Interview: See NASA's asteroid-chasing satellite through the eyes of its principal investigator
On September 8, NASA will launch another of its "hit-an-impossibly-small-object-with-an-even-smaller-object" missions. The space agency will be launching it's OSIRIS-REx satellite, named through a combination of letters that defines its mission: Origins, Spectral Interpretation, Resource Identification, Security and Regolith Explorer.
The REx, which measures 20.25 ft (6.2 m) long, 8 ft wide (2.43 m), and 10.33 ft (3.15) high, will be rendezvousing with asteroid Bennu, a space rock whose orbit ranges between 0.8 and 1.6 au away (an astronomical unit [au] is the distance from the Earth to the sun) and has an equatorial diameter of about 1640 ft (500 m). The satellite will map the asteroid in great detail using spectroscopy and radio signals, and will snatch a sample of its regolith – or soil – using a robotic arm and grabber that will shoot gas at the surface and suck up resulting dust particles under 2 mm in size.
That soil will be returned to Earth in a sealed capsule in 2023 where it will be analyzed for signs that it contains amino acids, the basic building blocks of life. If that's the case, NASA believes it will have solved at least part of the mystery of how life first started on Earth.
That pretty much covers all of the items in the satellite's acronym except for "Security."
According to NASA, Bennu has about a one in 2,500 chance of colliding with Earth late in the 22nd century, so the space agency wants to get a greater understanding of how the asteroid moves. To accomplish this, they'll be studying the Yarkovsky effect on Bennu (see video below), a rotational effect caused by parts of the asteroid heating and cooling.
Till now, much of what we know about OSIRIS-REx has come from NASA's mission page and occasional press releases. On Saturday, the space agency opened the clean room at the Kennedy Space Center in Florida to press and made the mission team available for questions. New Atlas was there and had a chance to catch up with Dante Lauretta, professor of planetary science and cosmochemistry at the University of Arizona, and the mission's principal investigator.
While you can learn some details about the satellite from the mission page and New Atlas' own coverage of OSIRIS, Lauretta gave us an even deeper look at what makes the spacecraft tick.
Here's what he had to say.
New Atlas: Is the black area on the satellite the arm that will grab the regolith sample from Bennu?
Yes. That black-wrapped area is where the gas bottles are that will hold the nitrogen. There are three of those that we open up one at a time for each sampling attempt. The sampler head is inside that long container. It's got two separation bolts that will fire when we get in the asteroid's vicinity and deploy it.
The reason that and only that portion of the spacecraft is wrapped in the black mylar is because we have a SamCam (sample camera) whose job it is is to image the TAGSAM (Touch-and-Go Sample Acquisition Mechanism) head as it makes contact with the asteroid surface and document the whole sample collection event. And if we were to use the germanium mylar that wraps most of the spacecraft, scattered light and the specular reflections would just dominate the image. You wouldn't be able to see the dark black asteroid in the background.
We actually did a lot of development and testing to come up with the right material that had the right thermal properties. Obviously to keep something cool, you don't want to wrap it in black material if you can avoid it because that's going to heat up really quickly, so it's got to be really insulated as well as have the right optical properties so the camera can get the money shot of the TAGSAM hitting the asteroid surface and collection the sample.
New Atlas: Are there any lights on the satellite?
No, we're going to rely on sunlight.
New Atlas: In our briefing your safety officer mentioned that there are small ordinances on the satellite. Can you explain what those are for?
Any separation that we do requires pyro bolts. So for example, during launch, the solar arrays are bolted onto the structure so they don't flap around during the vibrations of launch. The TAGSAM is attached at the elbow and at the wrist joints for the same reason, so it doesn't oscillate during launch. The launch cover also has separation bolts on it.
For the actual TAGSAM head, after we've got the sample, we're going to bring the whole collector back – it just goes into the sample-return capsule. There's a capture ring in there with some clamps that grab it. Pyro bolts will fire and tube cutters cut the gas lines so the arm pulls away leaving the head behind. And then finally, the sample-return capsule is deployed and there are pyro bolts that separate that.
New Atlas: What about opening and closing the capsule and some of the other mechanics on the satellite?
There are a lot of motors on the spacecraft. We've got two axis gimbals on the solar array so that we can deploy it and keep it pointing at the sun, but we can also bring them down into that "Y" configuration you've seen – that's our classic attitude for sampling. It's basically to keep them as far away from the surface of the asteroid as possible. So there are motors that drive those. There are also motors that drive each of the three joints on the TAGSAM arm, and there are motors that open and close the sample-return capsule like a clamshell.
New Atlas: And they'll all rely on the satellite's solar power to operate?
Everything is powered by the solar arrays. We have two batteries that store up power. So for example, when we're doing the tag event, those arrays are not on the sun necessarily – it will depend on where exactly the sample site is – so we have enough battery power for the tag.
But for the most part, we don't go into battery because, unlike an Earth orbiter or a Mars orbiter, we're never going into eclipse; we're always visible to the sun. And the reason we wanted those two axes on the solar arrays is because of all the different observation angles that we want to get. We can always keep our arrays on the sun no matter what our attitude is relative to the asteroid.
New Atlas: How are you steering?
On the bottom are the main engines. There are four of them with 200 newtons of thrust each. In addition, on the bottom main propulsion ring there are two low-thrust-reaction engines than give 0.2 newtons of thrust, so that just shows you the different amount of pushes we need to give the spacecraft. When we're doing our big deep-space maneuvers, we use the main engines. When we're just trying to trim orbit around the asteroid, we use those low-thrust engines.
There's also another thruster that's a trajectory-correction-maneuver thruster. So when we're firing on the main engines, if there's any kind of imbalance, the TCMs keep you aligned along the vector that you want to fire on. And then in each corner there's a set of two attitude-control thrusters.
So there are 16 thrusters total with full redundancy on all of them across the vehicle, and they're there to maintain attitude and help point the spacecraft, but more often we'll be using our reaction wheels, which are like mechanical flywheels. We have four of those.
If you spin a flywheel one way, the spacecraft will spin the other way to conserve angular momentum. So those reaction wheels will build up momentum over the course of the mission and you have to get rid of that at some point, or else the wheel will be spinning too fast. You do that by firing the attitude-control thruster in the opposite direction the wheel is spinning and you dump the momentum through the chemical propellant reaction.
New Atlas: And what is the propellant?
Hydrazine. It's a monopropellant system, so all of the thrusters have a catalyst bed that heats up and you just flow the hydrazine gas over that and it decomposes and releases its chemical energy and then goes out the nozzles.
New Atlas: Well, thanks for your time and good luck with the mission.
This video shows how the Yarkovsky effect – a phenomenon that will be studied by OSIRIS-REx – works.