The destination of ESA’s Hera mission for planetary defense – a tiny asteroid moonlet – has finally received its official name. After years of informal nicknames and temporary designations, the smaller of the Didymos asteroid pair has been formally christened ‘Dimorphos’ by the International Astronomical Union.

A near-Earth binary asteroid system, named after the Greek word for ‘twin’, Didymos’s main body measures about 780 m across, with its previously nameless moonlet about 160 m in diameter, approximately the size of Egypt’s Great Pyramid.

In 2022, this moonlet will be the target of NASA’s Double Asteroid Redirection Test (DART), the first full-scale demonstration of an asteroid deflection technology for planetary defense. ESA’s Hera mission will be launched two years later, to perform a close-up survey of Dimorphos, along with its parent asteroid, following DART’s impact.

“Dimorphos is Greek for ‘having two forms’,” says Kleomenis Tsiganis, a planetary scientist at the Aristotle University of Thessaloniki and member of both the DART and Hera teams, who suggested the name.

“It has been chosen in anticipation of its future status as the first celestial body to have its ‘physique’ intentionally altered by human intervention, the kinetic impact of DART. Hence, it will be known to us by two, very different forms, the one seen by DART before impact and the other seen by Hera, a few years later.”

DART’s kinetic impact into Dimorphos is expected to alter its orbit around Didymos as well as create a substantial crater, which will be studied by the Hera spacecraft when it arrives several years later. The DART impact itself will be recorded by the Italian-made LICIACube CubeSat, deployed from DART several days earlier, with longer-term effects studied by telescopes on Earth’s surface and in space.

“In the past we’ve referred to it as Didymos-B, or Didymoon, but the target of our mission finally has an official name, confirmed by the International Astronomical Union, IAU,” comments Ian Carnelli, managing Hera for ESA.

“It is one more small step towards making our destination seem like a real place, just as astronomers around the globe are observing what is currently just a minuscule dot in the sky to gather all the practical details they can, in support of DART and Hera mission planning. We can’t wait to see what is really looks like from up close and we are excited to plan close proximity investigations for at least six months in 2027.”

The Hera mission will also deploy two CubeSats to perform additional detailed investigations explains Hera mission scientist Michael Kueppers: “The Juventas CubeSat will be the first ever spacecraft to use a low-frequency radar to scan the interior structure of an asteroid. Understanding the interior structure is a fundamental step in the full interpretation of DART’s impact with Dimorphos.”

Naming an asteroid pair

Didymos was first spotted in 1996 by Joe Montani of the Spacewatch Project at the University of Arizona. Originally assumed to be a single body, the asteroid system didn’t yet meet the criteria for an official name at the time; at minimum, observers have to consistently trace an asteroid’s orbit and confirm that it won’t merely fade away and get ‘lost’ before filing for a formal designation.

Then in 2003, Petr Pravec, a planetary astronomer at the Ondřejov Observatory in the Czech Republic, was tracking the brightness of this then-nameless asteroid when he recognized a pattern consistent with a satellite orbiting the main body.

He comments: “I am very excited that the satellite of asteroid Didymos that we discovered is going to be a target of the first planetary defense technology demonstration mission just 19 years later. I’m looking forward – along with the rest of my colleagues in the DART and Hera Remote Observations Working Group – to contributing to the success of these missions with further photometric observations of the binary system.”

Across the world, Lance Benner, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California, and Mike Nolan, a planetary scientist currently at the University of Arizona, in Tucson, Arizona, found similar evidence using radar facilities at Arecibo and Goldstone observatories. Pooling their findings together pointed to the existence of a binary asteroid.

The work done by Pravec, Benner, Nolan, and other astronomers was enough to earn a name for the binary system. The original discoverer, Montani, retained the naming right to suggest a name to the IAU. In the following year, he selected ‘Didymos’, which was quickly approved.

After Didymos was identified as the target for the DART and Hera missions, the team went one step further and decided to pursue a proper name and identity for the asteroid’s moonlet as well, helping distinguish the mission target from its larger parent. 

With DART a little over a year away from launch, Rivkin and the DART observation team worked with Pravec, Benner and Nolan— relying on the name suggested by Tsiganis — to first route the proposal through the Minor Planet Center, and then the IAU for the final stamp of approval.

“You could think it is just a name, but it is more than that”, says Julia de León, a planetary scientist at the Instituto de Astrofísica de Canarias, in Tenerife, Spain, and one of the chairs of the Hera Remote Observations Working Group.

“We are putting all of our efforts in trying to know as much as possible about Dimorphos before the impact. Having a name makes this tiny, distant moon, more close and in certain way, more real.”