How To Get To Bennu and Back

OSIRIS-REx is a mission to visit asteroid Bennu, pluck a sample from its surface, and bring it back to Earth. It is a journey across the vast emptiness of space, spanning over seven years. The journey will be made by a spacecraft travelling at enormous speeds across the Solar System, capable of performing precision maneuvers that result in gentle contact with the asteroid surface, and culminating in the safe landing of a sample return capsule in the Utah desert.Image

How does Bennu orbit the Sun?

Bennu is on an Earth-crossing orbit that circles the Sun every 1.2 years with an orbital velocity over 28 km/s (almost 63,000 miles per hour – mph). Bennu approaches the Earth every six years. At the time of its discovery in 1999 Bennu came within 2.2 million kilometers (1.4 million miles) of the Earth (~5.7 times the average distance to the Moon). The closest approach that we can predict occurs in 2135 when Bennu will fly 300,000 km (186,000 miles) over the surface of the Earth, well inside the orbit of the Moon.

Bennu also gets very far away from the Earth. In a single orbit Bennu travels over a billion kilometers (more than 620 million miles) around the Sun. At its furthest point it is over 340 million kilometers (211 million miles) away from the Earth. When OSIRIS-REx is at Bennu it will take over 18 minutes for a signal transmitted from the Earth to reach the spacecraft (and another 18 minutes for the spacecraft signal to be transmitted back to Earth).

How will we get to Bennu?

In order to rendezvous with Bennu, OSIRIS-REx will leave the surface of the Earth on an Atlas V rocket in the 411 configuration. The Atlas V rocket uses a Russian-built RD-180 engine burning kerosene and liquid oxygen to power its first stage and an American-built RL10 engine burning liquid hydrogen and liquid oxygen to power its Centaur upper stage. The 411 configuration adds a single strap-on solid booster rocket to the first stage.

The Atlas V rocket will launch OSIRIS-REx with a hyperbolic escape velocity of 5.4 km/s (over 12,000 mph). Once in space, OSIRIS-REx will perform a series of Deep Space Maneuvers, changing velocity by another 0.52 km/s (1,163 mph).  These maneuvers, combined with an Earth-gravity assist one year after launch, will send OSIRIS-REx on a trajectory to rendezvous with Bennu.

Asteroid rendezvous requires not only being in the same place at the same time but also moving at the same speed and in the same direction. OSIRIS-REx will perform a series of braking maneuvers, slowing down by 0.53 km/s (1,186 mph), resulting in a relative approach velocity of 20 cm/s (~0.45 mph).

OSIRIS-REx is basically formation flying with Bennu during the asteroid encounter. The total change in velocity is just over 20 cm/s (0.45 mph), miniscule compared to the large maneuvers required to arrive and depart from the asteroid vicinity. The encounter culminates in the touch-and-go maneuver when OSIRIS-REx contacts the surface of Bennu for a total of five seconds, at a velocity of 10 cm/s (0.22 mph), and collects the sample.

How will we get home?

The window for departing Bennu opens in March 2021. At this time OSIRIS-REx will fire the main engines and leave Bennu with a speed of 0.32 km/s (716 mph). This burn will place OSIRIS-REx on a ballistic trajectory that intersects the orbit of the Earth in September 2023. Four hours before atmospheric entry OSIRIS-REx will jettison the sample return capsule then perform a deflection maneuver of 17.5 m/s (39 mph) that places the spacecraft on a stable orbit around the Sun. The sample return capsule will hit the top of the atmosphere with a speed of 12.4 km/s (27,738 mph). The heat shield removes over 99% of the initial kinetic energy. After entry the SRC will free-fall until it reaches an altitude of ~3 km (1.9 miles), at which point the parachute will deploy, bringing the capsule in for a soft landing in the Utah desert, seven years after launching on the journey to Bennu and back.


