Lucy and Psyche – Why Go to Asteroids?

Some 3.2 million years ago, a female hominin lived in the Awash Valley in Ethiopia.  Her fossilized skeleton was discovered in 1974 and she was given the name Lucy, after the Beatles song Lucy in the Sky with Diamonds, which had played repeatedly during the expedition.  The discovery of Lucy has done much to increase our understanding of the origins of the human race.  The new NASA mission, named after Lucy, is anticipated to increase our understanding of the origins of the Solar System.  That, along with the Psyche mission, will explore asteroids as part of NASA’s Discovery program.

Lucy reconstruction. Credit: Wiki Commons/Natural History Museum, Washington, D.C.

The Discovery program was initiated in 1992 as part of an effort to provide “faster, better, cheaper” missions.  This was in response to funding cuts in the early 90’s as part of an overall effort that eventually balanced the federal budget.  These missions typically cost around $400-500 million as opposed to Flagship missions such as Cassini which cost $1.5 billion.  Prior Discovery missions have included Mars Pathfinder, the Messenger mission to Mercury, and the Kepler mission that unearthed thousands of exoplanets.  The initial Discovery mission was NEAR, which was the first to land on an asteroid.  With Lucy and Psyche, the Discovery program is returning to its roots.

In grade school, when we learn about the Solar System, we are typically presented with a model of the planets with the main asteroid belt located between Mars and Jupiter.  Lucy will flyby one asteroid in the main belt but then move on to examine six Jupiter Trojan asteroids.  These asteroids are in the same orbit as Jupiter but are situated in the L4 and L5 Lagrange points.  These are located 60 degrees behind and ahead of Jupiter in its orbit.  At these points, the gravitational tug from the Sun and Jupiter are equal and stable.  That is, if an asteroid moves away from this point, the resultant gravitational forces will tug it back in.  This is why these regions serve as collection points for asteroids.

Trojan asteroids lead and trail Jupiter in the L4 and L5 Lagrange points. Credit: NASA/LPI

Jupiter is not the only planet that shares its orbit with Trojan asteroids.  Mars and Neptune have been discovered to have these, along with Earth in 2011.

Why study asteroids?  These objects are remnants, or if you will, fossils from the formation of the Solar System 4.6 billion years ago.  Chunks of asteroids make their way to Earth in the form of meteorites, but these become contaminated by the Earth’s atmosphere which can compromise the data received.  Going directly to the source provides a pristine environment to examine the primordial Solar System.  Lucy’s complex trajectory will enable visits to both the L4 and L5 asteroid swarms and to various asteroid types.

Lucy trajectory from launch in 2021 to final asteroid visit in 2033. Credit: SWRI

Lucy will visit C, D, and P type asteroids.  These types are broken down as follows:

C type or carbonaceous – as the name suggests, these asteroids are thought to be rich in carbons.  About 75% of known asteroids fall into this type.  These asteroids reside in the outer main asteroid belt and beyond.

D type – rare in the main belt and mostly found among the Jupiter Trojans, these asteroids are very dark and reddish in color.  The red color could indicate the presence of organic material.

P type – located in the outer main asteroid belt and beyond, these are among the darkest objects in the Solar System.  Even redder than D types, it is thought P type asteroids are very rich in organic material.

The reddish tint to these asteroids are thought to be caused by an organic material referred to as tholins.  This substance is common in the outer Solar System but cannot exist on Earth as oxygen breaks it down.  However, when the Earth was formed the atmosphere was mostly carbon dioxide.  Oxygen did not come on the scene until plant life developed to produce it via photosynthesis.  Tholins could have been present on Earth in its early history and played a role in the formation of life as it breaks down into amino acids in water.  Understanding how the Trojans formed could help us understand how organic material was delivered to Earth to bring about life.

Pluto’s moon Charon. The red spot at the pole is caused by methane gas escaping Pluto then trapped by Charon and converted to tholin via chemical reactions sparked by UV radiation. Credit: NASA/JHUAPL/SwRI

The origins of the Trojan asteroids are unclear.  They may have formed near where they are now, or they may be Kuiper Belt objects ejected towards Jupiter as it migrated after the formation of the Solar System.  It is hoped Lucy can solve this part of the puzzle in the early Solar System’s history.  The instrument package on Lucy will record surface composition and geological characteristics.  While Lucy will go to the Jupiter Trojans, Psyche, named after its target, will travel to the more familiar asteroid belt.

Psyche is unique in that, rather than consisting of rocky material, appears to be the left over metallic core from a protoplanet.  The core was exposed by a series of impacts cracking open the outer layers.  The Psyche mission will allow us to take a look at a planetary core that we cannot do here on Earth.  The metallic cores of planets are formed by the process of differentiation.

When a planet is being formed, heating processes caused by gravitational compression, impact events, and radioactive decay allows for melting and differentiation between heavier and lighter elements.  The heavy iron and nickel sinks to the core while lighter silicates rise to the surface.  On Earth, this has led to four distinct inner layers, the inner core, the outer core, the mantle, and crust as seen below.

Credit: JPL

What we know about the Earth below the crust is through study of seismic waves.  The deepest hole drilled into the crust was the 7.5 mile Kola Superdeep Borehole started by the Soviet Union in 1970 and abandoned in 1992.  This was not even halfway through the crust.  Eventually, as one digs deeper, the temperature rises to the point where drilling equipment is damaged.  As we cannot get anywhere close to the mantle yet, much less the core, this is where the Psyche mission comes in.

The Psyche asteroid is thought to be the left over core from a Mars sized protoplanet.  It was a Mars size protoplanet that crashed into the Earth shortly after its formation creating the Moon.  Psyche is scheduled to launch in 2023, flyby Earth in 2024 and Mars in 2025 for a gravity assist to bump up its velocity to reach the asteroid in 2030.  Then, the mission will provided planetary scientists the first look into what Earth’s core might look like if we could venture to the center of the planet.

The Lucy and Psyche missions will hopefully, allow us to learn about the origins of the Solar System, Earth, and thus, humanity.

*Image atop post is artist rendition of Lucy (left) and Psyche (right) missions.  Credit:  SwRI and SSL/Peter Rubin.

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