DART (Double Asteroid Redirection Test) Mission
DART is a NASA space probe with the goal to demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon for planetary defense purposes. The mission is intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with Earth. A demonstration of an asteroid deflection is a key test that NASA and other agencies wish to perform before the actual need of planetary protection is present. DART is a joint project between NASA and the JHU/APL (John Hopkins University/Applied Physics Laboratory) of Laurel MD, with support from the NASA centers: JPL (Jet Propulsion Laboratory), GSFC (Goddard Space Flight Center), and JSC (Johnson Space Center). As of summer 2018, the DART mission is in Phase B, led by JHU/APL and managed by the Planetary Missions Program Office at MSFC (Marshall Space Flight Center) for NASA's PDCO (Planetary Defense Coordination Office). 1) 2)
Under the auspices of the PDCO, an international cooperation between NASA and ESA was formed, named the Asteroid Impact & Deflection Assessment (AIDA). For AIDA, NASA provides the DART (Double Asteroid Redirection Test) mission element. 3)
Figure 1: Overview of the DART mission concept (image credit: JHU/APL)
DART is a planetary defense-driven test of one of the technologies for preventing the Earth impact of a hazardous asteroid: the kinetic impactor. DART's primary objective is to demonstrate a kinetic impact on a small asteroid. The binary near-Earth asteroid (65803) Didymos is the target for DART. While Didymos' primary body is approximately 800 meters across, its secondary body (or "moonlet") has a diameter of about 150 m, which is more typical of the size of asteroids that could pose a more common hazard to Earth.
The DART spacecraft will achieve the kinetic impact by deliberately crashing itself into the moonlet at a speed of approximately 6 km/s, with the aid of an onboard camera and sophisticated autonomous navigation software. The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, enough to be measured using telescopes on Earth. The kinetic impact will occur in October of 2022 during a close approach of Didymos to Earth.
DART completed Phase A in mid-2017,Phase B in mid-2018, and is currently in Phase C with the mission Critical Design Review (CDR) scheduled for June 2019.
Even as a low-cost, focused planetary science mission, DART will return fundamental new information on the mechanical response and impact cratering process at real asteroid scales, and consequently on the collisional evolution of asteroids with implications for planetary defense, human spaceflight, and near-Earth object science and resource utilization. DART will return unique information on an asteroid's strength, surface physical properties and internal structure. Numerical simulation studies will support Earth-based optical and radar observations of the DART impact event. A CubeSat, a potential contribution from the Italian Space Agency (ASI), is under consideration to image the ejecta and the DART impact site. 4) In addition, the ESA Hera mission concept is being considered, which would observe the impact site a few years after the DART mission is completed. 5)
DART Mission Rationale:
Four primary strategies are identified as sufficiently mature to warrant consideration as approaches to mitigate an asteroid impact threat [6)]: civil defense warning, sheltering, and evacuating populations; impulsive deflection by a stand-off nuclear explosion; gradual orbit change with a nearby massive spacecraft (the "gravity tractor" concept); and impulsive deflection via a sudden addition of momentum (the "kinetic impactor" concept). DART is designed to be the first demonstration of a kinetic impactor for planetary defense.
DART is needed because there are key unanswered questions about the kinetic impactor technique and because it, like the other primary strategies for asteroid deflection, requires some level of validation and demonstration before it is considered viable for implementation in the event of an impact emergency. Studies of asteroid mitigation 7) suggest kinetic impactors are useful in situations where an asteroid deflection Δv of mm/s to cm/s would be appropriate, namely, when an impact threat of up to hundreds of meters size is identified years to decades before the Earth impact date. This warning time is sufficient for the kinetic impactor deflection to cause the asteroid to miss the Earth.
The imparted Δv from a kinetic impactor is a small fraction of a typical Near-Earth Object's heliocentric speed, which is several tens of km/s, and measuring this small change is a challenge (e.g., it was a cost driver for the former ESA Don Quijote mission concept. The innovation used by DART to overcome this challenge is to target the secondary member of a binary asteroid system for the kinetic impactor demonstration. The Didymos system undergoes periodic dimming episodes called "mutual events" as the primary and secondary move in front and behind each other. These episodes can be precisely timed. Asteroid satellites typically have orbital speeds of tens of cm/s around their primaries, and a change of orbital speed by mm/s from the kinetic impactor changes the binary orbit period by an amount easily measured by an accompanying spacecraft or by ground-based telescopes by analyzing the timing change of the mutual events.
