Pioneering spacecraft from NASA and the Japanese space agency promise to reveal two unexplored asteroids later this year, officials said Wednesday, beginning surveys that will culminate in daring descents to capture samples for return to Earth, where eager scientists await a hands-on look at the specimens.
NASA’s OSIRIS-REx spacecraft and the Japan Aerospace Exploration Agency’s Hayabusa 2 mission are in the final months of their journeys to asteroids Bennu and Ryugu, two rocky worlds that to date have only been studied through the lenses of telescopes.
The Hayabusa 2 and OSIRIS-REx missions were developed, built and launched separately aboard Japanese H-2A and United Launch Alliance Atlas 5 rockets in December 2014 and September 2016, but they share many of the same objectives, and scientists intend to collaborate on their results.
The samples will tell scientists more about how the solar system formed, how planets grew and evolved, and perhaps how life took hold on Earth.
The Japanese probe is on track to arrive at its destination, asteroid Ryugu, by early July to kick off a year-and-a-half of up-close exploration and mapping, culminating in three touch-and-go manoeuvres to pick up rock and dust from the asteroid, including one sample excavated from beneath Ryugu’s surface.
Hayabusa 2 also carries four robotic landers — three built in Japan and the French-German Mobile Asteroid Surface Scout, or MASCOT — that will hop across Ryugu to multiple locations, returning imagery and other scientific measurements.
The biggest of the landing drones, the European-supplied shoebox-sized MASCOT robot, is set to make its descent in September or October after deployment from the Hayabusa 2 mothership.
Since its launch from Japan’s Tanegashima Space Center in 2014, Hayabusa 2 has fired its ion engines for two multi-month campaigns to target Ryugu. The low-power thrusters burn xenon gas to produce minuscule levels of thrust, but they can operate for months at a time, reshaping Hayabusa 2’s trajectory.
Hayabusa 2 zipped by Earth in December 2015, using the planet’s gravity for an extra kick toward Ryugu’s orbit. The asteroid takes an elongated path around the solar system, and it typically is located farther from the Sun than the Earth. Ryugu briefly dips inside Earth’s orbit on each 1.3-year lap around the Sun, making it a potentially hazardous asteroid in our celestial neighborhood.
The Japanese asteroid probe ignited its ion propulsion system again Jan. 10 for the final major maneuver before arriving at Ryugu, according to Yuichi Tsuda, JAXA’s project manager for Hayabusa 2.
The spacecraft will continue thrusting until June 5, when it is positioned around 4,000 miles (about 6,000 kilometres) from Ryugu, then the probe’s conventional hydrazine-fueled rocket jets will fine-tune its approach.
“We are just six months before our arrival, and we have just started the final ion engine burn a week ago, and that is the most critical one,” Tsuda said Wednesday in a presentation to NASA’s Small Bodies Assessment Group, a community of scientists who specialize in asteroid and comet research.
Hayabusa 2’s camera should begin tracking Ryugu in May to help controllers navigate the craft toward its target, according to Tsuda.
The roughly $300 million Hayabusa 2 mission is a follow-up to JAXA’s Hayabusa spacecraft, which launched in 2003 and arrived at asteroid Itokawa in 2005. Despite technical difficulties, which prevented Hayabusa from collecting the mission’s planned specimens, the hard-luck spacecraft returned a canister to Earth in 2010 with microscopic samples plucked from Itokawa’s surface.
Asteroid Ryugu has never been visited by a spacecraft before, but long-distance telescopic images indicate the object has a diameter of approximately 2,950 feet (900 metres), with a gravitational field 60,000 times weaker than Earth’s.
Because of the tenuous gravity, Hayabusa 2’s approach and landings on Ryugu will be more akin to a rendezvous with another spacecraft than landing on another planet.
Ground-based observations suggest Ryugu is near-spherical in shape, and scientists classify it as a C-type asteroid, a type of object that is primarily made up of carbon and appear almost as black as coal.
Hayabusa 2 will initially park itself 20 kilometres, or about 12 miles, from the asteroid for a comprehensive survey with a set of spectrometers, cameras, and other sensors to map the tiny world.
Tsuda said the probe is scheduled to arrive at the 12-mile hold point from Ryugu between June 21 and July 5, depending on tracking and navigation during the final approach.
“We do not know much about Ryugu, so the asteroid operations schedule is strongly dependent on the rotation state, thermal distribution and chemical distribution, and shape properties of the asteroid,” Tsuda said.
“We’ll start by lowering the altitude to 5 kilometres (3 miles) for higher-resolution (images), and then between September and October, we will try to deploy two rovers, MINERVA 2 and MASCOT.”
The battery-powered MASCOT lander was developed by the same French-German team that led the Philae mission to touch down on comet 67P/Churyumov-Gerasimenko after release from Europe’s Rosetta spacecraft in November 2014. MASCOT’s flight plan calls for it to return data for about 18 hours after deployment from Hayabusa 2.
Three touch-and-go maneuvers are planned by Hayabusa 2 in October, February 2019 and April or May of 2019. The last descent will try to scoop up underground material from Ryugu.
“Because this is the final year before arrival, we have done extensive practice to be ready for the arrival and proximity operations,” Tsuda said.
