dslauretta

The good news is that we are reusing the capsule design from the NASA Stardust mission. The only modification that is required is on the main deck to accommodate our sample collection device, which is different from Stardust. This is a minor modification - the SRC is one of the easy parts!

high five

We are using a device called the Touch-and-Go Sample Acquisition Mechanism (TAGSAM). The basic concept is to contact the asteroid surface with a large air filter (something that would look right at home sitting on top of a carburetor from a '57 Chevy), then blast the surface with a pulse of high-purity nitrogen gas. The gas agitates and fluidizes the regolith, which expands into the TAGSAM device. If we fill the collection chamber - we have ~2 kilograms of material. TAGSAM can collect particles up to 3-cm across. We also have contact pads on the outer edge of TAGSAM. We will collect particles less than 1 millimeter as long as we touch the surface.

I do not worry about getting hit by an asteroid on a daily basis. It is much more dangerous to cross the street - which I do worry about.

My favorite Christmas is Happy X-mas (War is Over) by John Lennon.

Target selection for OSIRIS-REx was originally driven by engineering constraints. First, we decided to use a Lockheed-Martin heritage spacecraft. That meant using solar power and trying to keep the thermal control system relatively simple. Using solar power limited how far out into the Solar System we could travel - setting a limit of 1.8 AU on the aphelion of the target's orbit. The thermal control limit constrained how close to the Sun we could go - limiting the perihelion of the orbit. Together these two constraints defined the semi-major axis and eccentricity of potential targets.

The next constraint was the total energy of the mission. We needed a target with relatively low delta-V (total change in velocity). We also needed a trajectory that limited the re-entry velocity of the Sample Return Capsule - since we are using a heritage design from the Stardust mission. These parameters limited the inclination of the asteroid orbit to less than 10 degrees.

These orbital constraints rapidly collapsed the number of potential targets to around 200 asteroids. The next constraint was on the size of the object. It turns out that asteroids smaller than ~200 meters tend to be rapid rotators - some spinning once every minute or so. We used absolute magnitude as a proxy for size - dropping the number of potential targets to about 20.

The final criterion was driven by science. We wanted a target that was likely to be rich in carbon and water - a carbonaceous asteroid. Of the twenty or so targets that met our dynamical constraints - only five were known to have low albedo and therefore likely to be carbonaceous. Bennu rose to the top of the list based on the extensive ground-based data set - particularly the fantastic shape model information that had been obtained from the Arecibo and Goldstone Planetary Radar telescopes.

There are three lines of evidence that constrain the average grain size on Bennu. First, in addition to the shape model, the radar astronomy also provided information on the radar polarization ratio. Basically, we transmit a beam with a specific circular polarization and measure how much of the returned energy comes back with the opposite polarization. These data show that the transition to radar roughness occurs at a scale smaller than lowest radar wavelength - 3 cm.

Next, we used the Spitzer space telescope to determine the average thermal inertia of the surface. Lower thermal inertia values mean smaller grain sizes. These data suggest that the average grain size on Bennu is on the order of a millimeter.

Finally, the asteroid shape reveals a prominent ridge at the equator - suggesting that there is loose material moving around on the surface and collecting at the geopotential lows (the valleys of Bennu) - which lie at the equator.

To get involved with OSIRIS-REx - come talk to me!

1. Asteroid 5819 Lauretta is continuing on its orbital trajectory through the Solar System. My astronomer friends snap a photo for me every once in awhile but there is no plan for a dedicated science campaign.

2. The OSIRIS-REx concept originated with Lockheed-Martin, who is always looking for new Principal Investigators for their planetary science missions. They approached Mike Drake, the original PI, in 2004 about collaborating on a sample-return mission. Mike invited me to be his Deputy at that time - which I gladly accepted. Mike and I worked on the mission concept for seven years before being accepted by NASA. Mike passed away in September 2011 - four months after winning the contract. I was promoted to PI at that time.

3. We can go faster to get to Bennu. However, we need to not only get to Bennu - but also go in the same direction at the same speed. Thus, if we get there more quickly, we need giant rocket engines and a lot of fuel to slow down for the rendezvous.

4. I came up with the name based on the mythology of Osiris as the bringer of life to the Nile Valley - Bennu represents the type of object that may have brought the seeds of life to Earth. It is also a crazy acronym - which fits in with the way NASA names their missions,

5. OSIRIS-REx will eject the sample return capsule four hours before the spacecraft hits the top of the atmosphere at 27,000 mph. The spacecraft will then perform a deflection burn and be placed into a stable heliocentric orbit that will not intersect any object of astrobiological interest (planetary protection requirement). It may be available for an extended mission at the discretion of NASA.

6. We have fragments of Chelyabinsk in our lab at the University of Arizona and are actively studying it. It snuck up on us because it came out of the Sun and it was a relatively small object - we are mandated by Congress to detect objects 140-m in diameter and larger - the Chelyabinsk bolide was ~20-m across.