NASA Spacecraft Refines Course Toward Big Asteroid Bennu

Space.com ^ | July 6, 2018 07:40am ET | Mike Wall,

[BenLurkin]

The OSIRIS-REx spacecraft aced its second major deep-space maneuver last Thursday (June 28), firing its engines to change its velocity by 37 mph (60 km/h), mission team members have confirmed.

"The thruster burn put the spacecraft on course for a series of asteroid-approach maneuvers to be executed this fall that will culminate with the spacecraft's scheduled arrival at asteroid Bennu on Dec. 3," NASA officials wrote in a statement Tuesday (July 3).

...

This material will make it to Earth in a special return capsule in September 2023. Scientists in labs around the world can then study the sample in detail, gathering data that should reveal clues about the solar system's early days and the role that space rocks such as Bennu may have played in delivering life's building blocks to Earth.

OSIRIS-REx isn't the only asteroid-sampling craft plying the heavens; Japan's Hayabusa2 probe arrived at the 3,000-foot-wide (900 m) asteroid Ryugu last week. Hayabusa2 is scheduled to grab its sample in December 2019, and the material should touch down here on Earth a year later

[Telepathic Intruder]

They’re probably just ordinary rocks.

[fieldmarshaldj]

*ping*

[ProtectOurFreedom]

But very special rocks...

But just how did the OSIRIS-REx team choose Bennu from the over 500,000 known asteroids in the Solar System?

Proximity to Earth
The closest asteroids to Earth are called Near-Earth Objects (NEOs). As the name suggests, NEOs are objects that orbit within 1.3 AU of the Sun. (1 AU = the distance between Earth and the Sun, or ~93 million miles) For a sample return mission like OSIRIS-REx, the most accessible asteroids for a spacecraft to reach are located between 1.6 AU and 0.8 AU. The ideal asteroid has an Earth-like orbit with low eccentricity and inclination. At the time of the mission’s asteroid selection in 2008, there were over 7,000 known NEOs, but only 192 had orbits that met these criteria.

Size
Asteroids with small diameters rotate more rapidly than those with large diameters. With a diameter less than 200 meters, an asteroid spins so rapidly that the loose material on its surface (regolith) can be ejected from it. The ideal asteroid has a diameter larger than 200 m so that a spacecraft can safely come into contact with it and collect a sufficient regolith sample. This size requirement reduced the number of candidate asteroids from 192 to 26.

Composition
Asteroids are divided into different types based on their chemical composition. The most primitive asteroids are carbon-rich and have not significantly changed since they formed nearly 4 billion years ago. These asteroids contain organic molecules, volatiles, and amino acids that may have been the precursors to life on Earth. Of the 26 asteroids left on the list, only 12 had a known composition, and only 5 were primitive and carbon-rich.

From these 5 asteroids, Bennu was selected. Bennu is a B-type asteroid with a ~500 meter diameter. It completes an orbit around the Sun every 436.604 days (1.2 years) and every 6 years comes very close to Earth, within 0.002 AU. These close encounters give Bennu a high probability of impacting Earth in the late 22nd century. Bennu’s size, primitive composition, and potentially hazardous orbit make it one of the most fascinating and accessible NEOs … and the ideal OSIRIS-REx target asteroid.

[alexander_busek]

"These close encounters give Bennu a high probability of impacting Earth in the late 22nd century."

It has a cumulative 1-in-2,700 chance of impacting Earth between 2175–2199.

- "101955 1999 RQ36: Earth Impact Risk Summary". NASA. Jet Propulsion Laboratory. 25 March 2016. Retrieved 20 March 2018.

I wouldn't call that a "high" probability.

Regards,

[SunkenCiv]

Thanks fieldmarshaldj.