them.
OSO 3, the one that launched March 8, 1967, and not the one that failed to orbit in August 1965, was more successful. It returned interesting solar data, for instance finding solar X-ray sources that weren't flares, determining that the chromosophere (visible surface) didn't necessarily heat up before a flare, and monitoring the change in the solar spectrum over the course of its 28-day rotation.
And the onboard gamma ray experiments told us a lot about the universe. For instance, the torrent of gamma rays streaming in from the universe is highly confined to the galactic plane, and particularly toward the Milky Way's core, which means it must be galactic in origin. OSO 3 also observed X-ray bursts from a star (maybe stars) that isn't the Sun: Scorpius X-1, later determined to be a neutron star, and Lupus XR-1 (which may or may not be the same source—the literature is unclear). The satellite stopped working just last month.
OSO 4 went up October 18, 1967, and was the first OSO to carry an international experiment—a University of Paris device that measures the Sun in the ultraviolet frequency that best shows solar activity ("Lyman-alpha"). Indeed, it was the first OSO to scan the Sun in ultraviolet at all. Also really cool is that its X-ray resolution is such that it could watch flares in X-ray wavelengths as sharply as we could see it on the ground in the visual spectrum, so scientists could make one to one comparisons.
You'll note the use of past tense—the satellite is still in orbit, but its tape storage failed in May 1968, and last month, OSO 4 was ordered into standby mode.
That brings us to the OSOs we haven't covered yet. OSO 5 went up on January 22, 1969, and has the ability to scan the Sun in the X-ray range more quickly and thoroughly. OSO 6 went up August 9. I don't have too much to say about them because it's too early for papers. NASA reports both did their jobs fine, and they're still operating. Like OSO 3 did, they not only study the Sun but also galactic X-ray sources…so stay tuned.
Small satellites are doing their part, too. For instance, Explorer 41, the latest in the Interplanetary Monitoring Platform series, launched June 21 into a high orbit that goes almost halfway to the Moon. The Sun this satellite examined has been unusually quiet, an expected trait of the "solar maximum"—the time in the Sun's 11-year cycle of highest output. On the other hand, low-energy galactic cosmic rays rates fluctuated more than usual, and interplanetary conditions appeared to be more disturbed. The satellite is still operating.
Finally, and only tangentially related to the Sun, there are the missions of Aurorae and Boreas, launched October 3, 1968 and October 10, 1969, respectively under the auspices of the European Space Research Organization (ESRO). They report on the brightness of Earth's aurorae, the composition and temperature of the ionosphere, and the charged particle environment in orbit. The first satellite is still working just fine, but Boreas went into a lower than expected orbit, and it reentered on November 23rd. Still, the mission was deemed successful.
Rocks to dig
Veering back into the manned space program, there was some exciting coverage during the Apollo 12 flight that I didn't have a chance to relate. As Conrad, Bean, and Gordon finish their three weeks in quarantine (joined on Dec. 2 by 11 scientists and technicians who had accidentally been exposed to lunar samples), this is a good time to talk about what we've learned from Moon rocks brought back by the Apollo 11 astronauts.
Walter Cronkite had, as a guest on his programming, Dr. John O' Keefe—a geologist at NASA's Goddard Space Center. The visibly excited O'Keefe stated that the most extraordinary aspect of the Moon rocks is that they are deficient in nickel and cobalt as compared to the Sun, that latter body presumably being representative of the nebula that originally coalesced and formed our solar system.
Why is that significant? Well, the Earth's crust is similarly lacking in nickel and cobalt (and other "precious metals" that dissolve easily in iron, collectively called "siderophiles"). We know Earth has a dense iron core because nothing else would account for the planet's mass with respect to its volume, and also, it explains why the planet has a magnetic field. While our planet was first cooling, it makes sense that the siderophiles melted and mostly sank to the center of the planet.
The Moon has no core—we know this because its density (volume divided by mass) is too low, and it has no appreciable magnetic field. That the Moon's surface rocks correlate to Earth's surface rocks, and because its density appears to be constant from crust to center, that suggests that the Moon was somehow formed from Earth's crust. It is, in fact, a piece of our planet's outer surface that somehow spun off into orbit and formed its own little, low-density world.
What causes this is still unknown. Perhaps the Earth was spinning so fast when it was formed that its middle flew off. Or maybe a rogue planet smashed into the Earth. What we do know is that the composition of the Moon rocks puts paid the hypothesis that the Moon formed separately from and at the same time as Earth, since we'd then expect its crust's composition to either be more like that of the Sun, or for our moon to have a dense core.
We also know that whatever created the Moon happened quite early in Earth's history. The lunar rocks have been dated as 4.6 billion years old. That's very close to the estimated age of the Earth. What I found particularly exciting is that the Moon rocks must be the very oldest rocks we've ever encountered, except maybe for meteorites. That's because erosion and vulcanism are constantly erasing the Earth's surface, and the oldest rocks I know of down here are somewhere around 3 billion years old.
As we continue to explore the cosmos, we shall find more data points with which to create an holistic view of the universe, something that would be impossible were we to stay Earthbound. I am happy that I live in the Space Age, when our scientific knowledge is expanding exponentially. Who knows what new discoveries 1970 will bring!
[New to the Journey? Read this for a brief introduction!]
