Deflecting the massive asteroid 101955 Bennu was the focus of recent research by a national planetary defense team. Bennu will make a very close approach to Earth on Sept. 25, 2135.
Credit: LLNL

Planetary defense specialists have taken a hard look at deflecting Earth-bound asteroids and scoping out the prospects to nudge a massive asteroid.

The effort is part of a national planetary defense collaboration between NASA, the Lawrence Livermore National Laboratory (LLNL) and the National Nuclear Security Administration (NNSA), which includes LLNL and Los Alamos National Laboratory.

Nailed by HAMMER?

At the heart of the assessment is use of a 9-meter-tall, 8.8-ton spacecraft — dubbed HAMMER (Hypervelocity Asteroid Mitigation Mission for Emergency Response vehicle) — to serve as either a kinetic impactor, essentially a battering ram, or as a transport vehicle for a nuclear device.

A possible mission for HAMMER: deflect 101955 Bennu, a massive asteroid with a diameter more than five football fields), tipping the scales at roughly 79 billion kilograms (1,664 times as heavy as the Titanic), circling the sun at around 63,000 miles per hour.

The 8.8-ton conceptual HAMMER spacecraft (right) is designed to fit within the Delta IV Heavy, the world’s second highest-capacity launch vehicle in operation, surpassed only by SpaceX’s Falcon Heavy rocket.
Credit: LLNL

Impact stats

More to the point. Based on observation data available, Bennu has a 1 in 2,700-chance of striking Earth on September 25, 2135, and it is estimated that the kinetic energy of this impact would be equivalent to 1,200 megatons (80,000 times the energy of the Hiroshima bomb).

“The chance of an impact appears slim now, but the consequences would be dire,” said Kirsten Howley, LLNL physicist and coauthor on a paper — Options and uncertainties in planetary defense: Mission planning and vehicle design for flexible response – published recently in the journal, Acta Astronautica.

“This study aims to help us shorten the response timeline when we do see a clear and present danger so we can have more options to deflect it. The ultimate goal is to be ready to protect life on Earth,” Howley explains in a LLNL press statement.

Gentle nudge

The preferred approach to mitigating an asteroid threat would be to deflect it by ramming a kinetic impactor into it, delivering a gentle nudge large enough and soon enough to slow it down and change its collision course with Earth, but not so large that the object breaks apart.

The new study helped quantify the threshold where a kinetic impactor would no longer be an effective deflection option. To evaluate this threshold, researchers focused on determining how many HAMMER impactors it would take to deflect Bennu.

Study results

The paper concluded that using a single HAMMER spacecraft as a battering ram would prove inadequate for deflecting an object like Bennu.

While recent simulations of nuclear deflection scenarios are not included in this paper – they will be included in a companion paper to be submitted for publication in the near future – the findings suggest that the nuclear option may be required with larger objects like Bennu.

The nuclear approach carries the potential to deposit much more energy into an object like Bennu, causing a greater change in speed and trajectory.

OSIRIS-REx spacecraft at Bennu.
Credit: NASA/University of Arizona

Bound for Bennu

If asteroid Bennu rings a bell, it’s the target of NASA’s Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer, mercifully shortened to OSIRIS-Rex. It is the space agency’s first asteroid sample return mission.

Bennu is a rare B-type asteroid (primitive and carbon-rich), which is expected to have organic compounds and water-bearing minerals like clays.

OSIRIS-REx launched Sept. 8, 2016, from Cape Canaveral, Florida and will arrive at Bennu on December 3 of this year. In March 2021, the window for departure from the asteroid will open, and OSIRIS-REx will begin its return journey to Earth, arriving two and a half years later on Sept. 24, 2023. The sample return capsule will separate from the spacecraft and enter the Earth’s atmosphere. The capsule containing the sample will be collected at the Utah Test and Training Range.

For more information on HAMMER and the new appraisal of planetary defense techniques — Options and uncertainties in planetary defense: Mission planning and vehicle design for flexible response – go to:

https://www.sciencedirect.com/science/article/pii/S0094576517307919#fig1

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