DART's target, the secondary member of  Didymos, will also allow the kinetic impactor demonstration to be conducted at a realistic scale for planetary defense. The target body, at a diameter of ~160 m, is large enough to be a Potentially Hazardous Asteroid (PHA) in its own right if it were a single body. There are an estimated ~6700 PHAs at diameter ~140 m or larger, 8) most of which have not yet been discovered.
DART will also answer a key question about the kinetic impactor technique, which is that the magnitude of the resulting deflection is highly uncertain, owing to the poorly understood contribution of recoil momentum from impact ejecta. The impact ejecta carries momentum back in the incident direction, so that the momentum transferred to the largest remaining fragment exceeds the incident momentum by a factor that may be as much as 3to 5. 9) Determinations of the momentum transfer from the DART impact will allow centimeter-scale laboratory experiments to be tested at sizes many orders of magnitude larger, and will help validate models and numerical simulations.
DART will be the first high-speed impact experiment at an asteroid and at a realistic scale for planetary defense, and the impact conditions and the physical properties of the projectile are well known. DART will determine, from terminal approach imaging, the impact location on the target asteroid, the local surface topography and the geologic context.
Figure 2: The Double Asteroid Redirection Test (DART): Hitting an Asteroid Head On (video credit: JHU/APL)
Figure 3: Fourteen sequential Arecibo radar images of the near-Earth asteroid (65803) Didymos and its moonlet, taken on 23, 24 and 26 November 2003. NASA's planetary radar capabilities enable scientists to resolve shape, concavities, and possible large boulders on the surfaces of these small worlds. Photometric lightcurve data indicated that Didymos is a binary system, and radar imagery distinctly shows the secondary body (image credit: NASA) 10)
The DART mission will impact the secondary member of the Didymos binary system during its close approach to Earth in October, 2022. For a period of several weeks around the time of the DART impact, Didymos is within 0.08 AU from the Earth and is bright enough for useful data to be obtained by small ground-based telescopes. The DART impact will change the binary orbit period by an amount sufficient to be quantified by ground-based observations.
Didymos satellite orbits its primary with a period of 11.9 hours, with a semi-major axis of 1.1 km, and a nearly circular orbit. 11) The primary has a diameter of 780 m, the secondary 160 m. The presence of a satellite has allowed the density of the primary to be estimated as 2.1 g/cm3 with ~30% uncertainty. Ground-based reflectance spectroscopy of Didymos shows it to be a member of the "S complex" of asteroids, the most common compositional group of near-Earth objects.
The impact of the ~560 kg DART spacecraft at 6 km/s will produce a binary orbit period change greater than 7 min (assuming that the incident momentum from the impactor is simply transferred to the target without enhancement). This change in binary orbit period can be measured within a week given expected observing conditions.
The DART spacecraft uses the NASA Evolutionary Xenon Thruster Commercial (NEXT-C) electric propulsion system, which allows for tremendous flexibility in trajectory design. The DART spacecraft will be launched on an Evolved Expendable Launch Vehicle (EELV) into a low-energy escape (possibly as a rideshare). 12) However, the mission can also use a higher-energy trajectory paired with a corresponding reduction in the operational time of the NEXT-C thruster. The Didymos encounter for the electric propulsion mission occurs in the same time frame and a similar geometry as for the chemical design (Ref.9). 13) The EELV selection is progressing with Launch Services Provider (LSP), and will constrain the large trajectory design space afforded by using the NEXT-C engine in the spacecraft design.
Table 1: DART Reference Mission Design
DART's primary launch window extends from mid-June 2021 through mid-October 2021. Three NEXT-C thrust arcs are currently planned for the interplanetary trajectory to target an asteroid flyby and arrival at the Didymos system. The DART reference mission design for launch on June 15, 2021 is summarized in Table 1 and shown in Figure 4.
