- 2660 - PIONEERS - exploring the universe? The Pioneer spacecraft reached escape velocity from the Sun and would ultimately leave the solar system but were slowing down more than the data predicted they would. Also if we were in a kind of gigantic 'bubble, where the density of matter was significantly lower than the known density for the entire universe, it would have consequences on the distances of supernovae and, ultimately, on determining the expansion of the Universe.
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----------------------------- 2660 - PIONEERS - exploring the universe?
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- Before Voyager 1 and 2 explored the outer solar system, Pioneer 10 and 11 paved the way. Launched in 1972 and 1973, respectively, these spacecraft were the first to transit the asteroid belt and the first to make close observations of Jupiter (both Pioneer 10 and 11) and Saturn (Pioneer 11).
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- Like their successors, the Voyagers and New Horizons, both Pioneers are past the orbit of Pluto and will continue speeding outward from the center of the solar system.
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- Powered by four plutonium-238 radioisotope thermoelectric generators each, the Pioneer spacecraft carried several scientific devices that allowed them to take photographs and make detailed measurements of the environments around Jupiter and Saturn and in the outer solar system.
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- The Pioneers also famously carried the Pioneer plaque, with a pictorial message for any extraterrestrials who may someday encounter the craft; this would give them information as to the origin of the probes.
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- The Pioneers had some unique design features that allowed them to be very precisely tracked by Earth-based receiving stations. Both Pioneers were spin stabilized (which means that they rotate around a central point to keep them from wobbling or changing trajectory) and required very few course corrections that could perturb their motion.
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- The combination of the type of transceiver aboard the spacecraft and ground-based Doppler tracking used on the missions allowed their acceleration to be tracked to an almost unbelievable accuracy of 10^–10 meters per second squared (m/s2).
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- The Voyager spacecraft, which were not spin stabilized and relied on more frequent use of thrusters, cannot be tracked to this level of precision.
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- The Pioneers are considered the most precisely tracked and navigated spacecraft to date. This astonishing accuracy with regards to measuring the acceleration of the Pioneers led to an unusual discovery. It was understood that the probes would slow down to some extent due to the Sun’s gravitational attraction.
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- But while the Pioneers had reached escape velocity from the Sun and would ultimately leave the solar system, by 1980 a researcher at NASA’s Jet Propulsion Laboratory noticed the Pioneers were slowing down more than the data predicted they would. This occurred in both Pioneers, manifesting as a sunward acceleration of 8.74 ± 1.33 × 10^−10 m/s2.
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- While this may seem like a very small difference in measured acceleration from predicted acceleration, it led to the Pioneers remaining thousands of kilometers closer to Earth per year than they should have been. The cause greatly puzzled the scientists who studied the problem. Even such a small, unexplained sunward acceleration suggested a possible breakdown in the principle of gravitation as described by general relativity.
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- Why were the Pioneers slowing down? Many factors were considered as possible causes for this unexplained deceleration, which became known as the “Pioneer anomaly“. Could our understanding of the fundaments of physics be flawed?
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- Could gravitational forces from the surrounding interstellar medium that fills the space between stars be different than had been expected? Could dust produce drag on the spacecraft? Was the anomaly possibly secondary to solar radiation forces?
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- Could our calculations regarding the exact position of Earth relative to the spacecraft be incorrect? Was something related to the spacecraft, waste heat, a faulty thruster, or a gas leak, causing the anomaly?
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- Beyond the theoretical problems facing researchers, another more concrete problem stood in their way: The technical information and recordings of Pioneer data needed to solve the problem were stored either on paper or on 7- and 9-track magnetic data tapes.
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- Identifying these data sources (some of which were literally in boxes under stairwells and in dumpsters on their way to being destroyed), recovering the hardware needed to read the tapes, and converting all of this information to modern media formats was a daunting challenge on several levels, most notably the cost and manpower involved.
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- When all of these possibilities were considered, one potential cause of the Pioneer anomaly repeatedly came to the fore: thermal effects, mostly originating from the four RTGs aboard each Pioneer. To fully analyze this possibility scientists used predicted and actual thermal measurements to create a highly accurate thermal model of the spacecraft.