  1. […] describe the journey to Bennu, including details on the speeds involved in my blog post How to Get to Bennu and Back. We will leave Earth with a hyperbolic escape velocity of 5.4 km/s (over 12,000 mph) on an Atlas V […]

  2. […] at the launch vehicle interface of approximately 61,000 pounds. This large force is the result of accelerating the spacecraft up to 6 “Gs”, or six times the acceleration due to gravity at the surface of the Earth. At the […]

  3. […] is seven-year mission to return samples from Bennu. This timeline is broken into three distinct segments. The first two […]

  4. […] Sample Return Mission. Our objective is to launch a spacecraft from the surface of the Earth, rendezvous with a near-Earth asteroid named Bennu, contact Bennu’s surface for five seconds to grab a handful of rocks, and return those precious […]

  5. […] flies past Earth every 6 years with a chance to impact the planet in the distant future.  We’ll travel over 660 million miles (1060 million km) to rendezvous with this space rock.  And once there, once we’ve characterized […]

  6. […] your chance: the space agency will be sending social network communications from users on its round-trip mission[2] to collect samples from the asteroid […]

  7. […] your chance: a space group will be promulgation amicable network communications from users on a round-trip mission to collect samples from a asteroid […]

  8. […] the launch loads for our baseline mission design, which defines the minimum amount of fuel needed to get to Bennu and back. In addition, we made sure to test the structure for the loads associated with a full fuel tank, […]

  9. […] Early in mission design, we looked at a variety of propulsion systems, including monopropellant, bipropellant, and solar-electric propulsion. Bipropellant systems, such as flown on Juno, add an oxidizing component to produce a more vigorous chemical reaction and hence more thrust. For Juno, the main propulsion system uses hydrazine as fuel and nitrogen tetroxide as an oxidizer. Solar electric propulsion systems, which are used on the Dawn and Hayabusa-2 missions, ionize the rare gas xenon and accelerate it through an electric field to gain thrust. Fortunately for us, our target asteroid Bennu has an orbit that makes it highly accessible. This feature allowed us to select the simplest, most reliable propulsion system to get to Bennu and back. […]

  10. […] The mural depicts the Egyptian god Osiris from whom the mission gets its name. It also includes multiple views of the spacecraft, the asteroid, and the round-trip journey to Bennu and back. […]

  11. […] Our special study focuses on a careful balance of four factors: the dry mass of the spacecraft, the wet mass of the spacecraft, the launch vehicle capability, and the delta-V budget of the mission. The dry mass of the spacecraft is the total mass of the OSIRIS-REx spacecraft when the propellant tanks are empty. The wet mass is the total mass once the hydrazine propellant is loaded into the fuel tank. The launch vehicle capability is determined by the total amount of thrust produced from the rocket based on our required hyperbolic escape velocity (5.41 km/s). The delta-V budget summarizes the total amount of energy needed to get to Bennu and back. […]

  12. […] Launch site operations for the spacecraft and instruments occur at the Kennedy Space Center. These operations including monitoring flight system health and performing required spacecraft fueling and maintenance operations. Once the spacecraft is fully fueled, we encapsulate it into the launch vehicle fairing then stack the entire assembly on top of the first and second stages of the Atlas V rocket. These activities lead up to our big day – when we fire up the main engines on the rocket and send OSIRIS-REx on his journey to Bennu and back! […]

  13. […] September 2015 marked an important milestone for the OSIRIS-REx mission. We are now less than one year to launch and only eight months away from shipping the spacecraft down to Kennedy Space Center for integration with the Atlas V launch vehicle. As I recently reported, the spacecraft is in great shape and is almost complete. Next month, we begin the “test” phase of the Assembly, Test, and Launch Operations portion of our development. Also in October, I will be visiting the United Launch Alliance facility in Decatur, Alabama to meet the team working on our rocket and get up close and personal with the vehicle that will send OSIRIS-REx on his journey to Bennu. […]

  14. […] The OSIRIS-REx project continues to progress seamlessly through the Assembly, Test, and Launch Operations (ATLO) phase. We are now done with the AT portion of this phase and have started LO. The test phase ended with the completion of the last major environmental test: the thermal vacuum test. With the test program complete, the team made the transition to the launch site at Kennedy Space Center. I was privileged to join the team in transporting the spacecraft to the launch site. I participated with great pride as we shipped our spacecraft halfway across the country on the first step of the journey to Bennu and Back. […]

  15. […] in two separate burns which will accelerate the spacecraft to a hyperbolic escape velocity of 5.4 kilometers per second (over 12,000 miles per […]

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