Figure 4: DART departs Earth using a low-energy escape trajectory. The NEXT-C engine executes three thrust arcs enabling a PHA flyby. Thrust arcs, depicted in orange, also show the thrust vector (image credit: JHU/APL)
The DART spacecraft is shown in Figure 5. The spacecraft has two ~7 m gimballed Roll-Out Solar Arrays (ROSA) to power the low-thrust engine and spacecraft components. The X-band communication system consists of two hemispherical low-gain antennas, and a gimballed Radial Slot Line Antenna (RLSA) for high-gain communication. An IMU and star tracker are used as the primary attitude sensors, and five digital Sun sensors provide Sun-direction information for safe mode. DART is carrying two propulsion systems: chemical and electric. The chemical system is currently used for launch clean up, trajectory correction maneuvers and attitude control. It is a monoprop hydrazine system that consists of twelve thrusters and uses helium as the pressurant. The electric propulsion targets the asteroid flyby and provides flexibility in launch options. It uses xenon for the propellant and its accommodation is a principal driver in the spacecraft and mission design. 14)
Figure 5: Two different views of the DART spacecraft bus. The DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav) is based on the LORRI high-resolution imaging instrument from New Horizons. The left view also shows the RLSA (Radial Line Slots Array) antenna with (solar arrays rolled up). The isometric view on the right shows a clearer view of the NEXT-C ion engine (image credit: NASA)
DART is a simple, low-cost spacecraft. The main structure of the spacecraft is a box with dimensions of roughly 1.2 x 1.3 x 1.3 meters, from which other structures extend to result in measurements of roughly 1.8 meters in width, 1.9 meters in length, and 2.6 meters in height. The spacecraft has two very large solar arrays that when fully deployed are each 8.5 meters long. DART will navigate to crash itself into Didymos B at a speed of approximately 6.6 km/s.
Demonstrated on the International Space Station previously, ROSA (Roll Out Solar Arrays) provides a compact form and light mass for launch that then deploy into two large arrays once in space, each extending 8.6 meters in length. 15)
Figure 6: ROSA during deployment outside the ISS in 2017 (image credit: NASA/JSC)
Figure 7: Overview of the DART spacecraft with the Roll Out Solar Arrays (ROSA) extended. With the ROSA arrays fully deployed, DART measures 12.5 meters (494 inches) by 2.4 meters (98.1 inches), image credit: NASA
NEXT-C (NASA's Evolutionary Xenon Thruster-Commercial): The DART spacecraft will utilize the NEXT-C solar electric ion propulsion system as its primary in-space propulsion system. NEXT-C is the next generation system that is based on the Dawn spacecraft propulsion system and was developed at NASA's Glenn Research Center in Cleveland, Ohio. By utilizing electric propulsion, DART is able to gain significant flexibility to the mission timeline and widen the launch window, as well as decrease the cost of the of the launch vehicle that gets the mission off Earth and into orbit.
Technology development and demonstration
Developing and maturing new technology is also a critical part of the DART mission. The principal technology development for the DART program has been the NEXT-C thruster. In addition to the ion thruster system, the Small-body Maneuvering Autonomous Rendezvous and Targeting Navigation (SMART Nav) system was a large technology effort in Phase B and included development of new FPGA-based avionics. This system is comprised of algorithms, software, and firmware running in the FPGA-based avionics, and it is used to estimate and control the spacecraft's asteroid-relative state. The SMART Nav algorithms and the spacecraft avionics successfully demonstrated TRL- 6. In addition, the Radial Slot Line Array HGA went through its own technology maturation effort and is now at TRL -6, and the engineering model for DART is currently in fabrication.
The spacecraft will be guided to hit the center of the asteroid using the SMART Nav algorithms. The spacecraft autonomously tracks the secondary, performing ΔV maneuvers, adjusting the spacecraft trajectory to ensure impact with the secondary. The spacecraft is in constant communication with ground via the RLSA, streaming images back to Earth at 3 Mbit/s. The high data rate is necessary in order to meet the requirement that the spacecraft take, process, and transmit at least one image in the last 17 seconds before impact.
