Pallas, our solar system’s third largest and wholly unexplored asteroid, is the target for a potential SmallSat NASA flyby mission for possible launch in 2022. This remnant protoplanet, in fact, remains the largest unexplored planetary body inside the orbit of Neptune.
Dubbed Athena, this small satellite mission spacecraft would be about the size of a minifridge, but potentially have a payload of up to 400 lbs. It would share the same launcher as Psyche, NASA’s planned robotic mission to the asteroid 16 Psyche. However, after launch, both missions would go their separate ways and Athena would unfold its various bits and solar panels and begin its journey two-year journey to Pallas.
“Because Pallas is unexplored, it is a more compelling destination now for a small mission like Athena that is designed to pluck low-hanging [scientific] fruit,” Arizona State University planetary scientist Joseph O’Rourke, Athena’s principal investigator, told me.
Pallas is fascinating as an individual object, he says, but perhaps even more compelling as a missing link between the dwarf planet Ceres and Vesta , which after Ceres, is the second most massive body in the main asteroid belt.
“We’re competing against 11 other SmallSat and CubeSat mission proposals with selections by NASA for the next round of development anticipated in mid-April,” said O’Rourke. “I’m tortured by optimism.”
Discovered serendipitously in 1802 during observations of Ceres, Pallas has long been an odd protoplanetary duck. Aside from a somewhat elliptical path around the Sun, it has an orbital inclination of almost 35 degrees. That means its orbit takes it up and down through the plane of the ecliptic, the imaginary plane of Earth’s own orbit around the Sun.
Lying at roughly 3 AU (Earth-Sun distances), Pallas crosses the orbit of both Ceres and Vesta , says O’Rourke but all three are in similar regions in the main asteroid belt.
What’s the origin of its odd inclination?
UCLA planetary scientist Christopher Russell told me that the likely origin of its high inclination is a close encounter with a similarly sized or larger asteroid.
If Athena is funded, its journey with the Psyche spacecraft payload would take it less than two years to reach Pallas. O’Rourke says the idea is to begin science observations within a few days of Athena’s closest approach to Pallas.
“We’d track the spacecraft’s radio signals during the encounter but probably spend more than a month afterward downlinking images,” said O’Rourke who notes that communicating with Earth from the main asteroid belt is not a trivial task for a SmallSat.
But the proposed launch has fortuitous timing in two respects, says O’Rourke. We encounter Pallas as it crosses the ecliptic, he says, and we use a Mars gravity assist, stealing some orbital energy to slingshot ourselves into the asteroid belt.
With a diameter of some 512 km and gravity only about 1/40th that of Earth, Pallas is known to have formed in a water-rich environment. Thus, Athena’s science goals include determining if Pallas might have a water-rich mantle and crust and learning more about high-velocity impacts on its surface.
Pallas is also the parent of a large impact family, says O’Rourke. This includes many near-Earth asteroids and in particular (3200) Phaethon, a small near-Earth rocky asteroid that is the source of the annual Geminid meteor showers.
What do we know about Pallas?
“We don’t know much,” Julie Castillo-Rogez, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL), and an Athena proposal team member told me. She says Pallas looks like a lot of asteroids with a dark surface reflectivity and evidence for hydration (in which water is chemically combined with minerals). But Castillo-Rogez says it does not look like Ceres which displays evidence for ammonia and carbonates. So, Pallas likely does not come from the same region of the solar system as Ceres, she says.
“We can’t really tell where and when Pallas formed,” said Castillo-Rogez, who notes that the current state of understanding of the origin of water-rich versus water-poor bodies is that the former formed farther away from the Sun and slightly later than the water-poor bodies. How far and early is yet to be determined, she says.
But O’Rourke says he expects that Pallas will provide fascinating examples of planetary processes on a small body; including volcanism, surface geology, and internal geophysics.
As for its age?
It almost certainly formed within roughly the first 100 million years of solar system history, and possibly much earlier, says O’Rourke. In fact, he says, Pallas seems “intact,” meaning that it does not have the irregular shape expected if it broke off a larger body.
But Castillo-Rogez says that while there is no evidence of organics at Pallas, however, Pallas’ low albedo could be evidence for the presence of carbon in its regolith.
As for what’s most puzzling about Pallas?
Recent observations by the very large telescope show the presence of at least one bright spot on Pallas, says Castillo-Rogez. “On Ceres, bright spots have been characterized as salt-rich deposits,” she said. “High-resolution imaging should tell whether we’re dealing with salt deposits or something else.”
However, salt deposits would represent evidence for protracted interaction between water and rock, says Castillo-Rogez. That would mean that Pallas could have hosted liquid water for long enough to enable chemical reactions that would create minerals and organics. But Castillo-Rogez says if Pallas ever harbored something on the order of an ocean, it would have been small and short-lived.
What about sending an orbiter and lander to Pallas?
O’Rourke says a flyby is the first step in exploring Pallas. Future orbiting, landing and sample return missions are all possible, he says, but require progressively more complex and thus expensive spacecraft.
“First-order characterization and geologic mapping of Pallas by a small, innovative mission like Athena are vital to planning detailed follow-up measurements for a future mission,” said O’Rourke.