Hayabusa 2’s team conducted operations training and a landing site selection simulation to rehearse how scientists and engineers will coordinate with each other during mission’s action-packed stay at Ryugu.
Japanese scientists say Hayabusa 2 will collect at least one gramme of material from Ryugu.
Another rover will be deployed from Hayabusa 2 for a landing attempt in mid-2019.
In December 2019, Hayabusa 2 will boost itself away from the asteroid. Its sample return canister will be ejected for a high-speed re-entry through Earth’s atmosphere and a landing in the Australian outback in December 2020.
OSIRIS-REx on course for Bennu
Meanwhile, NASA’s $1 billion OSIRIS-REx mission is on the final leg of its two-year journey to asteroid Bennu, a miniature world about 1,600 feet (500 metres) across. Bennu is a rare kind of object — scientists know it as a “B-type” asteroid — and is likely loaded with lots of carbon, the backbone of organic molecules.
In August, the solar-powered voyager will begin its final approach to the asteroid, and eventually slip into orbit. After a thorough survey of Bennu with OSIRIS-REx’s spectrometers, cameras and laser altimeter, scientists will decide where to snag a sample.
Dante Lauretta, OSIRIS-REx’s principal investigator, said Wednesday that the mission is ready for prime time at Bennu.
“OSIRIS-REx has been in flight for about 16 months now,” he said at the semi-annual SBAG meeting held at NASA’s Ames Research Center in California. “We’ve had a very productive outbound cruise so far.”
The spacecraft got a gravitational boost from Earth in September, following a flyby trajectory similar to the one used by Hayabusa 2 in 2015.
Like asteroid Ryugu, Bennu spends most of its time outside of Earth’s orbit. But it crosses Earth’s orbital path twice on each 1.2-year circuit of the sun, making it a low-risk impact hazard.
Lauretta said OSIRIS-REx is healthy on the final months of its trip to Bennu, with engineers only monitoring a minor concern with outgassing coming from the craft’s sample return canister heat shield and backshell.
Ground controllers detected a small amount of unexpected thrust coming from the sample carrier last year, and engineers determined the force was caused by outgassing of water absorbed by the sample return capsule during launch preparations in Florida. Water retention is common on spacecraft being prepared for launch, officials said.
“While this small thrust would not be a problem for other missions, the gravity at the target asteroid Bennu is low enough that even this small amount of thrust could make orbital operations more difficult for OSIRIS-REx,” mission officials wrote in a status update.
Controllers have steered the spacecraft to point the sample return capsule toward the sun multiple times since October, using solar heating to bake out the water. Lauretta said the outgassing has diminished as predicted.
“We fully expect that we’ll be able to dry out these components, and this should not be an issue when we get to the asteroid,” Lauretta said. “But this is something that we’re paying attention to very closely, and continuing to try to mitigate this unexpected event.”
Built by Lockheed Martin, OSIRIS-REx is short for the Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer. Its objective is to snag at least 2.1 ounces — 60 grammes — of surface material, and return the specimens to Earth in September 2023.
OSIRIS-REx’s approach phase officially begins Aug. 17, Lauretta said, at a range of about 1.2 million miles (2 million kilometres) from Bennu.
Detailed images of Bennu should be acquired by OSIRIS-REx in October. Analysts will look for evidence of debris or moons in the asteroid’s vicinity before OSIRIS-REx’s arrival.
All but one of OSIRIS-REx’s instruments were tested when the spacecraft flew by Earth in September, verifying the sensors were ready for Bennu. A laser altimeter instrument provided by the Canadian Space Agency will be activated after the probe gets to the asteroid.
“We are now about seven months away from optical acquisition of Bennu,” Lauretta said. “We expect that to occur on Aug. 17, 2018, kicking off about a three-month-long approach phase, and then getting us into the preliminary survey in late November and early December of 2018. That will be the first real resolved imaging of the asteroid.”
The asteroid has a surface area of about 200 acres — 0.78 square kilometres — and some of OSIRIS-REx’s instruments will capture data with centimetre-scale resolution, or better. The observations will add context to the sample OSIRIS-REx will return to Earth, and identify resources that might be valuable for astronauts in the future.
During the next phase of the mission, in late 2019, mission scientists will narrow down targets for OSIRIS-REx to go down and snatch up a piece of Bennu. A final decision by top NASA management will pick the sampling site based on several factors, primarily to avoid damaging the spacecraft and to maximise the likelihood of capturing primitive, pre-biotic material, the mission’s scientific payoff.
The all-important sampling attempt is scheduled for July 2020, when OSIRIS-REx will extend a robot arm to touch Bennu.
OSIRIS-REx will depart Bennu some time between March 2021 and April 2022 — managers have some flexibility — and drop the capsule containing the precious asteroid samples for a blazing-hot re-entry over the Western United States on Sept. 24, 2023.
Specialists at a military training ground in Utah will stand by to retrieve the samples.
But first OSIRIS-REx, like Hayabusa 2, has to home on its destination.
“It’s going to be a very exciting year for us,” Lauretta said. “The team is very excited and very ready for Bennu, and to begin the science campaign for this mission.”
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