The NEXT-C electric propulsion system drives numerous elements of the spacecraft design. The large ROSAs are needed to generate sufficient power to operate the NEXT-C thruster at the DART-specific throttle levels. The Power Propulsion Unit (PPU) supplies power to the NEXT-C thruster, and its thermal dissipation is comparable with the dissipation of the rest of the spacecraft. Accommodation of the PPU necessitated the addition of the heat pipes to the structure and a doubler to the PPU panel to redistribute the heat. A dual-axis gimbal on the thruster and single-axis gimbals on the solar arrays are required to keep the NEXT-C thrusting in the desired direction while producing manageable reaction torques on the vehicle and remaining power positive. The NEXT-C system is controlled by a DART-developed flight software application that runs on spacecraft avionics.
Figure 8: Illustration of the deployed DART spacecraft (image credit: JHU/APL)
• June 27, 2019: There are models and simulations, but nobody knows exactly what is going to happen after NASA's DART impactor crashes into the smaller of the two Didymos asteroids at 6.6 km/s – humankind's first full-scale deflection test for planetary defense. 16)
- It will take detailed telescope and radar observations from Earth to find out, complemented by a close-up survey to be performed by ESA's Hera mission.
- The collision itself takes place in late 2022. Meanwhile, PhD student Harrison Agrusa from the University of Maryland – as part of a larger team studying the dynamics of the Didymos system – is among the most qualified people to make an educated guess.
- Harrison has been simulating the interaction between the fridge-sized DART spacecraft and smaller 160-m diameter Didymos asteroid hundreds of times, run on his university's powerful computing cluster.
- His simulations recreate the 780-m diameter main Didymos asteroid and its orbiting ‘Didymoon' as a collection of small spheres – like the rubble piles that researchers believe these bodies to resemble – then apply the equivalent force of the DART impact.
Figure 9: DART mission profile. NASA's Double Asteroid Redirect Test, DART, mission is the US component of AIDA, intended to collide with the smaller of two bodies of the Didymos binary asteroid system in October 2022. ESA's Hera mission will then perform follow-up post-impact observations (image credit: NASA)
- "The interesting thing, depending on where DART hits and how hard, is that we can see a pronounced wobble triggered as a result," explains Harrison.
- "We've compared four different simulation codes to study this post-impact swinging back and forth and seen the same effect recur in all of them, even with conservative estimates of DART's momentum transfer."
- In asteroid researcher terms this effect is known as ‘libration' – the same term used for the wobble of the Moon as seen from Earth, which means that different parts of the lunar surface can be observed over time.
Figure 10: Simulating Didymos asteroids. PhD student Harrison Agrusa from the University of Maryland has been simulating the interaction between the fridge-sized DART spacecraft and smaller 160-m diameter Didymos asteroid hundreds of times, run on his university's powerful computing cluster. His simulations recreate the 780-m diameter main Didymos asteroid and its orbiting ‘Didymoon' as a collection of small spheres – like the rubble piles that researchers believe these bodies to resemble – then apply the equivalent force of the DART impact (image credit: University of Maryland–H. Agrusa)
Figure 11: Modelling Didymoon's post-impact libration. PhD student Harrison Agrusa from the University of Maryland, as part of a larger team studying the dynamics of the Didymos system has been simulating the impact of the DART spacecraft on the smaller body. The result is that the impact imparts a pronounced side to side movement – known as a libration – to the smaller body. ESA's follow-up Hera mission would observe this libration in close-up, in order to better constrain the efficiency of DART's momentum transfer (video credit: University of Maryland–H. Agrusa)
- Like the Moon, the smaller ‘Didymoon' is expected to be tidally locked to its parent at the present time, although it has not yet been confirmed with ground-based observations. Long-range measurements of distant lightcurves – gradual patterns of light shifting over time – or radar imagery do not give enough detail.
- In the same way, any wobble imparted to the asteroid by DART's collision will not be visible from Earth. It will take close-up observations after Hera's arrival to be sure.
