Atlas V begins OSIRIS-REx’s round trip to the asteroid Bennu

by William Graham

NASA’s OSIRIS-REx spacecraft has began a seven-year round trip to the asteroid Bennu on Thursday, beginning its journey via a launch aboard the United Launch Alliance (ULA) Atlas V. Liftoff from SLC-41 at the Cape Canaveral Air Force Station was on time, at the start of a 115-minute window that opened at 19:05 local time (23:05 UTC).

Atlas V – OSIRIS-REx Mission:

The Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer, or OSIRIS-REx, is NASA’s first sample-return mission to an asteroid, and – excluding the Apollo program – its third sample return mission beyond Earth orbit after the Genesis and Stardust probes.

Japan’s Hayabusa spacecraft was the first mission to return an asteroid sample to Earth, completing operations in June 2010.

OSIRIS-REx is the third mission of NASA’s New Frontiers program, following the New Horizons spacecraft which flew past Pluto last year and Juno, which arrived in orbit of Jupiter in early July.

New Frontiers is a class of intermediate missions between the low-cost Discovery program and NASA’s flagship missions such as the Mars Science Laboratory.

2016-09-08-151100The OSIRIS-REx mission is expected to last seven years, culminating with the reentry of the spacecraft’s return capsule in September 2023.

About two and a half years of the mission will be spent at the asteroid (101955) Bennu, from August 2018 to March 2021, with sampling of the asteroid expected to occur in 2020.

Bennu was discovered on 11 September 1999 by scientists at the Lincoln Laboratory’s Near Earth Asteroid Research (LINEAR) program using the Experimental Test Site (ETS) telescope at White Sands, New Mexico. It was provisionally named 1999 RQ36.

An Apollo-class near Earth asteroid orbiting between 134 and 203 million kilometers (83.4 to 126 million miles; 0.897 to 1.36 astronomical units) from the Sun, Bennu has an orbital period of 437 days and a rotation period of 4.29 hours.

2016-09-08-151257The asteroid has a radius of approximately 246 meters (807 feet) and its mass has been estimated at between 60 and 77.6 billion kilograms (59 to 76 million Imperial tons; 66 to 86 million US tons).

Bennu is classified as a B-type asteroid, within the wider C-group of dark-coloured carbonaceous asteroids.

The B-type is believed to be a particularly primitive class of asteroid, dating back to the early Solar System, so studying and sampling it could provide scientists with a snapshot of conditions when the Solar System was forming.

The asteroid was selected as the destination for OSIRIS-REx due to its proximity to Earth, being sufficiently large and with a slow enough rotation rate to make sample collection practical and the scientific interest in its carbonaceous composition.

2016-09-08-151459The goals of OSIRIS-REx’s mission are to return a sample of Bennu to Earth for analysis, to conduct a spectral analysis of the asteroid’s surface and characterise its composition, to investigate changes in the asteroid’s orbit due to the absorption and reemission of radiation – a phenomenon known as the Yarkovsky effect – and to study the structure of the asteroid’s regolith.

Professor Dante Lauretta of the University of Arizona is the principal investigator for the OSIRIS-REx mission, while the mission is being managed by NASA’s Goddard Space Flight Center.

The spacecraft was constructed by Lockheed Martin Space Systems.

*Click here for additional articles on this spacecraft*

OSIRIS-REx will be powered by a pair of solar arrays, generating up to three kilowatts of power at perihelion. Unfuelled, the spacecraft has a mass of 880 kilograms (1,940 pounds) and at launch it will weigh 2,110 kg (4,650 lb). The sample return capsule – the only part of the spacecraft designed to return to Earth – has a mass of 46 kilograms (100 lb).

2016-09-08-151355As a sample-return mission, OSIRIS-REx’s primary objective is to collect a sample of Bennu and deliver it to Earth. The Touch-and-Go Sample Acquisition Mechanism, or TAGSAM, will be used to achieve the sampling part of the objective, while a reentry capsule will facilitate its safe return to Earth.

TAGSAM consists of a sampler mounted on an extensible 3.35-meter (11-foot) arm. OSIRIS-REx will manoeuvre close to the asteroid without landing and deploy the arm to bring the sampler into contact with the surface. This will use a nitrogen jet to loosen surface material for collection. The spacecraft is carrying sufficient nitrogen to attempt this process three times if an insufficient amount of material is initially captured. At least sixty grams (2.1 ounces) of material is expected to be collected.

