Planetesimal Formation - The Stephen W. Hawking Center for Microgravity Research and Education

Planetesimals are the building blocks of planets. Asteroids and comets are leftover planetesimals from the time of formation of our own solar system. The formation of km-sized or larger planetesimals remains an open problem in planet formation theories. Once objects are larger than ~10 km, gravity helps these objects grow into planets. Condensation and electrostatic surface forces can explain the growth of mm to cm-sized objects in the nebula. These “pebbles” may grow through collisions or create local gravitational instabilities to form larger planetesimals. Both gravitational instability, which forms planetesimals directly through local collapse of patches in the disk, and pairwise accretional growth of particles face difficulties producing planetesimals in the protoplanetary disk environment as it is currently understood. It is possible that some combination of these processes took place, depending on the local conditions in the protoplanetary nebula. A major source of uncertainty in the accretional growth model is the behavior of small objects and aggregates of dust colliding at the low speeds expected (~0.1 – 10 m/s). We study these collisions and the formation of planetesimals through experiments and numerical simulations.

This image of the comet 67P/Churyomov-Gerasimenko was taken by the European Space Agency’s Rosetta mission. The comet is a leftover planetesimal from the origin of the solar system. Its ice sublimates when it nears the Sun. Objects such as this are the leftover building blocks of planets. Their formation is an active area of research. Credit: European Space Agency.

Experiments that focus on Planetesimal Formation include:

Snow Line Impact Mechanics is a project that George Hatcher has been working on in the lab. The snow line is the distance from the Sun where water vapor condenses into solid ice. Farther from the Sun than the snow line(currently between Mars and Jupiter) solid ice is abundant and may help in the accretion of smaller objects into the larger building blocks of planets…

TRACE is a low velocity collision experiment designed to characterize and understand the collisional properties of specific materials in microgravity. TRACE uses the Drop Tower platform to obtain near zero gravity and simulates accretion by colliding aggregates made of regolith simulants. Collisions take place at varying speeds and aggregate strengths and are recorded on a high speed camera…

COLLIDE-3 is a Suborbital Experiment that consists of one (1) Impactor Box System (IBS) contained within an aluminum vacuum chamber, a video recording system, and a logic unit. These components are all mounted to an aluminum baseplate. The experiment is performed with pre-determined impact parameters (target material, impactor mass, and impactor velocity)…

Nanorocks is a 1.5U Nanorack payload that flew to the ISS on Space-X 4 in early 2014. The experiment chamber consists of 8 sample cells containing different populations of mm-scale particles. The samples are periodically agitated to induce low-velocity collisions which are recorded on video.

Q-PACE (CubeSat Particle Aggregation and Collision Experiment) is a 3U CubeSat that will observe a set of 0.1 mm to cm-sized particles colliding at the very low speeds made possible by its microgravity environment in orbit around the Earth. Q-PACE is scheduled for the next launch of the Virgin Orbit LauncherOne rocket, targeting before the end of 2020…

CODA (Collisions of Dust Aggregates) is a drop tower experiment in which cm-scale particles are launched at each other at low speeds to study the effects of different particle types on collision outcomes. CODA experiments are run inside a vacuum chamber that falls in our laboratory-scale drop tower facility which provides 0.7 s of free-fall. This enables lower impact velocities than what we can achieve with the table-top COBRA experiment…

The Collisional Accretion Experiment (CATE) was designed to study the accretion of small particles onto a larger body in a vacuum, as well as in microgravity conditions. This experiment allows further insight into the particle interactions in proto-planetary disks and in planetary ring systems. A macroscopic target object is be released via a spring system on one end of an experiment tube and traverses a cloud of dust particles at a low speed…