September 20, 2018

Two NASA probes catch first glimpses of primordial targets


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On Aug. 17, the OSIRIS-REx spacecraft obtained the first images of its target asteroid Bennu from a distance of 1.4 million miles (2.2 million km), or almost six times the distance between the Earth and Moon. This cropped set of five images was obtained by the PolyCam camera over the course of an hour for calibration purposes and in order to assist the mission’s navigation team with optical navigation efforts. Bennu is visible as a moving object against the stars in the constellation Serpens.
Credits: NASA/Goddard/University of Arizona

Two NASA spacecraft — the OSIRIS-REx asteroid sample return mission and the New Horizons probe at the edge of the solar system — have captured their first looks at their targets as they approach a pair of primordial relics for the first time.

The imagery will improve in the coming months, before each mission’s target finally becomes fully resolved late this year.

Launched in September 2016, OSIRIS-REx is approaching asteroid Bennu, a dark, carbon-rich world measuring approximately 1,640 feet (500 meters) in diameter. The New Horizons spacecraft, on an extended mission after flying by Pluto in July 2015, has its sights set on a distant, frozen world nicknamed Ultima Thule nearly 4 billion miles (more than 6 billion kilometers) from Earth.

Nearly two years after its launch, the OSIRIS-REx spacecraft has commenced its final approach to asteroid Bennu. The probe aimed its long-range imager toward the asteroid and captured a series of pictures Aug. 17 from a distance of around 1.4 million miles (2.2 million kilometers), revealing Bennu as a faint dot moving against a field of background stars.

“I know Bennu is only a point of light here, but many of us have been working for years and years and years to get his image down,” said Dante Lauretta, principal investigator on the OSIRIS-REx mission from the University of Arizona.

The imagery taken Aug. 17 showed OSIRIS-REx was right on track to arrive at Bennu on Dec. 3, when it will begin a series of close passes over the asteroid to map the object for the first time, before entering orbit around Bennu on Dec. 31.

“There was a lot of good news in this image for us,” Lauretta told reporters in a conference call Friday. “First of all, the asteroid is right where we thought it was, so it’s there and it’s waiting for us. The spacecraft was also where it was supposed to be, and pointing in the (right) direction.”

OSIRIS-REx has worked flawlessly since leaving Earth in 2016, officials said, successfully making a brief return to its home planet last year for a gravity assist flyby that re-routed its orbit around the sun toward Bennu.

Bennu is currently about as bright as a 13th magnitude star, far too dim to be visible with the naked eye — if someone was riding along with OSIRIS-REx. But the asteroid will get brighter as the spacecraft makes its approach, finally filling OSIRIS-REx’s camera view in late November and early December.

OSIRIS-REx stands for the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer, a $1 billion mission designed to bring back to Earth at least 2 ounces (60 grams), and potentially more than a pound, of rock specimens from Bennu.

“First and foremost, we are an origins mission,” Lauretta said. “We are seeking to return samples of a carbonaceous asteroid that we believe dates back to the formation of our solar system.”

Simulated cratering and topography are overlaid on radar imagery of asteroid Bennu. Credit: NASA/GSFC/UA/Mike Nolan-Arecibo Observatory/Bob Gaskell-Planetary Science Institute

When the solar system formed more than 4.5 billion years ago, chunks of rock and ice collided as they circled the sun like the balls on a billiard table, eventually building up planets. The leftovers became asteroids and comets, and scientists believe Bennu still harbors the basic organic molecules that were present in the early solar solar system, the stuff that may have helped seed life.

The spacecraft carries three cameras — one for long-range viewing, a color camera for mapping, and another imager to take pictures as OSIRIS-REx collects samples from the asteroid’s surface. The rest of OSIRIS-REx’s suite of science instruments includes a thermal emission spectrometer to detect heat coming from the asteroid, a visible infrared spectrometer to locate minerals and organic materials, a laser altimeter provided by the Canadian Space Agency to create topographic maps, and a student-built X-ray spectrometer to identify individual chemical elements present on the asteroid.

As the craft zeroes in on Bennu, scientists will use its cameras to survey the region around the asteroid for debris, which could pose a hazard to OSIRIS-REx when it moves closer. So far, ground-based observations of Bennu using a high-power radar have discovered no such debris, raising hopes that the corridor toward the asteroid is safe.

In October, controllers at Lockheed Martin, which built OSIRIS-REx, will unlatch the probe’s robotic arm from its launch restraint for the first time. Over several days, the ground team will command the arm to bend its joints and jettison a launch cover over the sample collection mechanism, which will release compressed air during a touch-and-go maneuver to force gravel and surface material into an on-board chamber for return to Earth.

OSIRIS-REx is closing its range toward Bennu at a relative velocity of around 1,100 mph, or nearly 1,800 kilometers per hour, according to Michael Moreau, OSIRIS-REx flight dynamics system manager at NASA’s Goddard Space Flight Center.

A braking burn using OSIRIS-REx’s liquid-fueled propulsion system in mid-October will slow the craft’s approach to just 11 mph (18 kilometers per hour).

“Ultimately, our last approach maneuver — AAM-4 — on Nov. 12 will adjust our approach to target arrival at a position that’s 20 kilometers (12 miles) away from Bennu on Dec. 3,” Moreau said. “This event will signify our official rendezvous with Bennu.”

