Deep Space Network

Excellent Partnership between Ulysses and the Deep Space Network

By: Dr. Peter T. Poon, Telecommunications and Mission Systems Manager

70-meter deep space station at Goldstone, California

Since the successful launch from the Space Shuttle Discovery on October 6, 1990, Ulysses has been in excellent partnership with NASA's Deep Space Network (DSN). With the world's premier network for deep space communications as a partner, Ulysses has been returning valuable science data for over 17.5 years. The DSN supports Ulysses by providing tracking support, telemetry reception and the transmission of commands to the spacecraft. In addition, the DSN network was instrumental in maintaining control of the spacecraft during its nutation, as described below.

The DSN is the largest and most sensitive spacecraft communications network in the world consisting of three deep space communications complexes located approximately 120 degrees of longitude apart around the globe: at Goldstone, California; near Madrid, Spain; and near Canberra, Australia. To illustrate the sensitivity, the DSN is capable of receiving signals from a spacecraft billions of kilometers away, which, upon arrival on Earth, can be as weak as a billionth of a billionth of a watt -- that is 20 billion times less than the power required for a digital wristwatch. Each deep space communications complex provides capabilities required to perform telemetry data processing, including signal reception and amplification, signal demodulation and decoding, and data extraction. It also provides a capability to send commands to the Ulysses spacecraft. All DSN complexes are linked to JPL via a world-wide communications network.

Artist's concept of Ulysses making a north polar pass. Image credit: NASA/JPL

A joint NASA-ESA project, Ulysses is the only mission that has been exploring the heliosphere in three dimensions (in addition to the dimension of time). The heliosphere is a bubble in space caused by the solar wind emanating from the Sun blowing into the interstellar medium, which consists predominantly of hydrogen and helium that permeates our galaxy. The goal of the Ulysses mission is to explore the Sun's environment as a function of the complete range of solar latitudes. Ulysses has transformed our view of the integrated Sun-heliosphere system. It has overcome the limitations of measurements restricted to the vicinity of the ecliptic plane as the only spacecraft surveying the environment above and below the poles of the Sun.

It takes Ulysses slightly over six years to orbit around the Sun. In its first orbit over the Sun, Ulysses charted the well-ordered solar minimum heliosphere arising from the nearly symmetric corona. The second orbit revealed the complex solar maximum heliosphere undergoing re-organization as the Sun ejected energy stored in coronal magnetic fields. Ulysses' observations of these ejections are defining how the space radiation environment is affected by the three-dimensional magnetic field. A fundamental Ulysses discovery is that energetic particles are far more mobile in latitude than expected. This result is linked to similar results for cosmic rays, pickup ions, and dust. There are at least occasional direct magnetic field connections across latitude. With a comprehensive range of scientific instruments, Ulysses has detected and measured solar wind ions and electrons, investigated galactic cosmic rays, solar X-rays, dust, and particles, and gamma ray bursts from the interstellar medium and universe. Ulysses has determined basic properties of the solar magnetic field; its mode of reversal, and how magnetic flux is carried from the Sun and into the interplanetary medium. It has been exploring the heliosphere as an integrated system and conducting investigations of the Sun, the local interstellar medium, and the universe.

The following is a sample of findings described in more than 1,300 scientific papers:

• The sun casts both fast and slow winds of solar particles; the speedier from the sun's poles, and the slower from the equatorial region.
• Fast solar wind originates in regions of low coronal temperature and slow wind in regions of high temperature contrary to expectation.
• Fast solar wind of 700 km/sec is continuously present in the Sun's polar caps except near solar maximum when the polar coronal holes disappear.
• The Sun's "bar magnet" dipole simply rotates through 180° from sunspot minimum to sunspot minimum to achieve the polarity reversal
• The radial magnetic field component is independent of helio-latitude.
• The flux of Galactic Cosmic Rays is essentially the same over the polar caps as at the solar equator. Cosmic rays would have had easy access to the polar caps except for the large amplitude waves characteristic of the fast high latitude solar wind.
• A north-south asymmetry in the heliosphere during solar minimum results in a displacement of the heliospheric current sheet and the cosmic ray equator by about 10°.
• Ulysses directly measured interstellar gas for the first time, hinting at the presence of a "bow shock" for interstellar radiation beyond the heliosphere.
• The abundances of neutral interstellar atoms (Hydrogen, Helium, Oxygen, Nitrogen, Neon) have been inferred from observations of their ionization and pickup by the heliospheric magnetic field.
• The lengths of comet tails can exceed an Astronomical Unit.
• Gamma radiation from a magnetar, a collapsed star characterized by exceptionally intense magnetic fields, was detected and the time dependence of the radiation observed.
• Large interstellar dust particles make up about 2 percent of all mass between stars.

Ed Massey, NASA Project Manager, second from left, Bruce Brymer, Ground Operations Manager, fifth from left, and NASA-ESA Ulysses team participating in a CACTUS meeting chaired by Peter Poon, Telecommunications and Mission Systems Manager

Nigel Angold, ESA Mission Operations Manager, first from right, Fernando Castro, ESA Spacecraft Operations Manager, fourth from right, and NASA-ESA Ulysses team participating in a CACTUS meeting chaired by Peter Poon

The joint NASA-ESA team overcame an important technical challenge due to wobbling of the spacecraft, called nutation, which was discovered soon after launch. The nutation was due to the flexing of the long axial boom of the spacecraft as it was unevenly heated by the Sun. The problem was solved by using the spacecraft thrusters in an unconventional way to correct the wobble, which often required extensive DSN coverage. The NASA-ESA team held regular meetings called Committee to Assure Complete and Timely Ulysses Support (CACTUS) to ensure that the nutation of the spacecraft would be well under control throughout the nutation period. Indeed, the spacecraft has been kept very healthy with excellent return of science data. Two pictures taken at a typical CACTUS meeting provide a glimpse into the excellent partnership between Ulysses and the DSN in the joint NASA-ESA mission.