DART Moves Target

One of the more striking of recent space projects proved to be a smashing success (pardon the puns) when data compiled by the US National Aeronautics and Space Agency (NASA) showed on October 11, 2022, that its Double Asteroid Redirection Test or DART hit and moved a distant asteroid sufficiently to prove the concept, namely that this technique can actually be used in real-life situations to protect the Earth from an asteroid on a collision course with our planet. The first stage of fruition of the project came on September 26, 2022, when the DART small spacecraft crashed into the Dimorphos asteroid 11 million kilometres away from Earth seeking to knock it off its normal trajectory. This Mission, costing around $320 million, is part of NASA’s larger Planetary Defence programme, a subject portrayed in different ways by several sci-fi books and movies in which scenarios have even included setting off nuclear explosions on incoming asteroids.

It needs to be clarified that DART’s targeted asteroid poses no threat to Earth whatsoever, but was selected to meet the requirements of a realistic test without undesirable side effects. The target asteroid, Dimorphos (pronounced die-morfos, or two forms in Greek), is part of a paired set with the considerably larger Didymos (pronounced di-di-mos or twin in Greek) around which the former orbits, with distance between the centres being about 1.18k. The orbit of the asteroid-pair around the Sun does not intersect with Earth’s orbit at any point, so there is no chance of collision. Didymos is around 780 metres wide, whereas its orbiting moonlet Dimorphos is believed to be about 160 metres across with mass of around 5 billion kg, roughly of a size that could pose a serious threat to Earth. (The orbits of the binary asteroids are such that they are in eclipse when looked at from Earth i.e. are one behind the other. In fact, therefore, Dimorphos has never been seen from Earth!) The impact of the DART spacecraft, with a mass of about 570kg, was expected to jolt the asteroid just slightly, altering its orbit by a small amount, bringing it closer to Didymos, but with no effect on the latter and causing no major alteration in the gravitational balance in that region of space.

The normal period of orbit of Dimorphos around Didymos has been estimated through Earth-based telescopes by measuring the variation in light as the binary-asteroids change their positions relative to each other. The same technique is being used after the crash to estimate the changed period of Dimorphos’ orbit after the impact.

Effect of DART crash

Ground-based NASA observatories, and those of other countries and space agencies, have been acquiring data on the orbiting period or frequency of Dimorphos around Didymos so as to estimate the effect of DART’s impact on the former. Before the impact, Dimorphos took 11 hours and 55 minutes to orbit its larger parent asteroid. NASA has now determined that the orbit has now changed to 11 hours and 23 minutes (with an error margin of plus or minus 2 mins), that is the orbit has been changed by a substantial 32 minutes. Before launch, NASA had defined that a minimum change in orbit of 1 minute 13 seconds (73 seconds) would be considered a success. By that standard, DART exceeded minimum expectations by over 25 times.

Observations from various sources are now focusing on measuring the efficiency of momentum transfer from the DART spacecraft travelling at 22,530 kmph to the target asteroid, Dimorphos.  Data is also being compiled and studied pertaining to the debris of many tons of rock particles and dust from the asteroid released into space by the impact. It is expected that the recoil from this stream of debris would have added to the push imparted to the asteroid by the DART spacecraft. Analysis of this data would also contribute to a better understanding of the physical characteristics of the asteroid.

The success of the DART experiment has given humanity a direction for building defences against an asteroid crashing into the Earth with potentially disastrous consequences. Even though chances of a large asteroid dangerously crashing into the Earth are quite low, the fact is that a massive asteroid, Chicxulub, did smash into our planet around 65 million years ago, probably leading to the mass extinction of dinosaurs and almost three-quarters of other species on Earth. It is therefore prudent to think about measures to ward off threats from asteroids. NASA and other agencies have found about 40% of asteroids larger than 500 feet wide that could pose a threat to our planet and continues searching for others. In 2019, more than 19,000 near-Earth asteroids had been discovered and tracked, while 30 more are discovered each week.  NASA is also developing a space telescope called the Near-Earth Object Surveyor, likely to be launched in 2026, designed to identify potentially dangerous asteroids.

Mission and target 

Early detection is clearly the first step to ward off threats, but the obvious question is, after detection, what? DART sought to test and demonstrate one potential answer, and has now proved the concept. Apart from the impact itself, there are other interesting aspects of the Mission and novel technologies with significant potential in the future.

In the actual test, starting from launch on 23 November 2021 aboard a SpaceX (Elon Musk’s space company) Falcon9 rocket, the refrigerator-sized DART spacecraft seems to have hit the football-field sized Dimorphos almost head-on, at a speed of about 23,760 kmph and an estimated 17 metres from the selected impact site. As DART came close to the crash time, with insufficient time left for signals to be sent to and from Earth to the spacecraft to direct it accurately, this operation was performed autonomously by DART’s on-board Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO), a high-resolution imager and processor aided by a SMART Nav (Small-body Maneuvering Autonomous Real Time Navigation) system derived from earlier missile systems. When close enough, SMART Nav distinguished between the two asteroids, identified the target Dimorphos, steered DART towards it, and sent back images as the crash neared.

What Mission Control saw, starting just a few minutes before the impact, was a series of second-by-second incredible close-up views of Dimorphos, before the screen went blank. These shots showed a surface littered with small stones or rocks, leading the scientists to believe that the asteroid is a collection of rubble held together loosely by gravity.

A CubeSat or tiny satellite made by the Italian space agency (Light Italian CubeSat for Imaging of Asteroids or LICIA), which detached itself from DART fifteen days before impact and followed it closely, has taken a series of close-up and wide-angle photographs from a safe distance of about 1000km, and a fast fly-by about 55km away 3 minutes after impact. These photographs would capture the impact site and the wider environment, the dust and other material thrown up on impact, the resultant plume, perhaps even the crater, all of which would take a few more days to reach back to Earth. Similar observations are also being made by the James Webb and Hubble telescopes, other deep-space observatories and several terrestrial observatories, such as the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii, which have also already observed the dust and debris plume of the DART impact.

Other technologies

While the main “planetary defense” goal of the DART Mission, with echoes of famous and dramatic Hollywood movies, has understandably grabbed the headlines and most of the media coverage, the Mission also incorporates several innovative and important technologies, and involves multiple collaborating agencies and companies from both the US and internationally.

DART uses NASA’s NEXT-C (NASA Evolutionary Xenon Thruster-Commercial) solar powered electric propulsion system, rather than the conventional tiny rocket thrusters for the steering maneuvers to the final crash point. Thrust is produced by acceleration of ions or electrically charged atoms from the xenon propellant, improving upon earlier versions tried out in the Dawn and Deep Space 1 Missions.       

Whereas the DART impactor craft itself is roughly a 1 metre cube, its main power source is a novel Roll-out Solar Array (ROSA) earlier demonstrated on the International Space Station in 2017. The flexible “wings” are rolled up during launch and, later in space, unfurl and straighten out to 8.5 metres length. This new version, flown in space for the first time by DART, would be used in future missions. Interestingly, a small portion of this solar array also demonstrates a new Transformational Solar Array with high-efficiency solar cells and reflective concentrators along the array length, delivering three times the power of current solar arrays. This may even obviate expensive nuclear power for electricity

All in all, a very interesting technology demonstration for “planetary defence” with, undoubtedly, many further tests to come.