Figure 12: PhD student Harrison Agrusa from the University of Maryland is part of a larger team studying the dynamics of the Didymos double asteroid system (image credit: H. Agrusa)
- Harrison has shown that this induced libration is closely related to the momentum transfer efficiency – in other words, Hera's measuring of the libration can be used to constrain the asteroid's deflection. Such a measurement is crucial to developing a usable, repeatable planetary defence technique.
- In addition, Harrison notes that the ability to measure any libration in the post-impact asteroid will also open up a valuable scientific opportunity: "The fundamental frequency of the libration will depend on the mass of the secondary, and how that mass is distributed throughout its interior – in the same way that the frequency of a pendulum's swing depends on its mass.
- "So measuring this effect will give researchers an important insight into the nature of Didymoon's interior, constraining our models. However, it is essential to have a spacecraft on location to make such a measurement."
- Harrison is part of the DART Dynamics Working Group, led by his PhD adviser Prof. Derek Richardson, tasked with performing dynamic modelling of the Didymos system before and after DART's impact.
- "As an undergraduate I interned at LLNL (Lawrence Livermore National Laboratory) in northern California, where I encountered some researchers working on planetary defence," explains Harrison. "I never even knew this was a field until then, but after that I decided I wanted to get involved."
- This summer Harrison returns to LLNL, where he will take advantage of their supercomputer facilities to perform full-scale impact simulations, modelling the ejecta material thrown off of the asteroid by the DART impact.
- "Overall, it's great timing for me," says Harrison. "When the DART mission ends with its impact in 2022, then my PhD does too. We'll get a first glimpse of the actual shape of Didymoon from DART and the LICIA CubeSat – provided by ASI, the Italian Space Agency – it will deploy before colliding. Then, within a few years Hera will be providing its data, so we can rigorously compare our models to reality."
- The Hera mission will be presented to ESA's Space19+ meeting this November, where Europe's space ministers will take a final decision on flying the mission.
Figure 13: Hera surveying Didymos. ESA's Hera mission concept, currently under study, would be humanity's first mission to a binary asteroid: the 780 m-diameter Didymos is accompanied by a 160 m-diameter secondary body. Hera will study the aftermath of the impact caused by the NASA spacecraft DART on the smaller body (image credit: ESA–ScienceOffice.org)
Figure 14: ESA's planetary defence mission. Hera will show us things we've never seen before. Astrophysicist and and Queen guitarist Brian May tells the story of the ESA mission that would be humanity's first-ever spacecraft to visit a double asteroid. The asteroid system – named Didymos – is typical of the thousands that pose an impact risk to our planet, and even the smaller of the two would be big enough to destroy an entire city if it were to collide with Earth. - Hera will help ESA to find out if it would be possible to deflect such an asteroid on a collision course with Earth. The mission will revolutionize our understanding of asteroids and how to protect ourselves from them, and therefore could be crucial for saving our planet. - First, NASA will crash its DART spacecraft into the smaller asteroid - known as Didymoon - before ESA's Hera comes in to map the resulting impact crater and measure the asteroid's mass. Hera will carry two CubeSats on board, which will be able to fly much closer to the asteroid's surface, carrying out crucial scientific studies, before touching down. Hera's up-close observations will turn asteroid deflection into a well-understood planetary defence technique (video credit: ESA – Science Office)
• April 29, 2019: There are so many important components involved in getting the Double Asteroid Redirection Test (DART) to Didymos. As a ground-based observer, I am particularly excited that telescopes are high up on that list of critical elements. I lead the DART observing working group and we have been hard at work for the past several months trying to obtain more information about the Didymos binary system and the orbit of "Didymoon" in particular, the DART spacecraft's target. This information is so critical to the mission because our job as observers is to note the change in the orbit of Didymoon after the impact of DART. We need to have a very firm understanding of the pre-impact orbit to understand how much we've changed the orbit. 17)
• August 30, 2018: The first-ever mission to demonstrate an asteroid deflection technique for planetary defense has moved into the final design and assembly phase, following NASA's approval on Aug. 16. 18)
- DART (Double Asteroid Redirection Test), being designed, built and managed by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, will test what's known as the kinetic impactor technique — striking an asteroid to shift its orbit — and take a critical step in demonstrating how to protect our planet from a potential impact.
