Storm Hunt: Spacecraft Quintet to Track Down Magnetic Field Tempests

NASA's five THEMIS satellites are shown just after deployment in this artist's interpretation. CREDIT: NASA.

They may seem small and boxy, but five new NASA probes have a lofty purpose: sifting through the Earth’s magnetic field for the stormy beginnings of the planet’s most dynamic auroras.

Set to launch in less than a month, NASA’s THEMIS spacecraft are designed to hunt down the origin of substorm -- tempests of high energy particles that accumulate somewhere in the Earth’s magnetic field, then flow back to the planet and supercharge its aurora borealis – also known as the Northern Lights [image].

“For more than 30 years the source location of these explosive energy releases has been sought after with great fervor,” said THEMIS mission principal investigator Vassilis Angelopoulos, of the University of California, Berkeley’s Space Sciences Laboratory, which built the spacecraft quintet for NASA with Swales Aerospace. “Knowing the trigger location will help us identify the correct physics in play.”

The impact of substorm particles – accumulated from the Sun’s solar wind – with the Earth’s atmosphere are responsible for the shimmering ripples and color changes that accompany some of the most spectacular auroras to date, THEMIS researchers said [image]. But the seemingly aesthetic space weather phenomena may also prove to be a building block for severe space storms capable of endangering astronauts in orbit, interfering with communications or harming satellites, they added.

A United Launch Alliance Delta 2 rocket is slated to launch the five THEMIS probes into orbit from Florida’s Cape Canaveral Air Force Station during a 19-minute window that opens at 6:07 p.m. EST (2307 GMT) on Feb. 15 [image]. The mission’s name – short for the hefty title Time History of Events and Macroscale Interactions during Substorms – is also taken from the Greek goddess of justice.

The THEMIS family

Each about the size of a dishwasher, the five THEMIS spacecraft are nearly identical, with any one designed to fill in the orbital slot of its robotic brethren [image]. Only four probes are required to complete NASA’s planned two-year and $200 million mission – the fifth is a spare – but wrangling them all into launch position is not without challenges.

“THEMIS is the first NASA mission to launch five spacecraft on a single vehicle,” said UC Berkley’s Peter Harvey, NASA’s THEMIS project manager, adding that the probes are stacked atop one another in a sort of space probe wedding cake. “This allowed to the probes to be independently carried into space.”

After launch, the first probe – dubbed Probe A – is designed to pop free of from its housing atop the wedding while the other four deploy like flower petals [image]. The spacecraft are then expected to extend a series eight booms that range from just three-feet (one meter) to 65 feet (20 meters) in length [image]. Each THEMIS probes comes equipped with 11 instruments that include a pair of telescopes, six electric field instruments, two different magnetometers and an electrostatic analyzer.

“Our philosophy was to keep the probes as simple as possible and leave that complexity on the ground,” Harvey said.

That added complexity includes some 20 All-Sky camera stations and a host of ground-based magnetometers scattered across Alaska and Canada to complement the THEMIS probes’ findings.

Rooting out substorms

In order to scan the Earth’s magnetic field and pinpoint the origin of substorms, THEMIS researchers plan to stagger their spacecraft in different orbits that range in altitude from 10 to 30 times the radius of the Earth (the planet’s radius is about 3,962 miles, or 6,378 kilometers) [image].

“The problem has been that the substorm starts from a single point in space somewhere near Earth and within minutes it progresses past the Moon’s orbit,” Angelopoulos said. “So a single satellite alone cannot identify the substorm’s time and point of origin. Multiple spacecraft in tightly choreographed orbits are needed.”

The spare THEMIS probe and two others are slated for a one-day orbit some 10 Earth radii out in space, while two others will take up stations about 20 Earth radii above the planet. The final spacecraft is destined for a four-day orbit that reaches out to a distance of 30 times that of Earth’s radius. Their flight path also brings the five THEMIS spacecraft into alignment with themselves and Earth every four days, when researchers hope will be a prime substorm hunting period.

“It is amazing that these tiny probes will travel over halfway to the Moon,” Harvey said.

The distribution allows the THEMIS probes to evaluate two candidate locations -- one near the halfway point between the Earth and Moon and another closer in -- for substorms. Mission scientists are hopeful they’ll record at least 30 substorm events during the two-year THEMIS mission.

“Right now, we don’t have a real contender for which one is going to go in which position,” Harvey said, adding that the near-identical probes will likely show their differences once in space. “The best instruments then will go in the best orbits.”

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