2016-09-08-151550Once sampling is complete, the sampler head will be stowed within OSIRIS-REx’s return capsule and detached from the arm.

The return capsule will remain attached until OSIRIS-REx makes its final approach to Earth in September 2023, at which point it will separate with the main spacecraft bus subsequently performing a course correction to avoid also entering the atmosphere.

The sample capsule will use an ablative heat shield to protect it as it reenters the atmosphere, descending under a parachute to a landing in Utah. Following landing, the capsule will be taken to the Johnson Space Center’s Astromaterials Acquisition and Curation Office for the study of its contents.

2016-09-08-151627A suite of instruments aboard OSIRIS-REx will aid in the location of a sample collection site, provide for additional study of the asteroid and give context to the samples returned.

The OSIRIS-REx Camera Suite, or OCAMS, consists of three imaging payloads; the 20 cm (8-inch) PolyCam will be used for high-resolution imaging, reaching resolutions of up to one centimeter (0.4 in) while the spacecraft is scouting for a site from which to take its sample.

MapCam will be used to produce high-resolution imagery of the asteroid’s surface and to search its proximity for satellite asteroids, while SamCam will observe the sample collection phase of the mission.

2016-09-08-151658The OSIRIS-REx Laser Altimeter (OLA) will use LIDAR to produce a three-dimensional map of the asteroid’s surface and to characterise its topography. Higher-resolution surveys will be conducted of candidate sample collection sites ahead of the sampling attempt.

The OSIRIS-REx Thermal Emission Spectrometer, or OTIS, will produce spectra for thermal emissions from Bennu, allowing for minerals present in the surface to be characterised. The OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) will analyse the light reflected by the asteroid – providing reflection spectra which can also be used to identify minerals present in the asteroid’s surface.

The Regolith X-Ray Imaging System, or REXIS, will use a technique called X-ray fluorescence spectroscopy to produce additional surface composition data.

By studying the wavelengths of secondary x-rays emitted when the asteroid’s surface is bombarded with incident high-energy radiation, the instrument will characterise the elemental composition of the regolith. This research will complement the mineral composition determined by the OTIS and OVIRS instruments.

Scientists also plan to study the Doppler shift affecting radio communication between OSIRIS-REx and Earth, accurately determining the spacecraft’s orbit and allowing Bennu’s mass to be inferred and gravitational field to be characterised.

2016-09-08-152111OSIRIS-REx began its mission atop United Launch Alliance’s Atlas V rocket, which was making its sixty-fifth flight since its 2002 debut.

The vehicle that launched OSIRIS-REx, tail number AV-067, flew in the Atlas V 411 configuration.

Like all Atlas V versions, this consists of a Common Core Booster (CCB) first stage powered by an RD-180 engine burning RP-1 propellant and liquid oxygen, with a Centaur second stage which uses cryogenic propellants; liquid hydrogen and liquid oxygen.

In this configuration, the rocket makes use of the single-engine version of Centaur, with one RL10C-1 providing thrust. The single-engine Centaur was introduced with the Atlas III in 2000 as an alternative to the twin-engine configuration the stage had used since the 1960s, and quickly became the standard.

2016-01-06-212720All Atlas V launches to date have used the single-engine model, although Boeing’s CST-100 Starliner – slated to begin launching late next year – will use a dual-engine Centaur.

At liftoff, a single AJ-60A solid rocket motor ignited to provide additional thrust in the early stages of first stage flight.

Thrust vectoring allows the rocket to remain under control while flying in this asymmetric configuration, which was used for the fourth time with Thursday’s launch.

At the nose of the Atlas, OSIRIS-REx was encapsulated within a four-meter (13-foot) Large Payload Fairing – at 12 meters (39 feet) in length this is the shortest of three available four-meter fairings.

2016-09-08-152336The Atlas V has been chosen to launch a number of NASA’s exploration missions, beginning with the Mars Reconnaissance Orbiter (MRO) in August 2005 and the New Horizons mission to Pluto in January 2006.

