Interstellar Boundary Explorer

• 1. Introduction

  ›

  On the 19th of October 2008, the Interstellar Boundary Explorer lifted off not from a launchpad but from beneath the belly of an airplane, somewhere over the central Pacific Ocean. The Pegasus XL rocket that carried it was dropped from a modified Lockheed L-1011 known as the Stargazer, which had flown the rocket from Vandenberg Air Force Base all the way to Kwajalein Atoll before letting it fall and ignite. It was a fittingly unconventional beginning for a spacecraft with an unconventional assignment: to see something no instrument had ever seen before, by looking in a direction most telescopes are not built to look.

  The edge of the Solar System is not a line you can photograph. It is a place where the outward rush of the solar wind runs headlong into the interstellar medium, the thin gas and dust that fills the space between stars. That collision zone is called the heliosheath. It is enormous, distant, and invisible to ordinary light. What IBEX was designed to detect were energetic neutral atoms, particles that carry a faint signal from that far frontier all the way back to Earth.

  What the spacecraft found when it began returning data surprised nearly everyone. There was a ribbon of unexplained brightness stretching across the sky. There was evidence that the heliosphere had no bow shock. There were neutral atoms arriving from outside the Solar System that were compositionally different from the Sun. Each discovery posed a question that the next round of data would need to answer.

• 2. A Spacecraft Built to Survive

  ›

  IBEX was assigned the designation Explorer 91, marking its place in one of NASA's oldest satellite lineages, and it was classed as SMEX-10 within the Small Explorer program, which emphasizes lower-cost, focused scientific missions. Small, however, did not mean fragile. The spacecraft was built on an octagonal base roughly 58 centimeters high and 95 centimeters across, with a dry mass of 80 kilograms and an instrument payload weighing an additional 26 kilograms. When fully fueled, the spacecraft weighed 107 kilograms, and the entire flight system at launch, including the ATK Star 27 solid rocket motor, came to 462 kilograms.

  The Orbital Sciences Corporation manufactured the satellite bus and handled spacecraft environmental testing. Power came from a solar array capable of producing 116 watts, though nominal operations consumed only 66 watts, with just 16 of those going to the scientific payload. Two hemispherical antennas handled communications, with a downlink rate of 320,000 bits per second and an uplink rate of only 2,000 bits per second.

  That communication speed, slow by almost any modern standard, was a deliberate reflection of what IBEX was actually doing. The satellite collects only a few particles per minute on average. IBEX-Hi, the higher-energy sensor built by the Los Alamos National Laboratory, detects around 500 particles per day. IBEX-Lo, built by the Lockheed Martin Advanced Technology Center to capture lower-energy atoms, records fewer than 100. There is simply no need for a high-speed connection when your subjects arrive so infrequently.

• 3. The Orbit Problem

  ›

  Getting a spacecraft to the edge of the Solar System is impossible with current propulsion. The next best option IBEX's designers chose was to place it as far from Earth as practical, so that it could make measurements outside the planet's magnetosphere. Charged particles within the magnetosphere would interfere with the detection of the faint energetic neutral atoms arriving from the heliosheath.

  The solution was a highly eccentric elliptical orbit ranging from a perigee of about 86,000 kilometers to an apogee of about 260,000 kilometers. The original orbit stretched from about 7,000 kilometers at its closest point to around 320,000 kilometers at its farthest, placing IBEX at roughly 80 percent of the distance to the Moon at its peak. IBEX performed its scientific observations from the high reaches of that orbit, beyond the magnetosphere. When it dipped back within 70,000 kilometers of Earth, it used that time for telemetry downlinks.

  In June 2011, mission planners adjusted the orbit significantly, raising the perigee to more than 30,000 kilometers. The new orbit has a period of one-third of a lunar month, a phasing chosen specifically to keep the spacecraft from passing too close to the Moon, whose gravity would otherwise perturb the orbit over time. The adjustment reduced fuel consumption and extended the spacecraft's useful lifespan to more than 40 years. The prime mission had ended in early 2011, but the spacecraft and its sensors remained healthy, and the mission continued into an extended phase.

• 4. The Ribbon No One Predicted

  ›

  When IBEX returned its first sky maps, scientists encountered something that existing models of the heliosphere had not anticipated. The data revealed a narrow ribbon of energetic neutral atoms, two to three times brighter than anything else across the entire sky. Principal investigator Dr. David J. McComas, then at the Southwest Research Institute in San Antonio and later affiliated with Princeton University, described the subsequent output as an incredible scientific harvest.

  The source of the ribbon remained unknown. The Sun is currently passing through a region called the Local Interstellar Cloud, sometimes referred to as the Local Fluff. Researchers proposed that changes in the shape of the ribbon over time could reveal how the heliosphere is interacting with that surrounding cloud. In October 2010, the second full set of IBEX observations showed that the ribbon had changed measurably in just six months, confirming that the boundary region is dynamic in ways that had not been appreciated.

  The ribbon was not the only surprise. IBEX detected neutral atoms arriving from outside the Solar System and found them to differ in composition from the Sun. It measured the Sun's speed relative to the local interstellar medium at 23.2 kilometers per second, revising the earlier Ulysses spacecraft measurement of 26.3 kilometers per second. That difference meant the pressure on the heliosphere was 25 percent less than previously estimated. Then, in July 2013, IBEX results revealed that the Solar System's heliosphere has a four-lobed tail.

• 5. Imaging the Invisible

  ›

  IBEX carries two instruments, both energetic neutral atom imagers, that together cover a wide range of particle energies. IBEX-Lo detects atoms with energies from 10 electron volts to 2 kiloelectron volts. IBEX-Hi detects atoms in the higher band of 300 electron volts to 6 kiloelectron volts. Together they span the energy range needed to sample the heliosheath from multiple angles.

  Each instrument is built the same way: a collimator that narrows the field of view, a conversion surface that turns incoming neutral hydrogen and oxygen into ions, an electrostatic analyzer that filters out ultraviolet light and selects ions of a specific energy, and a detector that counts and identifies what arrives. The effective field of view for each sensor is roughly 7 degrees by 7 degrees, making each one, in effect, a single-pixel camera. A Combined Electronics Unit controls the voltages and records the data from both sensors.

  Because IBEX spins in a Sun-oriented, spin-stabilized orbit, it sweeps that narrow field of view across the entire sky over the course of six months. The result is a full-sky map of ENA intensity at multiple energy levels, updated twice a year. Most of the ENAs being mapped are generated in the heliosheath itself, the collision zone between solar wind and interstellar gas. By 2012, those maps had supported more than 100 published scientific papers, with data available through the Southwest Research Institute's public website and the NASA Space Physics Data Facility's Heliophysics Data Portal.