SSL / Research / REXIS

REXIS:
REgolith X-Ray Imaging Spectrometer


In 2016, NASA will launch OSIRIS-REx, the third spacecraft in its New Frontiers Program. OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security REgolith Explorer) will explore the asteroid 1999 RQ36, characterizing it, and ultimately returning a sample of the asteroid’s uppermost layer (its regolith) back to Earth.

Flying on OSIRIS-REx will be REXIS, an instrument largely designed, developed, and built by students at MIT and Harvard. REXIS (REgolith X-Ray Imaging Spectrometer) is a collaborative effort between MIT’s Space Systems Lab, EAPS Department, Kavli Institute, and Lincoln Laboratory; Harvard College Observatory; NASA Goddard Space Flight Center; and Lockheed Martin. The purpose of REXIS is to collect and image fluorescent X-rays emitted by asteroid 1999 RQ36, thereby providing spectroscopic information regarding the chemical makeup of the asteroid regolith and the distribution of features over its surface.

REXIS SAMPLE

OSIRIS-REx during sample acquisition

Why Asteroids and Why 1999 RQ36?

Asteroids are an attractive target for future space exploration missions for a number of reasons, each of which is reflected in the acronym “OSIRIS”. First, asteroids are some of the most primitive bodies in the Solar System, often representative of the material found in the protoplanetary disk over 4 billion years ago (“Origins”). In this regard, it is paramount to understand more precisely their chemical makeup (“Spectral Interpretation”). Moreover, many asteroids are believed to be rich in resources such as water and precious metals (“Resource Identification”); taking advantage of these resources could have implications for developing technologies here on Earth, or for future Solar System exploration missions. Finally, a number of asteroid belong to a class of space objects known as “Near Earth Asteroids” (NEAs) or “Near Earth Objects” (NEOs). These objects have orbits that are within that of Mars and that oftentimes cross the orbit of Earth itself, thereby posing a potential impact hazard (“Security”).

Clockwise from left: size of 1999 RQ36 compared to the Great Pyramid of Giza; the orbit of 1999 RQ36 compared to those of the planets of the inner Solar System; artist’s conception of the protoplanetary disk.

How does 1999 RQ36 fit into this framework? Based on near infrared measurements, 1999 RQ36 is thought to belong to a class of asteroids known as carbonaceous chondrites, believed to be amongst the most primitive of asteroid types. Understanding its composition and comparing it to meteorite samples on Earth with a similar makeup will shed light on the evolutionary history of the asteroid. Furthermore, orbital measurements have indicated that 1999 RQ36 has a roughly 1 in 1800 chance of hitting Earth in the next century. With a diameter of roughly 500 meters, a mass of about 140 million tonnes, and an orbit that crosses Earth’s, 1999 RQ36 poses a potentially great risk to Earth. By measuring thermal radiation pressure-induced orbital perturbations (the so-called Yarkovsky Effect), OSIRIS-REx enable scientists to better predict the future path of the asteroid and more precisely determine its likelihood striking Earth.

Asteroids are an attractive target for future space exploration missions for a number of reasons, each of which is reflected in the acronym “OSIRIS”. First, asteroids are some of the most primitive bodies in the Solar System, often representative of the material found in the protoplanetary disk over 4 billion years ago (“Origins”). In this regard, it is paramount to understand more precisely their chemical makeup (“Spectral Interpretation”). Moreover, many asteroids are believed to be rich in resources such as water and precious metals (“Resource Identification”); taking advantage of these resources could have implications for developing technologies here on Earth, or for future Solar System exploration missions. Finally, a number of asteroid belong to a class of space objects known as “Near Earth Asteroids” (NEAs) or “Near Earth Objects” (NEOs). These objects have orbits that are within that of Mars and that oftentimes cross the orbit of Earth itself, thereby posing a potential impact hazard (“Security”).

Selection criterion for 1999 RQ36 as the target asteroid

How Does REXIS Work?

REXIS, shown on the right, takes advantage of a phenomenon known as fluorescence in order to image and determine the chemical composition of the surface of 1999 RQ36. Fluorescence occurs when electrons in atoms or molecules become excited after absorbing incoming radiation. When the electrons become excited and de-excited, they release a photon; the energy of these photons is characteristic to the atom or molecule in question. In the case of 1999 RQ36, the sun irradiates the asteroid surface, the regolith of which fluoresces photons towards OSIRIS-REx and the REXIS instrument. By collecting these photons and determining their energies, we can determine the elemental abundances present on the surface of the asteroid.

REXIS collects photons using a charge-coupled device, or CCD; the REXIS CCDs are known as CCID-41s, and are developed by MIT Lincoln Laboratory. CCDs are photosensitive devices that generate charge when irradiated; the charge is proportional to the energy of the incoming radiation, thereby allowing us to determine the energy of the incident radiation. REXIS is concerned with detecting photons primarily in the 0.5 – 7.5 kiloelectronvolt (keV) range.

REXIS images emitted X-rays using a technique known as coded aperture imaging. Coded aperture imaging works much like a pinhole camera. When light is shined on a pinhole, the direction of the incident light can be determined by the position of the shadow cast by the pinhole. Coded aperture imaging utilizes this same principle, but makes use of many holes arranged in a pattern on a mask made up of X-ray transparent material (in the case of REXIS, stainless steel and gold). Depending on the shadow cast on the CCDs by the X-rays (and with an appropriate reconstruction algorithm), the direction of the fluorescent radiation from the asteroid can be determined and mapped back to a position on the asteroid surface. By tying the energy of the X-rays to elemental abundances, a map of the distribution of elements on the asteroid surface can be made. REXIS will identify the distribution of elements on the surface of 1999 RQ36 with a spatial resolution of 50 meters or better.

REXIS’s location, along with other instruments, on OSIRIS-REx

The REXIS Team: Students Design, Build and Lead

The REXIS team strives to design and build a robust instrument through the innovative MIT-Harvard Conceive, Design, Implement, Operate (CDIO) Curriculum, in which students take a lead role in instrument development. The REXIS team is made up of graduate and undergraduate students at MIT and Harvard, as well as research scientists and faculty at both institutions. As students graduate, multigenerational continuity is achieved through faculty, scientist, and graduate student mentors, and REXIS takes advantage of both heritage designs, world-class facilities, and oversight by senior staff with flight experience on programs such as STS, ISS, Chandra, Suzaku, and HETE-II.

The REXIS Team

Where Do I Go For More Information?

For more information, contact REXIS PI Professor David Miller (millerd@mit.edu) or Instrument Manager Dr. Rebecca Masterson (becki@mit.edu). More information on OSIRIS-REx and 1999 RQ36 can be found at osiris-rex.lpl.arizona.edu.