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- They first modeled the effects of heating from the Sun on the trailing face of the Pioneers. Then they turned to heat sources on the spacecraft. Heat from the RTGs radiated toward the leading edge of the spacecraft, in the direction of its motion, and heat from the electronics box also primarily radiated in this same direction. This generated a sunward (backward) pressure on the entire spacecraft.
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- As a result, the overall heating of the craft, when all factors were included, was asymmetric. Ultimately that radiation forces from the differential heating of the spacecraft, known as thermal recoil forces, from the RTGs and electronics were enough to explain the entire Pioneer anomaly.
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- Additional analysis showed that the Pioneer anomaly also appears to be decreasing in intensity with time, likely as a result of the RTGs’ radioactive decay and slow decline in power output.
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- In the end, the theory of relativity was not overthrown, our understanding of physics is not flawed, the Pioneer anomaly was explained to people’s satisfaction, and, for now, the question has been put to rest.
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- But the story of the Pioneer anomaly shows how exacting measurements in science can generate new questions, challenge old ideas, and stimulate new ways to solve complex problems. Here is an even bigger problem to solve far beyond our Solar System
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- The Earth, solar system, the entire Milky Way and the few thousand galaxies closest to us move in a vast "bubble" that is 250 million light years in diameter, where the average density of matter is half as high as for the rest of the universe.
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- This is the hypothesis advanced by a theoretical physicist from the University of Geneva to solve a conundrum that has been splitting the scientific community for a decade: At what speed is the universe expanding?
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- Until now, at least two independent calculation methods have arrived at two values that are different by about 10% with a deviation that is statistically irreconcilable.
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- The universe has been expanding since the Big Bang occurred 13.8 billion years ago—a proposition first made by the Belgian canon and physicist Georges Lemaître (1894-1966), and first demonstrated by Edwin Hubble (1889-1953). The American astronomer discovered in 1929 that every galaxy is pulling away from us, and that the most distant galaxies are moving the most quickly.
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- This suggests that there was a time in the past when all the galaxies were located at the same spot, a time that can only correspond to the Big Bang. This research gave rise to the Hubble-Lemaître law, including the Hubble constant (H0), which denotes the universe's rate of expansion.
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- The best H0 estimates currently lie around 70 (km/s)/Mpc (meaning that the universe is expanding 70 kilometers a second more quickly every 3.26 million light years). The problem is that there are two conflicting methods of calculation.
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- The first is based on the cosmic microwave background: This is the microwave radiation that comes at us from everywhere, emitted at the time the universe became cold enough for light to be able to circulate freely (about 370,000 years after the Big Bang).
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- Using the precise data supplied by the Planck space mission, and given the fact that the universe is homogeneous and isotropic, a value of 67.4 is obtained for H0 using Einstein's theory of general relativity to run through the scenario.
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- The second calculation method is based on the supernovae that appear sporadically in distant galaxies. These very bright events provide the observer with highly precise distances, an approach that has made it possible to determine a value for H0 of 74.
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- These two values carried on becoming more precise for many years while remaining different from each other. It didn't take much to spark a scientific controversy and even to arouse the exciting hope that we were perhaps dealing with a 'new physics.
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- To narrow the gap the idea that the universe is not as homogeneous as claimed, a hypothesis that may seem obvious on relatively modest scales. There is no doubt that matter is distributed differently inside a galaxy than outside one. It is more difficult, however, to imagine fluctuations in the average density of matter calculated on volumes thousands of times larger than a galaxy.
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- If we were in a kind of gigantic 'bubble, where the density of matter was significantly lower than the known density for the entire universe, it would have consequences on the distances of supernovae and, ultimately, on determining H0.
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- All that would be needed would be for this "Hubble bubble" to be large enough to include the galaxy that serves as a reference for measuring distances. By establishing a diameter of 250 million light years for this bubble, the physicist calculated that if the density of matter inside was 50% lower than for the rest of the universe, a new value would be obtained for the Hubble constant, which would then agree with the one obtained using the cosmic microwave background.
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- The probability that there is such a fluctuation on this scale is one in 20 to one in 5, which means that it is not a theoretician's fantasy. There are a lot of regions like ours in the vast universe.
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- The best part of astronomy is learning something you don’t agree with. What are you missing? What more is there to learn? It is like picking up small pebbles of knowledge up on the beach with the whole ocean of the unknown extending over the horizon.
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- March 23, 2020 2677
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