This mosaic compares the size of asteroid Bennu, to scale, with other asteroids and comets visited by spacecraft. Credit: NASA/University of Arizona/ESA/JAXA

Navigators will analyze imagery beamed back from OSIRIS-REx to precisely locate the asteroid and fine-tune the spacecraft’s rendezvous.

OSIRIS-REx will follow a pre-planned trajectory, with the help of multiple course correction burns, over the poles of Bennu in December, allowing scientists to precisely measure the asteroid’s mass for the first time. The data will help scientists and navigators better understand Bennu’s tenuous gravity field, critical information needed to chart OSIRIS-REx’s orbit around the asteroid.

Bennu’s gravity will ensnare OSIRIS-REx into orbit Dec. 31, and the asteroid will become the smallest planetary body ever orbited by a spacecraft. Due to the asteroid’s weak gravity, OSIRIS-REx will move around Bennu at a speed of just one-tenth of a mile per hour, or about 5 centimeters per second.

Orbital speeds are determined by the gravitational pull of the parent planet or star. For comparison, objects in low Earth orbit have to travel at around 17,500 mph (7.8 kilometers per second) to avoid falling back into the atmosphere.

“It’s Bennu’s size and small mass that make the navigation challenges on this mission unprecedented, really,” Moreau said.

Comet 67P/Churyumov-Gerasimenko, which was orbited by the European Space Agency’s Rosetta spacecraft from 2014 through 2016, is between five and 10 times larger than Bennu, depending on how you measure. Asteroid Ryugu, where Japan’s Hayabusa 2 spacecraft is currently exploring, is nearly twice the size of Bennu, and Hayabusa 2 will not enter orbit around it.

OSIRIS-REx will survey Bennu for more than a year before attempting a touch-and-go descent in July 2020 to snag samples from the asteroid. The spacecraft will depart Bennu and head back to Earth, releasing its sample carrier for re-entry and landing in Utah in September 2023.

Scientists will take the samples to an ultra-clean facility at NASA’s Johnson Space Center in Houston for detailed analysis.

New Horizons gets first look at Ultima Thule

When it zips by Ultima Thule at the outer frontier of the solar system Jan. 1, NASA’s New Horizons spacecraft will catch a much more fleeting glimpse of its target than the years-long campaign planned for OSIRIS-REx.

Like Bennu, Ultima Thule — officially known as 2014 MU69 — is a leftover from the solar system’s chaotic formation. Located in the Kuiper Belt, a ring of relatively small proto-planets beyond the orbit of Neptune, Ultima Thule will become the most distant object ever visited when New Horizons arrives on New Year’s Day.

More than three years past its historic encounter with Pluto, New Horizons spotted Ultima Thule with its telescopic camera Aug. 16, earlier than predicted.

The figure on the left is a composite image produced by adding 48 different exposures from the News Horizons Long Range Reconnaissance Imager (LORRI), each with an exposure time of 29.967 seconds, taken on Aug. 16, 2018. The predicted position of the Kuiper Belt object nicknamed Ultima Thule is at the center of the yellow box, and is indicated by the yellow crosshairs, just above and left of a nearby star that is approximately 17 times brighter than Ultima. At right is a magnified view of the region in the yellow box, after subtraction of a background star field “template” taken by LORRI in September 2017 before it could detect the object itself. Ultima is clearly detected in this star-subtracted image and is very close to where scientists predicted, indicating to the team that New Horizons is being targeted in the right direction. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

“Our team worked hard to determine if Ultima was detected by LORRI at such a great distance, and the result is a clear yes,” said Alan Stern, New Horizons’s principal investigator from the Southwest Research Institute in Boulder, Colorado. “We now have Ultima in our sights from much farther out than once thought possible. We are on Ultima’s doorstep, and an amazing exploration awaits!”

Astronomers observing Ultima Thule from the ground using the object’s occultation of stars have concluded the mini-world likely has an elongated shape, and it may consist of two or three separate components, including a small moonlet orbiting nearby. Scientists are still crunching data from another stellar occultation observation earlier this month in hopes of narrowing down Ultima Thule’s shape and size, which is estimated to be around 20 miles (30 kilometers) across, assuming it is a single object.

Discovered in 2014 by the Hubble Space Telescope, Ultima Thule is still incredibly faint in New Horizons’s viewfinder, and scientists had to analyze 48 individual images to locate the target amid a field of brighter stars. New Horizons is still around 100 million miles (160 million kilometers) from Ultima Thule, but is closing its distance by nearly 800,000 miles (1.3 million kilometers) each day.

The high-speed flyby with Ultima Thule, scheduled for 12:33 a.m. EST (0533 GMT) on New Year’s Day, will give New Horizons just one chance to record imagery of the faraway world, a billion miles beyond Pluto. New Horizons does not have the fuel to slow down and linger in Ultima Thule’s vicinity.

The newest images from New Horizons are the farthest ever taken, breaking a record set NASA’s Voyager 1 spacecraft in 1990, when that probe turned back toward the inner solar system to take the famous “Pale Blue Dot” image of Earth.

“The image field is extremely rich with background stars, which makes it difficult to detect faint objects,” said Hal Weaver, New Horizons project scientist and LORRI principal investigator from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. “It really is like finding a needle in a haystack. In these first images, Ultima appears only as a bump on the side of a background star that’s roughly 17 times brighter, but Ultima will be getting brighter – and easier to see – as the spacecraft gets closer.”

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Follow Stephen Clark on Twitter: @StephenClark1.


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