Other planetary missions launched by the Atlas V have included the Lunar Reconnaissance Orbiter (LRO) and LCROSS in June 2009, the Juno mission to Jupiter and the Curiosity and MAVEN Mars probes.

Atlas V was developed by Lockheed Martin to compete for the US Air Force’s Evolved Expendable Launch Vehicle (EELV) program, which saw it developed alongside Boeing’s Delta IV.

Production and operations of both vehicles – along with Boeing’s older Delta II rocket – was taken over by United Launch Alliance upon its formation in December 2006. ULA is currently in the process of phasing out the Delta IV and developing a new rocket, Vulcan, to replace both EELVs.

2016-09-08-152404Thursday’s launch began with ignition of the first stage’s RD-180 engine 2.7 seconds in advance of the planned T-0 mark in the countdown. Ignition of the solid rocket motor and liftoff occurred 1.1 seconds after the zero mark.

Ascending from Cape Canaveral’s Space Launch Complex 41 (SLC-41), AV-067 began a series of pitch and yaw manoeuvres after 6.7 seconds of flight to establish an 89-degree launch azimuth taking it almost due East over the Atlantic Ocean.

The vehicle reached Mach 1 – the speed of sound – 56.9 seconds after launch before passing through the area of maximum dynamic pressure, or Max-Q, 11.7 seconds later.

The AJ-60A booster is designed to burn for around ninety seconds, after which its thrust tailed off. Its spent casing remained attached until the two-minute, nineteen-second mark when conditions were optimal for its jettison.

2016-09-08-152441The first stage burned for four minutes and 2.8 seconds before shutting down its RD-180 engine, an event designated Booster Engine Cutoff (BECO).

Six seconds after BECO the spent Common Core Booster separated and the Centaur began its prestart sequence, culminating in ignition – designated Main Engine Start 1 (MES-1) ten seconds after staging.

Centaur’s first burn lasted eight minutes and 3.7 seconds, establishing an initial parking orbit. Following the conclusion of the burn at Main Engine Cutoff 1 (MECO-1) the Centaur coasted for 21 minutes and 25.7 seconds.

After its coast phase concluded, Centaur restarted for a second burn. Lasting six minutes and 50.4 seconds, this burn propelled OSIRIS-REx to Earth escape velocity and into heliocentric orbit.

2016-09-08-152540Spacecraft separation occurred a quarter of an hour after the end of the second burn, at 55 minutes and 38.6 seconds elapsed time, over the Indian Ocean.

Centaur underwent blowdown and safing 26 minutes and 20 seconds after separation, with the Atlas’ mission concluding one hour, 56 minutes and 58.6 seconds after liftoff.

Following launch, OSIRIS-REx will perform a series of manoeuvres as it journeys towards the asteroid Bennu. This will include a flyby of Earth which is scheduled to occur on 22 September next year in order to gain a gravity assist.

The spacecraft is currently slated to arrive at Bennu in August 2018, remaining there until March 2021. The sample return capsule is expected to return to Earth on 24 September 2023 while the parent spacecraft will remain in orbit around the Sun.

2016-09-08-152000The OSIRIS-REx launch is the first from Cape Canaveral after the explosion of a SpaceX Falcon 9 rocket during fuelling for a static fire test on 1 September.

The explosion occurred at Space Launch Complex 40 (SLC-40), adjacent to Atlas’ SLC-41. Although they have always been numbered separately, the two pads were both originally part of the Titan Integrate-Transfer-Launch (ITL) complex along with a shared processing facility which has since been demolished.

At the time of the explosion OSIRIS-REx was already atop the Atlas V and enclosed within its payload fairing, having arrived at the Vertical Integration Facility (VIF) on 29 August.

The Atlas was inside the VIF when the Falcon explosion occurred a little over a mile away. Despite this, NASA and ULA are confident that the payload and rocket suffered no damage and the Atlas launch has continued on schedule.

Thursday’s launch is the fifth Atlas V launch of 2016 and the eighth of the year for United Launch Alliance (ULA), who have also launched three Delta IV rockets.

ULA’s next launch is currently scheduled for 16 September with another Atlas carrying the commercial Worldview 4 imaging satellite into orbit. That mission will launch from Vandenberg Air Force Base in California.

(Images: NASA, ULA, Lockheed Martin and L2 – including render from L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)

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