- DART's target is the asteroid Didymos, a binary system that consists of Didymos A, about one-half mile in size, and a smaller asteroid orbiting it called Didymos B, about 530 feet (~160 m) across. After launch — scheduled for spring/summer 2021 — DART will fly to Didymos (Greek for "twin") and use an APL-developed onboard targeting system to aim itself at Didymos B. Then the spacecraft, about the size of a small car, would strike the smaller body at approximately 3.7 miles per second.
- "With DART, we want to understand the nature of asteroids by seeing how a representative body reacts when impacted, with an eye toward applying that knowledge if we are faced with the need to deflect an incoming object," said APL's Andrew Rivkin, who co-leads the DART investigation with APL's Andrew Cheng. "In addition, DART will be the first planned visit to a binary asteroid system, which is an important subset of near-Earth asteroids and one we have yet to fully understand."
- The kinetic impact technique works by making a very small change in the orbital speed of the target asteroid. DART will demonstrate the kinetic impact technique and will measure the effect of the DART impact. Observatories on Earth will determine the resulting change in the orbit of Didymos B around Didymos A, allowing scientists around the world to better determine the capabilities of kinetic impact as an asteroid mitigation strategy.
Launch: In April 2019, NASA has selected SpaceX to provide launch services for the DART mission. The DART mission currently is targeted to launch in June 2021 on a Falcon 9 rocket from Space Launch Complex 4E at Vandenberg Air Force Base in California. By using solar electric propulsion, DART will intercept the asteroid Didymos' small moon in October 2022, when the asteroid will be within 11 million kilometers of Earth. 19)
Sensor complement: DRACO
DRACO (Didymos Reconnaissance and Asteroid Camera for OpNav)
The DART payload consists of a high-resolution visible imager to support the primary mission objective of impacting the Didymos secondary. The DART imager is required to support optical navigation on approach and autonomous navigation in the terminal phase. The imager is derived from the New Horizons LORRI instrument 20) and will use a 20 cm aperture Ritchey-Chretien telescope with 0.5 arcsec/pixel. The DART imager will determine the impact point within ~1 m, and it will characterize the pre-impact surface morphology and geology of the target asteroid and the primary to ≤50 cm/px. 21)
Observation campaign strategy
Telescopic measurements of the Didymos system are a critical piece of the DART mission. The DART impact is expected to create a several-minute change in the period of Didymos B's orbit around Didymos A. Observing opportunities occur in spring 2019 and winter 2020-2021 prior to a 2022-2023 opportunity that runs for several months and includes the impact time itself. Observations prior to the impact will serve to establish as precise a baseline as possible for the undisturbed state of the Didymos system.
The observing campaign will focus on photometric lightcurve measurements. By precisely measuring the timing of Didymos B eclipsing and being occulted by Didymos A via these lightcurves, and the change in that timing, Δv imparted on Didymos B will be determined.
During the impact period Didymos will be at roughly V magnitude of 14-15 as viewed from Earth. At this brightness, telescopes as small as 1 meter in aperture can obtain useful data. Dozens of such telescopes exist, which mitigates the risk of bad weather or equipment failure at specific sites. Larger telescopes (4 meter apertures or larger, available on 5 continents) can be used to enable optical coverage by those facilities for 20 out of 24 hours per day near the impact date, weather permitting. The orbit period of Didymos B is 11.92 hours, so a little over two orbits occur per Earth day. The placement of the large telescopes is such that all orbit phases are covered by at least one telescope per 24 hours.
In addition to ground-based telescopes, spaceborne observatories will be used when possible. The Hubble Space Telescope is capable of observing during the impact period if it is still operating, and the limits on tracking rates for the James Webb Space Telescope allow observations 6 weeks before and after the nominal impact date.
Finally, the proximity of Didymos to Earth at the time of the 2022 impact will also allow the use of the Goldstone and Arecibo radars, perhaps in conjunction with the Green Bank radio telescope, to make measurements of the Didymos system. These measurements, along with potential optical studies of ejecta evolution, will provide a fuller understanding of the consequences of the DART impact.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).