On April 15, 2010, the President challenged NASA to “break through the barriers” to enable the “first-ever crewed missions beyond the Moon into deep space” by 2025. One of these barriers is navigation technology.In the 18th century, the advancement of clock technology and resulting improvement in navigation fidelity brought us to the New World that we now call home. In support of NASA’s push to explore new worlds beyond low Earth orbit (LEO), and to serve a variety of national needs, the NICER (Neutron star Interior Composition ExploreR) team proposes to use the International Space Station (ISS) to validate a revolutionary navigation technology.
The NICER/SEXTANT (NICER is the name given to the instrument for the SMD proposal, but NICER and SEXTANT are the same instrument) concept uses a collection of pulsars— stellar “lighthouses”—as a time and navigation standard just like the atomic clocks of the Global Positioning System (GPS). Unlike GPS satellites, NICER pulsars are distributed across the Galaxy, providing an infrastructure of precise timing beacons that can support navigation throughout the Solar System. Since their discovery in 1967, pulsars have been envisioned as a tool for Galactic navigation (Figure 1). An NICER system measures the arrival times of pulses through the detection of X-ray photons; a sequence of measurements is then stitched together into an autonomous on-board navigation solution.
NASA’s plans for distant exploration demand breakthrough navigation tools. The best current capabilities are resource-intensive and degrade as explorers recede from Earth. At Mars, they yield crossrange spacecraft positions to a few kilometers, but impose scheduling burdens on the Deep Space Network (DSN). For critical applications such as orbit insertion at Jupiter and beyond, the current state-of-the-art is pushed to its practical limit. NICER complements the existing navigation toolbox, promising three-dimensional position accuracies better than 500 m anywhere in the Solar System. Ultimately, a small (~0.1 m3), low mass (<~10 kg) NICER package will offer a cost effective on-board navigation option for the redundancy and reliability required for human exploration beyond LEO, and will enable deep-space missions that are not feasible with Earth-based tracking.
X-ray pulsar timing applications address the navigation and exploration goals of the National Space Policy of June 28, 2010, and NASA, DoD, and NIST are investing in technology development to exploit them. As celestial clocks, pulsars offer a new time standard that can be independently generated anywhere. The DoD is exploring applications enabled by a network of spacecraft with synchronized clocks, including mitigation of vulnerabilities in GPS. DARPA has funded the X-ray Timing (XTIM) program, which, in collaboration with the ISS NICER experiment, will demonstrate distributed time-synchronization using celestial sources.
Figure 1: NICER/SEXTANT brings to practical reality the concept of a pulsar based map, first used on the Pioneer Plaque to encode (radial lines at left) the Sun’s location in the Galaxy .
NICER (Neutron Star Interior Composition Explorer) is a SMD science mission chosen to go into Phase A. NICER represents a science aspect of SEXTANT, where looking at many of the same Neutron Stars we need to enable pulsar navigation, scientists can also learn about the densest objects in the universe and study extreme physics.
The combination of the science and technology demonstration represented by SEXTANT reflects a true cost sharing between very different parts of NASA and the US Government.
The selection notice is at: