How did our Moon form? The leading hypothesis, the Giant Impact Theory, proposes that in the formative years of the Solar System, a Mars-sized protoplanet crashed into Earth. Debris from the collision, a mixture of material from both bodies, spun out into Earth orbit and coalesced into the Moon. Soon, this theory will be tested, perhaps answering the question of how our Moon was born. Two identical NASA spacecraft are preparing to enter areas in space known as the Lagrangian points where remains of this mystery protoplanet may be hiding. The spacecraft duo, called Solar Terrestrial Relations Observatory, or Stereo, will pass by the L4 and L5 points where the gravity of the sun and Earth combine to form gravitational wells where asteroids and space dust tend to gather.
During their journey, the two spacecraft will use a wide-field-of-view telescope to look for asteroids orbiting the region. Scientists will be able to identify if a dot of light is an asteroid because it will shift its position against stars in the background as it moves in its orbit.
The Giant Impact Theory explains many aspects of lunar geology including the size of the Moon’s core and the density and isotopic composition of moon rocks. A modification of the Giant Impact Theory is the “Theia hypothesis,” a brainchild of Princeton theorists Edward Belbruno and Richard Gott.
“About 4.5 billion years ago when the planets were still growing,” said Michael Kaiser, Stereo project scientist at Goddard Space Flight Center, “a hypothetical world called Theia may have been nudged out of L4 or L5 by the increasing gravity of other developing planets like Venus, sending it on a collision course with Earth. The resulting impact blasted the outer layers of Theia and Earth into orbit, which eventually coalesced under their own gravity to form the moon.”
The 18th-century mathematician Joseph-Louis Lagrange realized there were five such wells in the sun-Earth system. The twin probes are approaching L4 and L5.
“These points may hold small asteroids, which could be leftovers from a Mars-sized planet that formed billions of years ago,” said Kaiser.
The theory explains puzzling properties of the moon, such as its relatively small iron core. At the time of the giant impact, Theia and Earth would have been large enough to be molten, enabling heavier elements, like iron, to sink to the center to form their cores. An impact would have stripped away the outer layers of the two worlds, containing mostly lighter elements like silicon. The moon eventually formed from this material.
Stereo’s primary mission is to give three-dimensional views of space weather by observing the sun from the two points where the spacecraft are located. Images and other data are then combined for study and analysis. Space weather produces disturbances in electromagnetic fields on Earth that can induce extreme currents in wires, disrupting power lines and causing wide-spread blackouts. It also can affect communications and navigation systems. Space weather has been recognized as causing problems with new technology since the invention of the telegraph in the 19th century.