U.S. and Japanese Probes Discover Asteroids Are Full of Surprises

Two asteroid sample return missions are showing scientists that Earth-based observations do not tell the whole story about the nature of these small bodies in the solar system.  Scientists and engineers on the U.S.-led OSIRIS-REx and the Japanese-led Hayabusa2 missions are having to scramble to adjust their sample acquisition plans now that the probes are at their destinations and finding unexpected environments.  For OSIRIS-REx, that includes particles spewing off of the surface in plumes. The mission’s top scientist, Dante Lauretta, calls it one of the biggest surprises of his career.

Planetary scientists from around the world are showcasing their research at the annual Lunar and Planetary Science Conference (LPSC) taking place in The Woodlands, Texas, near Houston.  OSIRIS-REx and Hayabusa2 were two missions that were subject of briefings today and NASA held its own OSIRIS-REx media teleconference this afternoon.

OSIRIS-REx’s full name is Origins Spectral Interpretation Resource Identification Security – Regolith Explorer.  It arrived at the asteroid Bennu on December 3, 2018 and entered orbit on New Year’s Eve.  Lauretta, the mission’s Principal Investigator from the University of Arizona, explained today that just 6 days after entering orbit, they realized Bennu was not what they expected.  Instead it is one of a rare class of “active asteroids” and is ejecting plumes of particles from its surface.  Scientists do not yet know why this happens.  The particles can be seen as specks to the right of Bennu’s edge in this image from OSIRIS-REx.

Some the particles continue out into interplanetary space, some go into orbit around Bennu, and some rain down on the surface.

Lauretta exclaimed the “discovery of the plumes is one of the biggest surprises of my scientific career.”  The OSIRIS-REx team determined the particles do not pose a threat to the spacecraft or the mission.

The surface is also a surprise.   Instead of a generally smooth surface with perhaps a few boulders, the very small asteroid, about 500 meters (1,640 feet) in diameter, is covered in small, medium and large boulders.

That will complicate obtaining a sample for return to Earth. OSIRIS-REx is equipped with a Touch-and-Go Sample Acquisition Mechanism (TAGSAM).  As the name implies, the spacecraft will not land on the asteroid, but briefly touch it with the TAGSAM head, expel nitrogen gas to disturb the surface and cause fine particles to enter TAGSAM’s sample container, then ascend back into orbit.  It must stay away from boulders that could damage the spacecraft and collect fine particles that can be ingested into TAGSAM.

The original plan was to find a hazard-free site with a 25-meter (82-foot) radius, but that looks unlikely now.  Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center and Coralie Adam, OSIRIS-REx flight navigator with KinetX, Inc. expressed confidence that the navigation and operations teams will be able to more accurately target the descent and achieve a “Bullseye TAG” in the summer of 2020.

Adam said they are now able to navigate using surface characteristics and have demonstrated “meter-level navigation accuracy which is even better than the Earth-based GPS navigation on your smartphone.”  She added that “our predicted navigation performance has been 10 times better than what we expected before we arrived at Bennu.”

The surface of asteroid Ryugu is not what Japanese mission planners anticipated either.  Hayabusa2 arrived at Ryugu last summer and also discovered a surface strewn with boulders.  The Japan Aerospace Exploration Agency (JAXA) delayed its first sample acquisition attempt for several months while it studied data from the Hayabusa2 spacecraft and three small landers/hoppers it deployed onto the surface last fall.

JAXA made that first sample grab in February and believes it was a success, although the amount of material collected will not be known until the sample return canister is recovered back on Earth in December 2020.

JAXA is now getting ready for the next experiment. On April 5 (Japan Standard Time), Hayabusa2 will send a “kinetic impact” device down to Ryugu’s surface.  Basically it is an explosive that will create a 10-meter diameter crater to expose subsurface material.

JAXA’s Hayabusa2 project manager Yuichi Tsuda told LPSC that a second sample acquisition attempt will be made a few months after they assess the crater, but they no longer are planning a third.   Hayabusa2 will begin its trip back to Earth in November-December of this year and they will not have time for that third attempt.

Hayabusa2 is JAXA’s second asteroid sample return mission.  The first, Hayabusa, returned particles of the asteroid Itokawa in 2010.  OSIRIS-Rex is NASA’s first asteroid sample return mission, but it has sent or is planning to send other probes to asteroids and dwarf planets, a category that includes Pluto and Ceres.  Spacecraft also observe them from Earth orbit.

Scientists study asteroids because they hold clues to the formation of the solar system 4.5 billion years ago.  Entrepreneurs are interested in the possibility of someday mining asteroids for their raw materials.

Some asteroids also pose hazards to Earth.  When they enter Earth’s atmosphere they are called meteors or “shooting stars” or “fireballs,” and are not uncommon. Usually they are harmless, but a particularly large one exploded over Chelyabinsk, Russia, in 2013, raising awareness of the potential dangers.  Another large meteor entered over the Bering Sea on December 18, 2018, but received less attention because it was not over a populated area.  Based on images captured by Japan’s Himawari-8 meteorological satellite, media reports are calling it the third largest meteor explosion in modern times after Chelyabinsk and the historic Tunguska, Russia event in 1908.